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1. e2tongl Terrinetior Ccuple Fig 8 5 Setup diagram for coupling factor measurement B Directivity measurement 1 Set the attenuator to 20dB 2 Read the SWR indicator Use this value as a reference Record the ttenuator setting 20dB in Table 8 1 A3 3 Change the coupler s orientation as specified in Fig 8 6 SWR Inicacator Square Wave senaratcr Jelecio Power Supply Matched Terminal on PN Dinde Modulator Gunn Oscillator Direckionpi coupler orickle Attenuatar Fig 8 6 Setup diagram for directivity measurement 4 Reduce the attenuation and increase the SWR indicator gain by 10dB or 20dB in 10dB steps untill the same value as in 2 is obtained Fill in Table 8 1 A4 with the attenuation of the attenuator The directivity is A3 4 n x 10 dB SAR Indica pr C Return loss measurement of a load Squore H Ge arter PN Ciods Meeulotar rysio Detector Vearoble Attenuator Ne E Slide Screw dina Qerectiana Turer Coupler Frequency Mater Fixed Ailenyotor Load Fig 8 7 Setup diagram for return loss measurement 1 Set up equipment as shown in Figure 8 7 2 Set the probe depth of the slide screw tuner to 5mm 52 3 Set the attenuator to ODb A5 Read the SWR indicator Use this as a reference 4 Change the attenuator to maximum attenuation
2. a constructed b equivalent circuit T A bp 3 c reverse biased d forward biased Fig 2 1 The construction and equivalent circuits of a PIN diode The PIN diode modulator has a diode connected across a waveguide The diode functions as a 21 modulator when the biasing conditions vary due to a sufficiently large squarewave low requency signal applied across the diode under the presence of a microwave signal inside the waveguide When the diode is reversed biased it does not affect the energy flow Full or partial removal of the reverse bias allows the diode to control the energy flow This type of modulator using an insulator junction between p and N offers excellent modulation characteristics due to the minimized rectification activities and harmonics generation during the modulation process B Crystal detector The crystal detector is a device capable of detecting microwave signals based on the square law characteristics Point contact germanium or silicon crystal diodes are the most popular type of crystal detectors Sometimes a bolometer is used for microwave detection although the device is mainly intended for microwave power measurements A typical crystal detector circuit and associated characteristic curves of a crystal detector are presented in Figure 2 2 and Figure 2 3 a and b In figure 2 2 the two filters input high pass and output low pass are to separate the microwave frequencies from the
3. 3 7 aD 0 at y 0 b OR ei 3 8 Applying 3 7 and 3 8 to 3 6 specifies the values of the characteristic constants Kx and Ky Kx n 0 1 2 a Ky m 0 1 2 Using the above relations and substituting Anm the solutions for D are nwX D Anm cos cos bE 3 9 For 0 1 2 mz0 1 2 and n m lt 0 The final form of Hz is now nzX DAY jo Hz Anm cos cos b 3 10 The associated transverse H fields are obtained from 3 10 and the Maxwell s equations in rectangular coordinates Ex Pon Anm cos sin ee 3 11 Ey pr Anm sin cos 02 3 12 ds E Anm sin cos e 007 7 4 3 13 Hy px Anm cos sin ee 3 14 It should be noted that na K2 2 mE S y the cutoff wave number for the nm th mode Also B Kc 2 and nmzO0 The significance of the integers n and m the value of n or m indicates the number of half cycle variations of each field component with respect to X and Y Another words each combination of m and n values represents a different field configuration or mode in the waveguide 2 TM modes Very much the same steps are required to obtain field solutions for TM modes except that this time Hz must be set to zero B Characteristics of rectangular waveguide 1 Cut off frequency and cut off wavelength The exponential form of e Pex represents a wave 30 traveling in the
4. INSTRUMENTS AT3000 3CM WAVEGUIDE EXPERIMENT SYSTEM Manual For Experiment SHENZHEN ATTEN ELECTRONICS CO LTD Quality Assurance SHENZHEN ATTEN ELECTRONICS Co Ltd guarantees quality of this product As for the period to warrant the quality of this product SHENZHEN ATTEN ELECTRONICS makes it a rule to maintain such assurance for 2 years from the purchase date If any failure is found during this guaranteed period please contact your sales representative if possible or SHENZHEN ATTEN ELECTRONICS Co Ltd directly Cautions for Warranty It is recommended for you to keep the followings when using this product so that you may be covered by our quality assurance 1 Before using the equipment you should have full understanding about how to handle and use the equipment as described in the manual 2 Before connecting power confirm if input voltage selected for the equipment is same as the power voltage 3 Rated fuse shall be used 4 Do not use or keep this product under heat and humid condition 5 SHENZHEN ATTEN ELECTRONICS Co Ltd is not responsible for assurance in the following cases a In case of damage caused by mistake or negligence or remodeling its performance b In case guaranteed period is terminated But when guaranteed period is terminated parts supply or failure repairing is provided on charge Such period is 5 years from the termination of guaranteed period TABLE OF CONTENTS Chapter 1 Introd
5. 5 Remove the shorting plate Connect arml to the directional coupler as shown in Figure 10 7 Connect a matched termination to arm3 Leave are4 open Arm4 is the E plane T Since the decoupling to arm4 is almost 30 40dB leaving it open should not affect th accuracy SWR Square Cunr Wave 4 Indiccto Genaral Oscillatar Sharting Crystal Pin Jiode variable plate Modulator Attenuator Directional Coup ar Fig 10 6 Initial setup for return loss measurement 6 Increase the gain of the SWR indicator in 10dB increment Decrease the attenuation As untill the same level as in 4 is obtained Record the results in Table 10 3 7 Repeat 5 and 6 using E plane T arm4 instead of H plane T arml Fower Meli ermraikn cut Oscilator Pla Dek aras E e a Wedutato Mecano D rcczi nal Loar Adaple Fig 10 7 Return loss measurement setup Table 10 3 Return loss measurement data Object Attenuation Gain increase of the Return loss As dB Ag dB SWR meter in 104 dB Absolute steps value Ao Ast nx 10 Armi Arm4 59 Smith Chart Impedance or Admittance coordinates 7 a m Uri Ov sse a v 7 ha Daz 2 mo ed Tuer FHER T ad 45 4 Li 16 E Ls
6. x didi Sin dom 6 5 C Measuring high SWR using a calibrated attenuator 1 Maximize the probe depth of the slide screw tuner 2 Move the porbe along the slotted line untill a minimum is observed 3 Set the variable attenuator to 10dB call this value AI Adjust the gain of the SWR indicator untill 3dB deviation is observed 4 Move the probe along the slotted line and adjust the attenuator untill the same maximum value as in the previous step Read the dB value call this value A2 and record it in Table 6 3 Table6 3 SWR measurement using a calibrated attenuator Probe penetration A dB dB SWR SIR E 5 Calculate the SWR using the following formular and fill in the Table 6 3 Az M 20 6 6 Repeat the procedure at the different probe depths S 10 45 Experiment7 Impedance Measurement 1 Objectives Learn the Smith chart and its application in determining unknown impedances 2 Theory In a transmission line with the characteristic impedance of Zo the reflection coefficient between the incident and the reflected signal is defined as where Z R jX load impedance connected to the line p le complex reflection coefficient The magnitude of the P l P 1 is the ratio of the amplitude between the incident and the reflected signal The angle 0 represents the angle of rotation of the phase at the point of reflection The voltage
7. 2 mismatch loss U lesl2 1 412112 where p lt the reflection coefficient of the source p the reflection coefficient of the load Although Psand p are not directly known in most cases the above equation is still useful in determining the maximum and the minimum mismatch power losses A Maximum mismatch loss This happens when the argument of Ps plus the argument of Py is equal to 180 degrees dredeslieil 0 SRRs SWRL 1 Then the maximum mismatch l y 4 SWR SPR Where SWRs SWR at the source SWR SWR at the load B Minimum mismatch loss This happens when the argument of Ps plus the argument of PL is equal to zero degrees 1 lesll art S Rs S RL 2 Then minimum mismatch t l es 1 1e c1 4 SWRs SFR 41 Example For SWRs 1 5 and SWR 1 25 Calculate the maximum and the minimum mismatch loss 1 5 x 1 25 1 1 1 102 Maximum mismatch loss 4 1 5 1 25 1 5 1 25 2 9 E Minimum mismatch loss 4 1 5 1 25 42 Experiment 6 Standing Wave Measurement 1 Objectives Learn how to measure SWRx using slotted line or SWII indicator 2 Theory At any point along a transmission line we can think of the electromagnetic field as a sum of two waveforms one is traveling toward the load incident and the other is traveling toward the generato reflected The reason for th
8. Replace the load with a short 5 Decrease the attenuation untill the reference level in 3 is obtained Record the attenuator position As In case it is necessary to change the range on the SWR indicator add the increased value to the position of the attenuator to get A6 6 The return loss AS n x10 dB Table 8 2 Data for return loss calculation As 5 Ao nx 10 dB dB dB 53 Experiment 9 Attenuation Measurement 1 Objective Learn attenuation measurement techniques of microwave components 2 Theory Although attenuation in general means reduction or decrease of something it specifically refers to the ratio between the input to the output power in microwave A GB 10log b 9 1 where P is the input power and P is the output power Insertion loss although it has mathematically the same expression as 9 1 has completely different meaning While attenuation is introduced in the system on purpose insertion loss is an undesirable situation The insertion loss is happening due to non ideal physical components in the system In microwave waveguides two different measurement methods are popular power ratio or RF substitution A Power ratio method The power ratio method is simply taking the power measurement with and that the microwave detector operates at different power level in each case causing errors due to the non Ilnearity of the device There
9. Setup equipment as shown in Figure5 5 Waveguide ta Gags Adapter Matched Guan Voricble p 5 Osciilotor Attenuclor Preclionel az ar Coupler Fig 5 5 Setup for power measurement using a directional coupler Do not alter settings on the power supply Gunn oscillator and the variable attenuator Take the reading of the power meter C Measurement of conjugate and Zo power Setup equipment as shown in Figure 5 6 S c San tede Suor Wave Moau ator Curu ctor Power Side lt Oirect ar Turer Coupler Wavyy sida t C tapier urn Gecillatar Fig 5 6 Setup for measurement of conjugate and Zo power Set the variable attenuator to 3dB 1 Modulate the output of the directional coupler with a square wave of 1000pps Adjust the pulse frequency to maximize the SWR meter Zo power 2 Do not change the signal power level Adjust the slide screw tuner for the minimum 39 indication on the SWR meter Use the most sensitive range of the SWR meter 3 Record the reading on the power meter conjugate power 4 Adjust the slide screw tuner to get the maximum indication on the power meter Record the reading D Modulated signal Setup equipment as shown in Figure5 7 1 Adjust the variable attenuator to 10dB 2 Adjust the output of the pulse generator to 0 volt peak to peak Offset the output by 0 SVdc Observe the power meter
10. z direction Let us re examine the relationship of B kK Kc 2 1 K gt Kc Thefjg is real and is indeed traveling in z direction propagating ii K Kc Thefg is imaginary and the propagation mode decays rapidly with distance z in the z direction resulting in propagation cut off The frequency separating the propagation and no propagation is called the cut off frequency Since 2 2 nx nz ac i we Ke oe 2 HET fe a ae 2X UME 3 15 Dx 1 Also from oue and replacing witho 2zf f then K cx Leth be the cut off wavelength when K Kc TE 22 _ 4 he 44 E A l 5 c 2 2 3 16 2 Wavelength in the rectangluar waveguide KK Bes qi 29 LetA be the guide wavelength From A and xy xy 29 3 17 E whereAis the wavelength in free space In order to support propagation in the waveguide A lt A is required This implies to the fact that A is greater thanAin the waveguides Another words the guide wavelength is longer than the wavelength in the free space 3 Phase velocity The phase velocity or the velocity of a constant phase point in waveguide is readily obtained from the frequency and the guide wavelength Vp f gt 4 31 It should be noted from 3 19 that the phase velocity in the waveguide can exceed the speed of light in free space which is 3 x 10
11. 4 gt 4 15 03 5 TE E TS 1 JC ARTT An gt RAD 41 Y SOLED I 1 iF s is gt te oP 4 4 terte t 3 1 z 1 3 5v d eo me MM 9 a 11 a 3 3 1 y 5 vs wi vi te 14 9 D LEH a i ER 13 23 rm E LES 8 m 60
12. Attenuclor Crystal Detector 54 2 Set the SWR indicator gain to either 30dB or 40dB range and adjust the indicator for OdB 3 Using a directional coupler add a matched termination to the setup as shown in Figure 9 2 4 Obtain the reading of the SWR indicator Calculate the actual coupling of the directional coupler 5 Repeat step 2 3 and 4 with the attenuator set at 15 and 10dB respectively 6 Repeat the above experiment using fixed attenuators SWR Indicator cryste Getectar maa Unknown Squore Wave Gene rotor PIN Diode Modulotar Vorlable Attenuator Gunn Oscillator Malched Termination Figure 9 2 measuring attenuation using the power ratio method C Measurement using RF substitution method 1 Connect the crystal detector to the PIN diode modulator See Figure 9 1 2 Set the variable attenuator to 20dB Adjust the SWR indicator s range switch and gain control so that the indicator can indicate OdB 3 Insert the directional coupler and connect the crystal detector to the auxiliary arm of the directional coupler See Figure 9 2 Without altering the SWR indicator setting adjust the variable attenuator untill the SWR indication is the same as before Record the attenuator setting This is the actual value of the attenuation of the directional coupler in this case 4 Repeat the above experiment using a fixed attenuator as a DUT D Measurement of
13. DC output High Pass Filter Low Pass Filtar Microwave Input Dioda De Output Fig 2 2 Typical crystal detector circuit 109 000 10 006 nt ima 1 000 Foward Voltage Reverse Voltage IV Fig 2 3 The V I characteristics of a crystal diode 22 In Figure 2 3 we are interested in finding the relationship between the diode current and the diode voltage In general the curves like the one in Fifure2 3 can be approximated by the Taylor series expressed in terms of the powers of the voltage i Bo a2V Normally the first three terms are sufficient to approximate the entire function If the voltage is expressed as V Acosat where is the amplitude and is equal to 2zf Substituting V into 2 1 yields i 2 2 S 2 2 Using 1 cos ot l cos2mt 1 2 3 2 i apta Acost 1 200 ae 2 4 Now the characteristics of the square Jaw becomes clear In equation 2 4 the DC component 4247 z p 5 cos2ut is contained inthe 2 term The second harmonics is expressed as Therefore we can say that the current in the detector is proportional to the square of the amplitude A of the microwave voltage This concept is only valid up to a certain signal level At higher signal level more terms may be needed in 2 4 and the di
14. SWR indicator gain for obtaining any convenient deflection 3 Apply 9V to the Gunn oscillator 4 Apply modulation voltage to the FIN diode modulator Adjust the offset voltage and the pulse frequency of the square wave generator to obtain the maximum deflection on the SWR indicator B Measurement of decoupling between H arm and E arm 1 Set the equipment as shown in Figure 10 5 2 Set the attenuator to 20dB 3 Select a range on the SWR indicator which gives a reasonable deflection on the indicator Adjust the gain control to a reference reading on the dB scale of the indicator 4 Remove the detector and connect the variable attenuator to arml 5 Connect matched terminations and power meters to arm2 and arm3 and connect the detector to arm4 Keep the power meter at off position 6 Increase the sensitivity of the SWR indicator in IOdB increment untill the same reference as in 3 is obtained The attenuation Az of the attenuator maybe reduced if necessary 57 Varigbie Attenyotor 7 Record the results to Table 10 1 exc pc c tica REN AN Cun FI Ciadr Fixar Yari du L 1 J ariab e Hybi f Ved ty lon Alter sate Atonu clo 1 pa 3 io crga japie Thee Yount Los Fower Meter Fig 10 5 Setup diagram for measurement of decoupling between H arm and E arm Table 10 1 Decoupling betw
15. at any given point on a transmission line is the vector sum of the incident and the reflected voltage waveforms The resultant voltage waveforms are called the standing wave pattern In fact when the two waveforms add phase it forms a peak at that point Likewise when the two waveforms add out of phase a valley or a minimum voltage is observed at that point voltage standing wave ratio VSWR is defined as Emax 1 1 lol VSWR fe RAGS ak dee Enin Lb rn i fol 1 2 The angle of rotation of the phase of the reflection coefficient at a distance of d from the load is determined by 02md n 7 3 The determination of the load impedance can be a three step process 1 Obtain data on the waveguide through well defined measurements 2 Determine the magnitude and the phase of the reflection coefficient 3 Calculate the load impedance The smith chart is used for the third process of determining the load impedance at any given point on the waveguide or a transmission line in general from known reflection coefficients The Smith chart is a graphical representation of the impedance transformation property of a length of transmission line The chart coordinates give the normalized resistance and reactance They are normalized to Zo The characteristic impedance of the waveguide The VSWR circles are usually not included but can be constructed as needed with a compass centered on the center point of the chart Notice
16. direction only 4 Auxiliary 3 Arm Main Arm Fig 8 1 Directional coupler The basic properties of the directional coupler is graphically presented in Fig 8 2 and Fig 8 3 Notice that one end of the directional coupler contains a matched termination a forward wave b reflected wave Fig 8 2 Sampling direction of a directional coupler The coupling coefficient and the directivity which are the most important figure of merit of a directional coupler are defined as the following coupling coefficient 10106 z dB 2 8 1 directivity 10log dB 4 8 2 3 4 nex DUT 1 eid Pr Fig 8 3 Return loss measurement When measuring return loss of a device the input signal is applied at port2 and the device under test DUT is connected to portl Then the return signal is picked up at port3 See Figure 8 3 The power at the detector when the coupling coefficient is C or 1OLogC dB P Us 000006 8 3 50 Pre Since the voltage reflection coefficient of the DUT is given byf should be known to make use of the expression If the DUT is replaced by a short all the input power is reflected back and therefore PI should appear at port3 The actual power at port3 is qual to P C The ratio of the two signals detected at port3 is P C C P 21 IpP 8 4 The ratio as expressed in 8 4 is called return loss The accuracy of he return loss measurement is dependent upon the directivity of the ou
17. probe depths The three different depths are required to be greater than the depth used in the above procedure Fill in the Table 6 1 44 S WR Ingicator Matened Slide Screw Ter inc Lic n Tuner Varlable Attenustor 7 xgd Attenuatar Table 6 1 Probe depth vs VSWR Probe depth mm VSWR B Measuring high range SWR 1 Maximize the depth of the probe of the slide screw tuner Large depth of probe is necessary for high SWR measurements 2 Move the probe along the slotted line untill a minimum is observed on the indicator 3 Adjust the gain of the indicator untill 3Db is shown on the Db scale If necessary reduce the attenuation of the variable attenuator 4 Move the probe along the slotted line untill OdB full scale is obtained on the dB scale Table 6 2 Recording the 3dB measurement 5 Record the position of the probe under the d Column in Table 6 2 Probe penetration d 4 1 min 2 min rg SWR mm mm mm mm mm 6 Repeat the above procedure while moving the probe toward the right and record the position of the probe under the d2 column Repeat the measurement at three different probe depths 7 Replace the slide screw tuner with a shorting plate Find the distance between two adjacent minimums The guide wavelength As is twice of the distance 8 Compute the SWR using the following formula xidi 2
18. that the distance scale on the outside periphery is normalized to the guide wavelength Usually the best way to learn the Smith chart is to try to solve actual problems Let s take a look at a sample problem example A waveguide is attached to a normalized load impedance of 0 6 j1 2 The guide wavelength Ai 15 42 millimeters a Find the impedance 10mm from the load See Figure 7 1 b Find the distance from the load where the first minimum of VSWR occurs Solution 46 a Refer to the Smith chart shown in Figure 7 2 1 Locate point A on the chart representing the load impedance of 0 6 j1 2 Toward Generator 11 2 Fig 7 1 Sample problem for the Smith chart 2 Draw a straight line from 0 to A This line intersects the distance circle outer most circle at 0 15A toward generator Traveling 10mm from the load is same as traveling l mm 42mm 0 2384 from the load 3 Locate a point on the distance circle which is equal to 0 15A 0 238A 0 388A Draw a straight line from the point to 0 4 Draw a circle with radius OA centered at 0 The impedance at point B represents the impedance of a point 10mm away from the load toward the generator The normalized impedance of point B is 0 38 j0 78 b The VSWR of 4 3 at point C is where the maximum of the VSWR pattern occurs The first minimum occurs at point D The distance between point A and point D is 0 5A 0 15A 0 35 Fig 7 2 Solving th
19. to the different properties of silicon and tungsten silicon has few surplus electrons but there are many free electrons in tungsten Therefore when a voltage is applied across the diode in such a direction as to force electrons to leave silicon and enter tungsten a very small current results In contrast when the direction of the voltage is reversed a large current flows from tungsten into silicon This is how the diode can be used for detection of microwave energy The diode is a fragile device it can be easily damaged from an excessive voltage The characteristic of a crystal detector or the relationship between the output voltage and current to the input voltage is such that the device follows a square law within a certain range of input power The square law characteristic means the output voltage is proportional to the square of the input voltage It can also be said that the output voltage is directly proportional to the input power Matched Terminator The matched terminator is essentially a matched load to a microwave transmission line As the standing 8 12 13 14 Reflecting Sheet waves occur due to impedance mismatches in the system the matched terminator is used to minimize the SWR in a system Coaxial Adapter Provides a match between a waveguide and a 50 ohm coaxial line The power flow can be in either direction However SWR in the adapter should be kept less than 1 2 Magic Tee or Hybrid Te
20. 8u sec 3 Experiment procedure Set up equipment as shown in Figure 3 2 SWR Cone Indientor Rellecting i Sheet Straight Ylavegu ee Guan PIN Mad Vorlable 1 Frequency Dac ulater Muculator Attenuator Mieter Fig 3 2 Equipment setup for measurements of frequency andA A Frequency measurement 1 Apply voltage to the Gunn oscillator Also apply 1KHz 2Vp p square wave to the PIN modulator 2 Adjust the variable attenuator to 10dB Set the SWR meter such that the meter indicates approximately the middle of the scale 3 Adjust the frequency of the square wave generator so that the SWR indication is maximized 4 Turn the frequency meter untill there is a significant drop on the SWR indicator Record the frequency in Table 3 1 De tune the frequency meter B Measurement of free space wavelength and guide wavelength 1 When the reflecting sheet is moved toward the open end of the waveguide with the reflecting sheet oriented to the waveguide with right angle then the standing wave pattern should vary due to the reflections from the plate This variance of the standing wave is detected by the probe in the slotted line Find two adjacent positions where the two detected values are minimum The distance between these two points corresponds to the half wavelength in free space Record the distance in Table 3 1 2 Cover the output of the slotted line with the shorting plate Vary the slott
21. CTOR 14 MATCHED TERMINATOR 15 WAVEGUIDE TO COAX ADAPTER 16 HYBRID TEE 17 REFLECTOR WITH STAND 18 SHORTING PLATE 19 WAVEGUIDE STRAIGHT SECTION 20 DVM Experiment 1 The Gunn Oscillator 1 Objectives The objectives of this experiment are to obtain the knowledge on the theory and the operation of the Gunn oscillator as source of microwave frequencies 2 Theory A The Gunn Effect The Gunn effect or the transfered electron effect which was discovered by Gunn in 1963 states that when a small DC voltage is applied across a thin slice of semiconductor materials as illustrated in Figure 1 1 it exhibits negative resistance under certain conditions Gunn used in his case gallium arsenide GaAs and indium phosphide InP Once the negative resistance is developed one can easily generate oscillations by connecting the negative resistance element to a tuned circuit One of the requirements in maintaining the negative resistance state in the semiconductor materials is to keep the voltage gradient across the material over 3000V cm The most appropriate tuned circuit to be connected the semiconductors for microwave frequencies is a tuned cavity Anode Guld Wire Geld Alloy Activa antasi n Layer MA Teu MESE dv lt Heat Sink Cathods Fig 1 1 The side view of an epitaxial GaAs Gunn semiconductor The Gunn effect which takes place only in n type semiconductor materials
22. Load Load Short g di d Load Penetration SWR Min Minima 2 d i d 2 g Impedance mm di mm 411 mm see note frequency GHz Note In the following measurements in case dr dg turns out to be positive move the probe toward the generator If the value is negative move it toward the load Also note that me is always less than 0 25 6 Remove the slide screw tuner and the matched termination from the setup Place a shorting plate to the slotted line 7 Obtain the distance ds and ds2 which correspond to two adjacent minimum VSWRs The guide wavelengthA is 2x ds d Note When measuring the minimum it may be helpful to increase the gain of the SWR indicator to obtain better accuracy 8 Repeat the procedure for two more different probe depths 49 Experiment 8 Basic Properties Of A Directional Coupler 1 ObJective Learn the basic properties of a directional coupler including the coupling coefficient and the directivity 2 Theory Directional coupler as shown in Figure 8 I is basically a sampling device of microwave signal The importance of the directional coupler as a sampling device is that it does not introduce reflections to the system The physical structure of a directional coupler be thought of as a transmission line with one input port but two output ports The directivity of the directional coupler allows energy coupling in one
23. SWR Supply Generator Indicctor Gune FIN Dlode Fixed Yari R s Crysta Oscillator Moduiatar Attenuator Attencator DELI Fig 2 8 b Setup diagram for measuring square law characteristics of a crystal detector 5 Select a range on the SWR meter Adjust the gain control of the SWR meter to obtain a convinient reading on the dB scale Once the range and the gain control are set do not touch the gain control 6 Vary the attenuator setting up to 20dB in IdB increment At each step record the SWR meter deflection in dB and the gain range in Table 2 1 26 Table 2 1 Input power vs attenuation and SWR reading Accuracy 2dB A dB SWR INDICATOR INPUT POWER DEFLECTION RANGE dBm dB dB DEFLECTION AND RANGE dB 27 Experiment3 Propagation Modes Wavelength And Phasevelocity In Waveguide 1 Objective Learn the theory of waveguide Experiment the propagation characteristics of microwave in free space as well as in waveguide 2 Theory A microwave waveguide is a hollow metallic pipe having rectangular or circular cross section In our experiments rectangular waveguides are used The following mathematical analysis are based on the rectangular waveguides Also it is assumed that the user has basic knowledge about the wave equation Circular waveguides can be analyzed in the similar way using cylindrical coordinate W
24. and record the meter reading 3 Adjust the output of the pulse to 2 volt peak to peak Set the offset to 0 volt Record the power meter reading Power Square Supply avc Gererctor r Garr FIN Cade Oserilleat gr Modu atar Waveguide to Coax Acop er Yor coe 4tlenuocor Mcurt ils ower Meter Fig 5 7 Setup diagram measuring the output power of a modulated signal 4 Replace the thermocouple mount and the power meter with crystal detector Connect oscilloscope to the crystal detector Adjust the vertical position of the scope to align the top of the square wave to the zero level on the screen The reason for this adjustment is that the output of the crystal detector is negative Therefore the power at the top of the pulse is actually less than the power at the bottom of the pulse 5 Reduce the attenuation of the attenuator untill the bottom of the square wave lines up with the zero level of the scope Record the change in attenuation E The dB scale Repeat the setup of Figure 5 4 with the variable attenuator set at 20dB 1 Vary the attenuator as specified in Table 5 I At each time record the power meter reading When the meter is switched into a new range make sure the meter is properly zeroed Table 5 1 Attenuator position vs Power meter reading Attenuator setting dB Power meter reading mW 10dB 12 15 20 4 Considerations of
25. at the frequency meter is calibrated in 10MHz increment Power Supply Power Veter Variahle Trequercy The Troe cupie Attanualar Helgi Mo ant Fig 1 7 Setup for measuring the oscillator output frequency vs supply voltage Waveguide la Coaz Adopter Tablel 3 Supply voltage vs measured frequncy voltage V Measured frequncy MHz 20 Experiment 2 Modulator And Crystal Detector 1 ObJectives Learn the basic theory and the operation of the PIN diode modulator Learn the basic theory and the operation of the crystal detector 2 Theory A PIN diode As is shown in Figure 2 1 a the PIN diode is constructed with a thin insulator sandwitched etween P and N materials hence the name PIN diode The thickness of the P and N is much heavier than the insulator In case of reverse bias condition the PIN diode is a high resistive and capacitive device in the microwave frequency It is an avalanche effect device under a orward bias condition an avalanche effect takes place in the insulator allowing holes from P and electrons from N to flow Thus the insulator becomes effectively conductive An equivalent circuit of a PIN diode is shown in Figure 2 1 b In Figure 2 1 c and d the equivalent circuit is modified to indicate the results of biasing R Ri junction resistance Ci junction capacitance 2 seaiconductor resistance Lp Package inductance Cp package capaci tance
26. ave telecommunications systems The superior characteristics of a microwave system comes from the fact that the microwave frequencies have highly directional propagation properties which are similar to those of light Also the high degree of noise immunity of the microwave frequencies in the atmosphere makes the microwave communication the top choice in the long distance communications The AT3000 3cm waveguide experiment system a very effective learning tool on the properties of microwave frequencies offers a variety of experiments centered around the following key components involved in the microwave frequency oscillation transmission through antenna and reception at the receiver Basic components included in AT3000 3cm waveguide experiment system Gunn Oscillator 10 Directional Coupler Optional components available 1 Power meter with thermocouple 2 Slide Screw Tuner 11 Horn Antenna 2EA 3 Slotted Line 12 Hybrid Tee 4 PIN Modulator 13 Wave to Coax Adapter 5 Crystal Detector 14 Waveguide 2EA 6 Frequency Meter 15 Reflector with Stand 7 Variable Attenuator 16 Power Supply 8 Fixed Attenuator 17 Coaxial Cable with Connector 9 Terminator 18 IKHz Square Wave Generator 2 SWR meter 1 2 Technical Descriptions Of Components Used In AT3000 1 Gunn Oscillator A Gunn oscillator named after Gunn who discovered the Gunn effect in 1963 generates microwave frequencies when a Gunn diode which is loose
27. d electron effect takes place at this area first due to a sufficient voltage gradient resulting in a negative resistance domain The domain as described above is considered to be unstable What is happening in such a domain is that as electrons are taken out of circulation at a fast rate the elctrons in front travel forward rapidly and the ones behind bunch up electron bunches This way the whole domain moves across the slice and toward the positive and with an average speed of about 107cm per second As the transfered electron effect takes place in a domain moving electrons to less conductive higher energy band fewer electrons are left behind in the conduction band making this region less conductive at this time As was explained in the previous section this causes the potential gradient to increase making the domain capable of traveling Thus this process of electron transfer and domain traveling repeats itself and is said to be self perpetuating When the domain reaches to the anode of the diode a pulse is applied to the associated resonant tank circuit resulting in oscillations In fact the oscillations of the Gunn diode is caused by the pulse arriving at the anode rather than the negative resistance property of the diode D Gunn Oscillator A picture of the Gunn oscillator used in AT3000 is shown in Figure 1 3 Fig 1 3 The Gunn oscillator in AT3000 Although the oscillator is designed to prevent spurious mode oscillations
28. dvantage of using thermocouple wires is the possibility of measuring the power by measuring the DC volts developed across the thermocouple wires The DC voltage is generated as the wires conduct the high frequency current and heat is generated across the two different metal contacts In contrast a power meter works as the voltage is applied to a meter and the meter is calibrated to indicate the power The usability of the thermocouple wires used to be limitted to low frequencies However The recent thin film technology offered a breakthrough in frequency There are thermocouple materials suitable for microwave applications such as bismuth and antimony These materials develop heat by absorbing the input power resulting in an electromotive force voltage Figure 5 3 shows the construction of the power meter RF Shielding Thermocouple Controlled Gain Mcunt Amplifier Two 5 6V Batteries TR 184 Fig 5 3 Construction of the thermocouple power meter 3 Experiment procedure A Direct measurement Setup equipment as shown in Figure 5 4 38 Wavequide lg Coax Adaptar Gunn Variable Therrriacouple 2scillaccr Attenuatar Mourt Fig 5 4 Direct power measurement setup 1 Without the signal activated set the range to xlmW and adjust the meter to zero 2 Turn on the Gunn oscillator Adjust the variable attenuator to 3dB Read the power meter and record the reading B Power measurement using a directional coupler
29. e A magic Tee is a four port device which is basically a microwave version of a hybrid coil of the type commonly used in telephone repeater circuits It has the properties that when properly terminated in external impedances power incident on any arm splits equally between the two adjacent arms but there is no power coupled to the opposite arm The magic Tee is an essential device in balanced mixers automatic frequency control circuits and impedance measurement circuits A mean of reflecting electromagnetic waves in free space when measuring the wavelength of a signal 15 Shorting Plate 16 Waveguide Straight Section When measuring the wavelength inside of a waveguide a shorting plate is used to create a short zero impedance at the open end of a wave guide A six inch straight section of waveguide used in measurements of the wavelength and the phase velocity inside a waveguide CHAPTER 2 USAGE 2 1 Cares For Use 1 In order to improve the measuring reliability the connection of each micro waves should be made correctly and should adhere to the following rules The central rectangle of waveguide should be kept in a line and the edge should be matched Two flange should be tightly jointed with four screws and nuts so that the micro wave doesn t leak The components of this experiment set should not be used for any other experimental purpose No drop or shock should be applied to every c
30. e load So far the analysis assumed that the waveguide was lossless In case the waveguide is lossy any traces on the Smith chart becomes a spiral rather then a circle In a lossy line the SWR increases when the point of observation moves toward the load and decreases toward the generator 48 3 Experiment procedure A Basic measurement 1 Set up equipment as shown in Figure 7 5 2 Completely unscrew the probe 3 Turn on the Gunn oscillator 4 Aply IKHz modulation signal to the PIN diode modulator 5 Measure the maximum and minimum value on the SWR indicator Also Measure the frequency of the oscillator Square Power SWR Indicoter Slotced Sice 1 ine i Tow Termimn ce Fig 7 5 Setup diagram of impedance measurement B Impedance measurement 1 Observe the SWR indicator deflection at the 40dB range Take the reading 2 Bring the probe of the slide screw tuner into the device such that the depth of probe is approximately 5mm 3 Move the probe along the slotted line untill a maximum deflection is observed on the SWR indicator 4 Adjust the SWR indicator untill the meter indicates 1 0 5 Move the probe along the slotted line until minimum deflection is observed Record the SWR value 651 and the depth of the probe d in Table 7 1 Table7 1 Data recording table for the impedance measurement PIN Diode Modulatar Vor cbie Actenuator Oscil olw Probe
31. e begin our analysis with the wave equation A Wave equation The reduced wave equation is expressed as 2 2 Vib 2 0 3 1 where 2 is the scalar wave function and the wave number is defined as K 3 2 In rectangular coordinate system 3 1 becomes e A K2 0 ax ay az 3 3 The rectangular coordinate system is established in this case such that z is the direction of propagation as shown in Figure 3 1 Y eee ee 22 21 Fig 3 1 Rectangular waveguide in rectangular coordinate system Our goal is to obtain a solution which has a form of 28 f z 3 4 where f is a function of z only and g is a function of x and y or other suitable transverse coordinates only Solving 3 3 for using the separation of variables method gives A cos KxX Aosin KxX Bicos K X Bosin K X J8az j Raz Cre par Cze n J8g Where KK K and 92 Bg 2 wavelength since the propagation takes place only in the z direction A wave propagating in the positive gz j Bgz Z direction is represented by e and therefore corresponds to a wave propagating in the negative z direction Three types of propagation modes are of particular interest for us TEM transverse electromagnetic modes In these modes both electric and magnetic fields are transverse to the direction of propagation Thus there is no field components in the direc
32. e example using the Smith chart When working on a determination of unknown impedance 1 is necessary to establish a reference plane which the impedance can be related to For example the input terminals of the unknown impedance can be a reference plane Once the reference plane is established the unknown impedance can be measured a Connect the unknown impedance to a slotted line then measure the VSWR and the position of minimum value 47 b Replace the unknown with a short at the output of the slotted line then measure the distance between two adjacent minimum values The distance obtained should be equal to 0 5A of the guide wavelength c Choose one of the minimum points as a reference See Fig 7 3 Output of the Slottarn Line Probe Minimum with Short Minimum wiih Fig 7 3 Fundamentals of impedance measurement Draw a VSWR circle on the Smith chart See Figure 7 4 The impedance at the voltage minimum is 1 SWR When a short is placed across the load the minimum of the VSWR moves toward the load See Figure 7 3 Therefore the impedance at the load is determined by drawing a straight line from a point d A away from the zero of the outer most circle to the center of the VSWR circle The intersection of the circle and the straight line represents the load impedance Fig 7 4 Finding the load impedance using the Smith chart Notice that the line impedance equals the load impedance at A s 2 Ai 3Aj 2 from th
33. e extremly high value of Q The other characteristics of the microwave resonator which make the analysis difficult are the circuit parameters vary depending upon the propagation modes Unlike the low frequency case the meaning of the voltage and the current in the tank becomes ambigious making the definition of Ro difficult Usually the Ro is defined in microwave as Je dz By 4 3 where E is the maximum potential between two points in the cavity and W is the power dissipated in the cavity The o Qo and Ro of simple cavities can be found from the dimensions of the cavity and the power loss on the cavity walls However due to the complexity of the cavity structure it is almost impossible to calculate Qo and Ro Instead actual measurements of a few parameters of the cavity make it possible to determine the entire characteristics of the device Let s consider a cavity system in Figure 4 2 a In Figure 4 2 a Zo is the characteristic impedance of the input waveguide Zo is the characteristic impedance of the output waveguide Rg is the internal resistance of the generator Ru is the load resistor An equivalent circuit of Figure 4 2 a can be developed by lettingZo Rg Zo RL See 33 Re Figure 4 2 b Fig 4 2 a Atransmission cavity Rg R SET Y o im Fig 4 2 b equivalent circuit of Fig 4 2 a In Figure 4 2 b the cavity is effectively a series resonant circuit coupled by two ideal transfo
34. e point where the SWR meter reading is minimum Read the dB indication on the meter P 5 Plug in the value obtained from 4 into the following equation to get the ratio Ps Nur v ble Aran ctor Fl ed Advenuul F 5 Diode baodulstor Gunn Jszillalor P 10108 Pp 6 SinceAPz1 Ps calculate AP 2 and convert it to dB example 1 X dB 101og AP Pg 7 Slowly rotate the frequency meter and find fl 2 andA f where the SWR meter reading indicates X dB as calculated above 36 Experiment 5 Power Measurement Objectives Learn different ways of measuring power Learn how to evaluate the accuracy of the power measurements 2 Theory In general the term power is defined as the time rate of transferring or transforming energy In case of microwave the energy is used in many different forms exchange of information over long distance heating a microwave oven or acceleration of particles in nuclear engineering etc The most common practice of power measurement at low frequencies is to measure the voltage an the current of the device under test then calculate the power from the lumped value of the circuit parameters However at microwave frequencies the difficulty arises due to the distributed nature of the circuit elements Another factor affecting is the reflection of the signal at whereever there is an impedance mismatch Two types of power m
35. e reflection is as was already discussed in the previous chapter due to the impedance mismatch Any open spot on the line is considered to be an impedance mismatch and becomes a cause of the reflection as well The amplitude and the phase of the reflected wave depend upon the load impedance The degree of the attenuation of the line affects the amplitude of the reflected wave also The only way the reflection can be eliminated is either the line is infinitely long or there is impedance match between the load and the transmission line A standing wave results from the two traveling waves in opposite direction As was discussed in the previous chapter the vector sum of the two waves create minimum and maximum points on the standing wave pattern Typical standing wave patterns in a lossless transmission line are shown in Figure 6 1 Resistive CapecHive tous Aesistiva J4 F iow Distanca trem we Losd nesasured along thee Ives Fig 6 1 Standing wave patterns in a lossless line In Figure 6 2 a voltage standing waveform in a transmission line having a characteristic impedance of Zo and a load impedance of 7 is shown Er m Ei Zu Diste ce from fhe Load Fig 6 2 Avoltage standing waveform In Figure 6 2 the complex reflection coefficient is 43 Er z Lo p Ei Sige Xe 6 1 Where Er reflected signal Ei incident signal Z complex impedance at a giv
36. e waveforms of the square wave modulation and detection is shown in Figure 2 7 L I T ee P max Figure 2 7 Square wave modulation and detection As one can see from Figure 2 7 the attenuator setting deviation A can be expressed as Pmax Vmax 20 log 2 9 Pmin Vmin AdB 10 log 25 B The square law characteristics of a crystal detector Squara Def lor Pit Diode M nci ator Meler Wur Waveguide i 2 Aoup iwr variazla A tenustor Fixed Attenv otar Gara Oscillator Fig 2 8 a Setup diagram for output power level setting 1 Set up equipment as shown in Figure 2 8 a Calibrate the power meter to zero at the X0 1 range Observe the meter for a few minutes Make sure the calibration is maintained 2 Apply voltage to the Gunn oscillator Also apply 1KHz square wave to the PIN modulator At this point modulation should take place 3 Set the variable attenuator to 0 The power meter should indicate between 0 02 and 0 15mW Refering to the conversion table on the power meter change mW reading to dBm For example 0 1mW is equal to 10dBm 4 As shown in Figure 2 8 b replace the waveguide to coax adapter the thermocouple mount and the power meter with a crystal detector and a SWR indicator AdJust the modulating frequency such that the deflection of the SWR meter is maximized Power Square Neve
37. eading is 6 3mW with the supply voltage of 8 5V Then the power in dBm l0log 6 3 8dBm attenuation to 8dBm The total power is Now convert thelldBm which is the Gunn diode output power back to the milliwatts Pout 11 z Pout 12 60V ink 16 log 4 Repeat step 3 and complete the Table 1 2 Table 1 2 Data for the supply voltage and the output Voltage V 4V T4 5V power meter meter mW power dBm attenuator dB Gunn diode dBm Gunn diode mW 5 Draw a graph showing the relationships between the supply voltage and the output power Figure 1 6 HERE 3 4 5 a 2 9 vedo Fig 1 6 Gunn diode supply voltage vs output power characteristics C Measurement of the oscillator output frequency vs supply voltage 1 Set up the equipment as shown in Figure 1 7 Set the supply voltage to 9V Set the attenuator to the maximum attenuation Switch the power meter 1 Reduce the attenuation untill the power meter reading is close to the right side of the meter scale approximately 0 8 to 1mW Slowly turn the frequency meter Observe a dip on the power meter when the frequency meter is exactly same as the frequency of the Gunn oscillator 2 From the lowest supply voltage at which oscillations occur to the maximum of 10V supply voltage vary the voltage in an increment of IV Each time fill in the Table I 3 Notice th
38. eaning of TSS is there has to be a minimum microwave power level to make the pulse ride above the noise The TSS of a detector is very much dependent upon the bandwidth of the amplifier which follows the detector since the noise amplitude on the scope is determined by the bandwidth One MHz bandwidth and 50dBm of TSS are typical values of a microwave detector Experiment Procedure A Square wave modulation 24 Square Wove cill Genergtor ogcode Gunn PIN Dioda l Variaole Crystal Cscillator MaduIctor Atte7uator Detector Fig 2 6 Setup diagram for square wave modulation 1 Set up equipment as shown in Figure 2 6 2 Apply 9v to the Gunn oscillator 3 Set the variable attenuator to 10dB 4 Adjust the square wave generator to 1KHz and 2Vp p output Connect the generator to the PIN modulator 5 Adjust the scope so that the top of the square wave aligns to the zero level on the screen 6 Adjust the attenuator so that the bottom of the square wave aligns to the zero level on the screen 7 Repeat above measurement for IVp p negative square wave 8 Calculate the modulation depth for the two modulating inputs of 2Vp p and 1 Vp p using the following equations Vmax AdB 20log 2 2 5 2 7 Vmin where A is the difference in the attenuator settings between step 3 and 6 where m is the modulation depth Vmax Vmin 1 as 5 5 2 8 Vmax 1 A sketch of th
39. easurements are involved in the microwave power measurements average power or peak power measurement The average power is the time average of the sum of the product of the instantaneous voltage and current over the time period This relationship is illustrated in Figure 5 I and a mathematical expression is given in 5 1 P NST RL Zo Relative Amplitude Tims Fig 5 1 Instantaneous power appearing at the load resistor l T Pave eT eidt where period instantaneous voltage 1 instantaneous current It should be noted in Figure 5 I that the frequency of the instantaneous power Pinst is two times of the frequency of e and i The average power in an alternating current circuit is expresse as Pasyv Erms Cos 5 2 where is the phase angle between E and I Now consider a duty cycled pulse as shown in Figure 5 2 The peakpower of the pulse is related to the average power of the pulse by the duty cycle of the pulse 37 Pavg Ppeak duty cycle where duty cycle T ne i r H i cu Power fime Fig 5 2 Peak power of a pulse Usually average power is involved in the microwave circuit which has a contineous signal source In contrast where pulsed signals serve as a signal source peak power is more meaningful way of expressing the power Let s take a look at the details of the thermocouple power measurement method The main a
40. ed line and locate a position where the detected output voltage is minimum From that point find another adjacent point where a minimum is detected again The distance between two points is the half of the guide wavelength Record the value in Table 3 1 Table3 1 Comparison of measured and calculated values of frequency free space wave length and guide waveleagth frequency Measured values Measured values Ag Calculated alues calculated values Ag 32 Experiment 4 Q Value And Bandwidth Of A Resonance Cavity 1 Objectives Learn the theory of a resonance cavity Experiment the relationship between Q and bandwidth Learn how to measure the fl of a resonance cavity 2 Theory Resonant circuits are of great importance for microwave oscillators tuned amplifiers and frequency measurements etc A resonant circuit such as the one shown in Figure 4 1can be defined in terms of R L C when the frequency is limited up to approximately 100 MHz E t Rs Figure 4 1 A resonant circuit In fig 4 1 Ro Qe Ro 2 55 Rs diode ehe OS 4 1 where Ro is the parallel equivalent resistance wo is the resonant frequency Qo is the Q of the resonant tank asol wo a Qo o EE Ra olto From 4 1 LC Rs 4 2 When the and L are know o Qo and can be easily calculated However in microwave circuits the analysis of the resonant cavity becomes difficult due to th
41. een arml arm4 ATTENUATION OF VARIATIONS OF THE SWR DECOUPLING THE VARIABLE METER GAIN in 10dB steps A z nx10 dB ATTENUATIOR n steps A dB A dB C Measurement of insertion loss of hybrid T refer to Figure 10 4 1 Connect the detector to the attenuator which is set at 20dB 2 Select a range on the SWR indicator which gives a resonable deflec tion on the indicator Adjust the gain control to a reference reading on the dB scale of the indicator 3 Remove the detector and connect the arml of the hybrid T to the attenuator 4 Connect the matched terminations and the power meters to arm3 and arm4 Also connect the detector to arm2 5 Decrease the attenuation A4 untill the same reference level as in 2 is obtained The insertion loss between arm1 and arm2 is A3 A4 6 For insertion loss between arml and arm3 repeat 4 and 5 7 For insertion loss between arm4 and arm2 repeat 3 4 and 5 8 Record the results in Table 10 2 Table 10 2 Insertion loss of different path ig TELE TWE Jf A t A3 dB Ag dB 1 2 20 1 3 20 4 2 20 D Return loss measurement of H arm 1 Set up equipment as shown in Figure 10 6 2 Turn on the Gunn oscillator Do not exceed 9V Adjust the square wave output for the maximum modulation 3 Set the variable attenuator to 20dB As 58 4 Set up a reference point on the SWR indicator
42. en point The evaluated at the load is Ss 6 2 p L Z Zo Then Emax Ei Er VSWR S HER zm ver 6 3 Therefore E S 1 5 6 4 3 Experiment procedure 1 Set up equipment as shown in Figure 6 3 Pawar Supp Gunn P N loda P Oscillator Maodulatar Fig 6 3 Setup diagram for SWR measurement 2 Set the Gunn diode supply voltage to 9V Do not exceed this value 3 Set the variable attenuator to lOdB 4 Set the SWR indicator range switch to 20 40dB Turn on the indicator 5 Turn on the Gunn oscillator 6 Apply the modulation signal to the PIN diode modulator 7 Adjust the SWR indicator frequency for the maximum meter deflection A Measuring low and medium range SWR 1 Move the probe of the slotted line and observe the SWR indicator meter deflection 2 Completely disengage the probe of the slide screw tuner At this point the VSWR indication should be very small less than 1 3 Therefore use the expanded scale for better reading 3 Move the probe in the slotted line untill a maximum deflection is observed on the SWR indicator Adjust the gain of the SWR indicator untill the expanded meter reading reaches 1 0 Both the coarse and the fine gain adjust are needed in the expanded scale 4 Move the probe to where a minimum deflection is observed Take the reading on the expanded scale and record it in Table 6 1 5 Repeat the above procedure with three different
43. equencies where the D l 4582 1 maximum power is reduced to one half This happens when r O wo fo L 28 Aw 4 9 where Af is the half power bandwidth 3 Experiment procedure A Measurement of Q values using the power measurement technique Waveguide ip C ox Adapte gunn Voriacle Frequerey nerrrocouple Oscillcter Attenuator Meter Mount Fig 4 4 Setup diagram of 0 measurement 1 Set up equipment as shown in Figure 4 4 2 Apply voltage to the Gunn oscillator Set the range switch of power meter to xlmW Adjust the variable attenuator for the maximum meter deflection Refer this value as Po 3 Turn the frequency meter slowly and find the power and frequency reading when the power meter reading is minimized Call these values Pg for power and fo for frequency 35 4 Slowly rotate the frequency meter Find two frequencies fl and f2 where the power reading is equal to AP 2 see Figure 4 5 Fig 4 5 The resonant curve of a resonant cavity B Measurement of Q using SWR method Sqacr i Sex Powar Gens dio idicater Frequs sy Cryztal Melon Fig 4 6 Setup diagram for measurement of using SWR method 1 Set up equipment as shown in Figure 4 6 2 Turn the Gunn oscillator on and set the attenuator to 10dB 3 Adjust the range switch and the gain adjust to obtain OdB on the SWR meter 4 Slowly rotate the frequency meter Find th
44. ese Directional Coupler Magic Tee also called as Hybrid Tee The following picture is an example of using a directional coupler and the arrows indicate the possible wave directions X a AM A 5 990 a gt i i n The directional coupler of picture a shows that in the case that the wave enters to Pl the same PI wave is transmitted to P2 and P3 However picture b shows an opposite case of a which the wave enters through P2 Although this wave is transmitted to P1 from P2 it is not transmitted to P3 If using these characteristics one M W antenna can be made for transmission and reception and the reflection wave in the waveguide circuit can be detected Experiment 10 EXT 10 provides the detailed information about Magic Tee component Please refer to Experiment 10 11 2 4 The Way To Use Power Meter Option AT3000 Microwave Trainer provides the M W Power Meter Model PM 3001 The following picture shows an example of using the Thermo Couple Power Meter PM 3001 Note PM 3001 Power Meter measures up to several 1uW through a thermo probe If the RF power over 1W is directly measured without attenuation the M W detection thermo probe may be damaged PM 3001 holds the frequency measurement range of 10MHz 12 4GHz and is made of three range like0 1 mW 1mW and 10mW Therefore when initially measuring measure by setting the 10mW range and gradually select a lower range according to
45. f the cavity is equal to the frquency ofthe waveguide there is a maximum energy transfer from the waveguide to the cavity This condition is indicated 5 by a large power drop on the power meter which is connected to the waveguide The actual frequency is obtained by reading the calibrated dial 4 Thermocouple Power Meter Optional A high quality thermocouple high frequency materials for the thermo junctions with low error rate can convert the microwave energy to a readily measurable DC voltage The DC voltage can be easily amplified then fed to a meter The meter indication is calibrated to represent the power level in the waveguide S jpe amp ae 4 4 4 Thermocouple Head Connection Cable Power Meter 5 Variable Attenuator A variable attenuator provides an attenuation by varying the degree of insertion of a matched resistive strip into a waveguide The variable attenuator is used to control a power level or to isolate a source from a load 6 Fixed Attenuator The purpose of the fixed attenuator used in AT3000 is to provide a fixed attenuation of 20dB The attenuation is obtained by insertion of a thin conducting absorber into a straight portion of a standard waveguide 7 Directional Coupler The directional coupler which allows directional coupling of energy in the waveguide is basically a sampling device of the microwave signal A directional coupler is consisted of two waveg
46. fore the measurement results need to be compensated For example when the Output power of the detector diode is maintained at less then I milliwatt level about O 3dB compensation is necessary for up to 20dB attenuation B RF substitution method The error associated with the detector in the above method is eliminated in the RF substitution method by first measuring the output power with the DUT then replacing the DUT with a calibrated variable attenuator By properly adjusting the variable attenuator to the same power level as before attenuation of the DUT then is simply the amount of attenuation of the variable attenuator So far in both methods we are based on the assumption that the errors due to the impedance mismatch is insignificant Sometimes the SWR indicator itself introduces e small amount of errors The AT3000 components are designed with less than 0 4dB of error rate 3 Experiment Procedure A Preliminary adjustment 1 Set up equipment as shown in Figure 9 1 Squure Wave Generotor PI Diode Madulotor Fig 9 1 Setup for preliminary adjustment 2 Turn the power supply on 3 Apply 1000pps modulation signal to the PIN diode 4 Adjust either the pulse repetition rate or the frequency on the SWR indicator for the maximum deflection on the SWR indicator B Measurement using the power ratio method 1 Set the variable attenuator to 20dB SWR Indicator Os llatear
47. is a result of the property of the semiconductor itself It is found that any parameters associated with junction or contact properties as well as voltage or current do not affect the Gunn effect Only the electric field is required to be above the threshold to maintain oscillation The Gunn diode is insensitive to the magnetic field and thus it does not respond to any incident magnetic field The frequency of oscillation is mainly determined by the time the electrons in a form of a bunch take to traverse the given slice of material B Negative resistance and transfered electron effect 15 The energy bands and energy levels of GaAs is shown in Figure 1 2 Notice that this material possesses an empty energy band at the top of the energy level The partially filled energy band is bellow the empty band When the material is doped with n type material there are excess electrons in the material ready to flow when a voltage is applied across the diode Emory Enargy Narrow Forbidden Band Gap 6 5 Partly Fillen Energy Hand Forbidden Energy Gap SSS gt Band Filled Energy Other Energy Randy and Gaps Belew Fig 1 2 The energy levels in a GaAs Gunn The current flowing in the diode is proportional to the voltage and the current is oriented toward the positive end of the GaAs The situation of the higher the voltage the larger the current is equivalent to positive resistance However when the
48. ld distribution inside of the hybrid T A view of the electric field with the input applied at portl is shown in Figure 10 3 a It s assumed that all arms of the hybrid T are properly matched 1 Fig 10 3 a Electric field with input applied at portl The field as shown in Fig 10 3 a is an even symmetry about the mid plane If the input signal is applied at port4 the signal splits equally to port2 and port3 but no part of the signal enters 56 Fig 10 3 b Electric field with input applied at In Figure10 3 b A sideview at the junction of the hybrid T is shown when the input signal is applied at port4 in TEIO mode The reason for no coupling to port1 is duo to reciprocity and the symmetry of electric and magnetic field The signal divides equal ly to port2 and port3 but the phase is 180 degree out of phase Arml is sometines refered to as H arm because it is in the plane of the magnetic field Arm4 is refered to as E arm for the similar reason Another place where the inputs can be applied to arm2 and arm3 at the same time In this case the vector sum of the two inputs will appear at arml At amd the vector difference of the two inputs will appear 3 Experiment procedure A Initial adjustnents Square SWR Wcve Generator Indicotor Gunn DIA Diode Fixed Crystol Oscilato Modulotor Attenuator Detector Fig 10 4 Initial setup diagram 1 Set up equipment as shown in Figure 10 4 2 Adjust the
49. level of the applied voltage reaches a sufficiently high value electrons instead of trying to travel even faster transfer to the higher energy band which is empty and less mobile As a result the current flow decrease exhibiting negative resistance phenomenon The electron transfer from a lower energy level to a higher level is called transfered electron effect If the voltage level is increased even further the mobility of the electrons in the higher energy band begins to improve resulting in an increased current Gunn Domains The frequency of the oscillations in the GaAs has to do with the formation and the transit time of the electrons which form themselves in electron bunches The negative resistance effect is an important factor in understanding the Gunn oscillator however the negative resistance effect alone does not explain everything that is happening inside the oscillator The other important element is the formation of domains or Gunn domains The amount of free electrons available in the GaAs depend upon the density of the n type material doped in the GaAs Since the density of doping is not necessarily uniform across the GaAs there are fewer free electrons where the doping density is low Fewer free electrons mean less conductivity and therefore the potential difference in such an 16 area becomes greater than the area where there are more free electrons Hence as the applied voltage is increased eventually the transfere
50. low values of attenuation 1 Set up equipment as shown in Figure 9 3 Malchsc Terminotion Square Sar Cone bor i Ircicatar 2 Shorlir4 i 1 perice Under ip E Fawer Supely Fised Aber ys lor Belted Live Voriopie A lus 2 4 0 Madulalo Cure Jscilialo Figure 9 3 setup diagram for the measurement of low values of attenuation 2 Set the variable attenuator to 20dB 3 Measure the input SWR of the device under test a directional coupler in this case 4 Deterzine the attenuation using the following expression A 10log xi tt 4 9 2 55 Experiment 10 Study Of A Waveguide Hybrid Tee 1 ObJectives Understand the basic principle of Hybrid T Learn measurement methods on Hybrid T characteristics 2 Theory A hybrid T or a magic T of Figure 10 1 is basically a microwave version of a hybrid coil of the type commonly used in telephone repeater circuits See Figure 10 2 2 Fig 10 1 Waveguide Hybrid T 4 0 9 Fig 10 2 Balanced bridge circuit When the bridge circuit is properly matched by external impedances the input signal applied at portl appears at port2 and port3 but no signal appears at port4 In the same manner when the input is applied at port4 then the signal appears at port2 and port3 but no signal appears at portl The above input to output relationship can be described in terms of the fie
51. ly coupled to a cavity is connected to a 8 10V DC power source The power output of the Gunn oscillator ranges from 5 to 20 milliwatts depending upon the supply voltage and other parameters of the oscillator It is recommended that output frequency of X Band of this manual s experiment procedures should be fixed 10GHz 2 PIN diode Modulator A PIN diode modulator utilizes the property of a PIN diode which is placed across a waveguide If the PIN diode is reverse biased he insertion loss of the diode is so small that it does not affect the energy flow inside the waveguide However when the reverse bias is removed either fully or partially the diode begins to control the energy flow thus creating an amplitude or pulse modulation effect Impedance matching is required to obtain the maximum power output Leaving the diode unbiased could be destructive to the diode when there is a signal flow in the system 3 Frequency Meter The basic working principle of the frequency meter in AT3000 comes from the high Q resonant characteristic of the resonant cavity which is attached to a waveguide The microwave signal in the waveguide is coupled to the resonant cavity through a small slot between the cavity and the waveguide The effective size of the cavity and thus the resonant frequency of the cavity is variable by moving in and out an adjustable plunger which has a calibrated dial knob assembly When the resonant frequency o
52. mismatch loss and maximum power transfer Mismatch loss is defined as the power loss in the system due to reflections In fact the impedance mismatch between the generator and the load causes multiple reflections which produce a random phase determined by the electrical equivalent length of the waveguide The random phase makes the power and attenuation measurements difficult due to the errors 40 occuring and the power level deviations When the SWRs are know for the source and the load the maximum and the minimum of the signal level deviations can be found assuming the attenuation of the waveguide can be ignored One way to determine the deviation values is to use a chart such as the one pple in Figure 5 8 FEFPEDUEPT AIR eect i DUE 57 z 8 tt eb ee eia Sims Ld LPR 5 99 709 LOAD OR SOURCE Sw 52 MR Figure 5 8 Conjugate mismatch loss chart The maximum power transfer takes place when the load impedance is equal to the complex conjugate of the source impedance For example a source with 50 j25 ohm impedance delivers the maximum power to the load when the load impedance is 50 j25ohm When the load impedance is not the conjugate of the source impedance the conjugate mismatch loss the available power of source power delivered to the load The above relationship can be expressed in terms of the reflection coefficient 11 pu
53. ode is no longer considered as a square law device In addition to the detector circuit of Figure 2 2 the diode itself can be expressed in terms of an equivalent circuit In Figure 2 4 a complete equivalent circuit of a detector is presented 23 3 Fig 2 4 Equivalent circuit of a detector In Figure 2 4 Ro and C represent the impedance of the junction and r is the body resistance of the diode A figure of merit of a detector is the voltage and current sensitivity of the detection function which is expressed as Open circuit voltage Rolac Voltage sensitivity c 2 5 Input power Pin Short circuit current Ro Current sensitivity 85 Ve 2 6 Input power Pin In order to maximize the output power it is necessary to match the microwave impedance of the diode to the characteristic impedance of the waveguide Another reason for the impedance matching is to minimize the reflection from the detector since the measurement accuracy is affected by the reflection The mininum signal level a diode can detect depends on the noise in the diode The ability of a diode to detect a signal in the presence of noise is called the tangential sensitivity TSS of a detector A graphical illustration of the concept of TSS is sketched in Figure 2 5 Signal O cillosco pe ere Fig 2 5 The TSS of a diode In Figure 2 5 a microwave signal which is pulse modulated is detected amplified and displayed on an oscilloscope The real m
54. omponent Keep away from humidity or heat Should avoid to use in a dusty area and should be kept in its storage case after use QN tA A Q N Check whether any foreign substance is attached on the entry exit of waveguide before the assembly for the experiment circuit construction of components and remove the foreign substance if existing 7 While the oscillator is oscillating the internal part of oscillator must not be observed through the output part Because the oscillator used in this experiment unit a relatively small power the output is not dangerous to other body parts but eyes can be permanently damaged 2 2 The Way To Use The Component With IN OUT Directions Among the 15 kinds of microwave components within AT3000 the input and output of wave course is classified as in the following Slide Screw Tuner Slotted Line The above two kinds hold the sliding location of 0 40 Since the flange with 0 on this adjustment scale becomes input and the flange with 40 scale becomes output the connection should be made correctly according to the experiment and route The following picture shows a connection example of Slotted Line 10 2 3 The Way To Use The Component With Directions Among microwave components there are components which has the pre set connection direction These components are not transmitted in one certain direction and send the wave only to a specific direction The following components are th
55. pler which describes how much of the input power at port2 leaks into ort3 For example a directivity of 40dB corresponds to a return loss of 40dB which in terms of reflection coefficient is 10 4 0 01 SWR in this case is SWR mo 1 02 3 Experiment procedure A Coupling factor measurment 1 Set up equipment as shown in Figure 8 4 Set the varible attenuator to 20dB Apply 1000pps modulation signal to PIN diode modulator and turn on the Gunn oscillator Read the SWR indicator Use this value as a reference Fill in Table 8 1 A with the value 20dB Power qm SWR Supply Ser erate Indicoter E Gunn Oscilla or Fig 8 4 Initial setup for coupling factor measurement 2 Replace the frequency meter with a directional coupler Fig 8 5 Move the crystal detector to the auxiliary arm of the coupler Table 8 I Data for coupling factor calculation F 4 1 UIN Cisde Crystal Detector Wavegu ide oVornlulclor Atcenucls Aj A3 As A4 dB dB dB dB dB nx10 dB 3 Adjust the variable attenuator untill the same reference reading as in 1 is obtained 4 Fill in Table 8 1 A2 with the attenuation of the attenuator The oupling factor of the directional coupler is AI A2 51 SHR Indicator Power pon re Crysta Supaly GRE do Delactar Gunn PIN Cudy Ver able Hatthec Oscillator Modu ato
56. rmers with 1 ni and n2 1 turns ratio The output power in this case is Po Ey2 4Zo A resonant curve is a relationship between the frequency and the output power of a resonant cavity Consider two points on the resonant curve like the one shown in Figure 4 5 Assume the points are several dB below where the maximum output power occurs Af 1 1 0 fo F fo a i Af 4 4 where F is the ratio between the maximum power and the power of the two points on the curve Af is the bandwidth of the cavity is the bandwidth coefficient The cavity portion of the Figure 4 2 b is presented in detail in Figure4 3 m Rs L R E nz Rc Fig 4 3 Equivalent circuit of the cavity portion of Figure 4 2 b The following relationships are obtained by carefully observing Figure 4 3 Geo L o 1 OL WoL R since ic Rent Reng B H 34 GboL R Qo l np2Rg R ng22R R sd Bu t Bz 4 5 n2 Ri R 2 Bi Where R and The impedance of the equivalent circuit is Qo Z R iapa 2 59 ao 4 6 The power delivered to the load is Pr Il ni R ni Eg n R 7 Z P ni n2 Eg RL L R2 1 81 B2 2 402 1 tao ni n2 Eg Ri PL R2 1 81 82 I 402 At resonance the imaginary portion of 4 6 becomes 0 ni nz Es Re R 1 81 82 4 8 PL The bandwith of the resonant circuit is defined as the difference of two fr
57. signal is proportional to the depth The user should be aware of an optimized depth in use of a slotted line since too shallow depth would make the detected signal too weak and too deep depth would substantially reduce the main signal power in the waveguide and may even cause field distortion 9 Slide Screw Tuner The primary use of the slide screw tuner is to match loads detectors or antennas to the characteristic impedance of the waveguide The mechanical structure of a slide screw tuner consists of a probe 7 mounted on a carriage which slides along a narrow and long slot on the feeding waveguide When the Adjusting micrometer is turned the depth of the probe varies The combination of the depth and the position of the probe causes reflection in the waveguide at a specific amplitude and phase Relation between probe s depth and micrometer s scale unit mm micrometer s 3 3 7 9 scale probe s depth 7 3 3 0 10 Crystal Detector 11 The crystal detector is located inside the waveguide walls which is joined to a coaxial connector The crystal detector is basically a diode assembly which responds to the electromagnetic field inside the waveguide The diode assembly is consisted of a small thin piece of silicon a thin tungsten wire and a case One side of the silicon is directly connected to the case and the other side is connected to the tip of the tungsten wire The diode action is due
58. the indication sensitivity of meter 2 5 The Way To Use A 1kHz Sine Wave Generator This oscillator by applying the sine wave bias voltage of 1kHz to Pin Modulator make possible to give a sine wave alteration to the microwave This alteration is helpful to various measurements such as its specific experiment practice of a simple alteration measurement of phase changes within a waveguide circuit measurement of reflection wave and measurement of attenuation amount The following shows a connection example of Pin Modulator and 1kHz sine wave oscillator o 5 27 POWER SUPPLY Tz z ae Generator f C3 9 S Lo M Sunn QSC 1 Pin Modulator 11 Wave guide 2 Slotted Line 12 Fixed Attenuator 3 Magic Tee 13 Terminator 4 Variable Att 14 Frequency Meter 5 Crystal Detector 15 Shorting Plate 6 Coaxial Adapter 16 IKHz Generator 7 slide Screw Tuner 17 Cord Screw 8 Directional Coupler 18 Power Supply 9 Gunn oscillator 19 Reflector Stand 10 Horn Antenna 20 Component Supporter CHAPTER 3 EXPERIMENTS LIST OF REQUIRED EQUIPMENT FOR PERFORMING EXPERIMENTS 1 GUNN OSCILLATOR 2 PIN DIODE MODULATOR 3 POWER SUPPLY AND SIGNAL SOURCE 4 OSCILLOSCOPE DUAL TRACE 5 S W R METER 6 FREQUENCY METER 7 THERMOCOUPLE MOUNT POWER METER 8 VARIABLE ATTENUATOR 9 FIXED ATTENUATOR 10 DIRECTIONAL COUPLER 11 SLOTTED LINE 12 SLIDE SCREW TUNER 13 CRYSTAL DETE
59. tion of travel TEM modes do not exist in waveguides TE transverse electric modes or H modes In these modes no electric field exists in the z direction However Magnetic field does exist in the z direction All the field components therefore maybe derived from the axial component Hz of magnetic field TM transverse magnetic modes or E modes In these modes no magnetic field exists in the z direction However electric field does exist in the z direction All the field components therefore maybe derived from the axial component Ez of electric field TE and TM modes are the modes of propagation in a hollow empty waveguide We will continue our efforts in mathematically deriving field components for TE modes The inside walls of the waveguide are assumed to be made out of perfect conductor Also the waveguide is assumed to be filled with perfect dielectric o 0 These conditions are necessary to simplify the field solutions 1 TEmn modes The equation 3 5 can be rewritten for Hz considering only for z direction Hz A1cosK X AosinK X BicosKyXBosinKyX Cie P De P an 3 6 The boundary conditions are applied at the waveguide walls the normal component of the 29 transverse magnetic field must vanish at the perfectly conducting waveguide walls Also tangential electric field also vanishes on the waveguide walls Then the requirements on D as was defined in 3 6 are an 0 at 0 Ox
60. tuning features are provided with the oscillator in case fine adjustments are necessary 3 Experiment Procedure Set up the equipment as shown in Figure 1 4 Iherm azoscle Gunn vongole Ise llatc Attenuator Power Meter Fig 1 4 Setup for measuring the current vs voltage characteristic of the Gunn diode A The current vs voltage characteristic 1 Set the voltage to 4 volts Set the variable attenuator to 10dB This will ensure proper isolation to the Gunn oscillator 2 Raise the voltage in 0 5V increment Measure and record the current each time in Table 1 1 Caution Do not exceed 10V Table 1 1 Current vs Voltage of the Gunn diode Voltage V 4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5 8 0 8 5 9 5 10 Current mA 3 Reduce the voltage to OV From Table 1 1 construct a V I curve as suggested in Figure 1 5 2 3 4 5 G 7 8 9 70 Voltage Figure 1 5 Current vs Voltage characteristic of Gunn diode B Measurement of the oscillator output power vs supply voltage 1 Turn the power meter on and calibrate the power meter to zero 2 Raise the Gunn diode voltage in 0 5V increment and record the power indication on the power meter and the attenuator setting 3 Convert the obtained power reading in miliiwatts to dBm Then add the attenuation in dB 18 to the dBm Example Assume the power r
61. uction 1 1 The module AT3000 3cm waveguide experiment system 1 2 Technical descriptions of components and instruments Chapter 2 Usage 2 Cares for use 2 2 The way to use the component with IN OUT directions 2 3 The way to use the component with directions 2 4 The way to use power meter option 2 5 The way to use a sine wave generator Chapter 3 Experiments Experiments 1 The Gunn oscillator Experiments 2 PIN diode modulator and detector Experiments 3 Propagation modes wavelength and phase velocity in a waveguide Experiments 4 Q and bandwidth of a resonance cavity Experiments 5 Power measurement Experiments 6 Standing wave ratio SWR measurement Experiments 7 Impedance measurement Experiments 8 Basic properties of a directional coupler Experiments 9 Attenuation measurement Experiments 10 Study of a waveguide Hybrid T CHAPTER 1 INTRODUCTION 1 1 The Model AT3000 3cm Waveguide Experiment System The AT3000 3cm waveguide experiment system is developed to provide with the users a comprehensive way of understanding the basic properties of microwave frequencies and also the easiest way of performing a number of microwave experiments using the popular X band frequencies 8 2 12 4 GHz The microwave radio communications network plays very important roll nowadays in our daily life For example high quality long distance telephone calls sometimes via communications sattelites are made possible using microw
62. uides combined together and coupled by holes at the joining section of the two Directional couplers are very popular in microwave systems where measurements of incident and reflected power are needed to determine the Standing Wave Ratio or SWR The directivity 6 which is a figure of merit of a directional coupler is a measure of how well the power can be coupled in the desired direction in the neighboring waveguide Usually one end of the neighboring waveguide containes a matched load which absorbs the energy headed towards undesired direction The directional coupler used AT3000 has a coupling factor of 10dB 3dB and a directivity of 40dB A directional coupler is represented by a graphical symbol of the following 8 Slotted Line In measuring the standing waves inside a waveguide a slotted line is used to probe the amplitude and the phase of the standing wave pattern Obtaining the standing wave pattern information allows us to determine the wavelength standing wave ratio and the impedance of the transmission line As the name implicates a slotted line has slot along the center line of the broad side of the wall An assembly consisting of a probe and a crystal detector is designed to slide along the open slot and as it does the probe samples the field in the waveguide while the crystal detector provides a rectified signal The depth of the probe into the waveguide is adjustable and the strength of the detected

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