HP 431C power meter
Contents
1. BATTERY CHARGE Instrument off Trickle charge applied to battery THERMISTOR MOUNT Accepts the thermis tor mount cable CALIB FACTOR Switch compensates for the Calibration Factor of the thermistor mount Calibration Factor values from 88 to 100 may be set in 1 steps RANGE Sets power range also includes a NULL position which in conjunction with the adjacent nullscrewdriver adjustment ensures that the metering bridge is reactively balanced Model 431C THERMISTOR MOUNT MOUNT RES A three position slide switch which sets the power meter to accommodate thermistor mounts of 100 ohm 200 ohm and 200 ohm balanced operating resistances ZERO and VERNIER Sets the meter pointer over the zero mark The VERNIER control is a fine adjustment of the ZERO control setting DVM A BNC type jack providing an output voltage linearly proportional to the meter indication A DC voltmeter with an input im pedance less than 10 ohms is required to minimize introduction of measurement error refer to Paragraph 3 49 RECORDER LEVELER A BNC type jack pro viding a DC voltage of low source impedance for a recorder or leveler amplifier In Option 02 instruments a thermistor mount connector is wired in parallel with the front panel connector Two mounts cannot be con nected simultaneously DC CALIBRATION This connector permits a DC input for power meter calibration and DC substitution method of power measurement2. 8 8 Figure 3 2 Mismatch Power Measurement Uncertainty 3 26 RF LOSSES AND DC TO MICROWAVE SUB STITUTION ERROR RF losses account for the power entering the thermistor mount but not dissipated in the detection thermistor element Such losses may be in the walls of a waveguide mount the center conductor of a coaxial mount capacitor dielectric poor con nections within the mount or due to radiation DC to microwave substitution error is caused by the dif ference inheating effects of the substituted audio bias or DC power and the RF power inathermistor The difference results from the fact that the spatial dis tributions of voltage current and resistance within the thermistor element not the same for audio DC and RF power RF losses and DC to microwave sub stitution error are generally combined for the sim plicity of analysis 3 27 THERMOELECTRIC EFFECT ERROR A mild thermocouple exists at each point of contact where the connecting wires join to the thermistor elements Each thermocouple creates a DC voltage Thus two ther mocouple voltages of opposite relative polarity are formed one at each junction to each thermistor element 3 28 Ideally each thermocouple voltage would be equal in magnitude sothat they cancel with no resultant effect onthe accuracy of power measurement In prac tice however each point of contact does not have iden tical thermocouple characteristics andin addition the temperatures at eac
3. CAUTION To avoid severe damage to the thermistor mount be carefulnot tomove the MOUNT RES switch while operating the RANGE switch 3 Set RANGE to 01 mW 4 Set POWER to LINE ON If instrument is to be battery operated rotate POWER to BAT TERY ON Adjust ZERO control for 25 to 75 of full scale on meter Rotate RANGE switch to NULL and adjust NULL screwdriver adjustment adjacent to NULL on RANGE switch for minimum reading Repeat steps 5 and 6 until NULL reading is within NULL region on the meter Set RANGE switch to the power range to be used and zero set the meter with ZERO and VER NIER controls Note Range to range zero carryover is less than 1 0 if the meter has been properly adjusted mechanically Step 1 above andthe instrument has been properly zero set electrically on its most sensitive range For maximum accuracy zero set the power meter on the range to be used 9 Set CALIB FACTOR switch to correspond with Calibration Factor imprinted on hpthermistor mount label Apply RF power at the thermistor mount Power is indicated on the meter directly in mW or dBm Figure 3 8 Turn On and Nulling Procedure 02316 2 Section Figure 3 8 CALIB FACTOR Connect thermistor mount and cable to THER MISTOR MOUNT connector Refer to Table 1 2 for recommended thermistor mounts and their frequency ranges Meter Mechanical Zero a With instrument turned off rotate meter adjustmen
4. Figure 3 9 DC Substitution 02316 5 Section Figure 3 8 CALIB FACTOR Connect thermistor mount and cable to THER MISTOR MOUNT connector Refer to Table 1 2 for recommended thermistor mounts and their frequency ranges Meter Mechanical Zero a With instrument turned off rotate meter adjustment screw clockwise until pointer approaches zero mark from the left Continue rotating clockwise until pointer coincides with zero mark If pointer over shoots continue rotating adjustment screw clockwise until pointeronce again ap proaches zero mark from the left Rotate adjustment screw about three degrees counterclockwise to disengage screw adjust ment from meter suspension Note When using an hp Model 478A or other 200 ohm un balanced coaxial thermistor mount the power meter should be zeroed and nulled with the RF power source turned off and connected to the thermistor mount If the RF power source cannot be turned off the power meter must be zeroed and nulled while the RF input connection of the thermistor mount is terminated in the same 10 kHz impedance as that presented bythe power source short open or 50 ohm These precautions are not necessary when waveguide mounts such as the hp Model 486A series or balanced 200 ohm coaxial mounts are used ak Set MOUNT RES switch to correspond to the operating resistance and type of thermistor mount used Model 431C RANGE 08m 431 2
5. OPERATING AND SERVICE MANUAL MODEL 431C POWER METER ad i DV Copyright HEWLETT PACKARD PAN Yor 1966 gt 1501 PAGE MILL ROAD PAL CALIFORNIA U S A 02316 5 00431 90016 Printed MAR 1967 Section Paragraphs 3 47 to 3 52 substitution measurement The hp Model 8402B Cali brator conveniently provides DC power and appropriate switching to perform DC substitution measurement with the Model 431C If the 431C is being used with a bal anced 200 ohm thermistor mount the 8402B must be used If the 431C is used with an unbalanced thermis tor mount such as hp Model 478A Coaxial or 486A Waveguide types the 8402B may be replaced with an 8402A Power Meter Calibrator 3 47 Although the DC substitution technique is the most accurate method of measuring RF power there are sources of error that must be considered The accuracy of DC substitution depends largely upon 1 how accurately substituted DC is known 2 how precisely the power meter reading is duplicated and 3 the actual operating resistance of the thermistor 3 48 SUBSTITUTION FUNCTION MEASUREMENT ACCURACY Voltmeter terminals are located on the rear panel of the 8402B Calibrator These terminals provide a means to monitor the magnitude of calibrator output currents by presenting a DC voltage proportional to the substituted current For the purpose of calcu lating a substituted power this voltage carries atotal uncertainty
6. Rotate RANGE switch to NULL and adjust NULL screwdriver adjustment adjacent to NULL on RANGE switch for minimum reading Repeat steps 5 and 6 until NULL reading is within NULL region on the meter Set RANGE switch to the power range to be used and zero set the meter with ZERO and VER NIER controls Note Range to range zero carryover is less than 1 0 if the meter has been properly adjusted mechanically Step 1 above andthe instrument has been properly zero set electrically on its most sensitive range For maximum accuracy zero set the power meter on the range to be used 9 Set CALIB FACTOR switch to correspond with Calibration Factor imprinted on hpthermistor mount label Apply RF power at the thermistor mount Power is indicated on the meter directly in mW or dBm Figure 3 8 Turn On and Nulling Procedure 02316 2 Model 431C TEMPERATURE COMPENSATED MICROWAVE THERMISTOR OSCILLATOR hp 418 486 Section III Paragraphs 5 53 to 5 57 RECORDER LEVELER J2 hp MODEL 4310 PONER METER X Y RECORDER Figure 3 6 Insertion Loss or Gain Measurement cease by noting a change of meter readingto zero The operating resistance of the detection thermistor ele ment is measured by reading the resistance deviation in percent directly from the switch setting that causes oscillations to cease 3 53 ADDITIONAL APPLICATIONS 3 54 A discussion of microwave
7. A number of factors affect the overall accuracy of power measurement Major sources of error are presented in the following paragraphs to showthe cause and effect of each error Particular corrections or special measurement techniques can be determined and applied to improve overall measurement accuracy The following are the major sources of error to con sider 1 Mismatch error 2 RF losses 3 DC to microwave substitution error 4 Thermoelectric effect error and 5 Instrumentation error 3 20 MISMATCH ERROR The following discussion uses the terms conjugate power 20 available power conjugate match and mismatch and 20 match and mismatch These basic terms are defined as follows Conjugate power is the maximum available power It is dependent on a conjugate match condition in which the impedance seen looking toward the thermistor mount isthe complex conjugate of the impedance seen looking toward the RF source A special case of this maximum power transfer is when both the RF source and the thermistor mount have the same impedance as the transmission line Zo available power isthe power a source will de liver to a Zo load Itis dependent a Zo match con dition in which the impedance seen looking into atrans mission line is equal to the characteristic impedance of the line 3 2 Model 431C 3 21 In a practical measurement situation both the source and thermistor mount have SWR and the source is seldom matched tothe
8. EE FEEDBACK _ CURRENT moll a i m a DC FEEDBACK 1854 0003 i i 1 t 1 1 1 1 8 Ll 1 25V UNREG NOTES 18V REG 1 DC VOLTAGE LIMITS GIVEN AT REFERENCE DESIGNATORS VARIOUS POINTS THRU CIRCUIT 1 THERMISTOR MOUNT 15 1 CONNECTED TO THE 4318 106 HO 125 FOR THESE MEASUREMENTS CRIOI 18 2 CHASSIS GROUND 102 103 101 105 GROUND ON THE PRINTED CIRCUIT BOARD DIFFERENTIAL 4 ALL VALUES IN OHMS AND AMPLIFIER qol i PICOFARADS UNLESS 01 103 105 13 144 OTHERWISE INDICATED 0042 0105 150 155 160 181 241370 5 FRONT PANEL 5101 102 ENGRAVING gt 8 Tiros REAR PANEL wt ENGRAVING ven UNASSIGNED RUIZ ROTARY SWITCH OR POTENTIOMETER RI43 7 DC 990 ERROR SIGNAL 1 5 8 VALUE SELECTED FACTORY UNREG AVERAGE VALUE SHOWN 9 0 VOLTAGE MEASURED WITH RESPECT TO I8V REG 10 TEST POINT COPYRIGHT 1962 8Y HEWLETT PACKARD COMPANY 4318 PWA METER I i PAGE 2 OF 2 _ i Figure 5 3 Power Meter Assembly
9. Factor value appropriate to the frequency of measure ment imprinted on the thermistor mount label With the proper setting the 431C compensates for the Cali bration Factor of the thermistor mount 3 4 Model 431C 3 36 Calibration Factor is applied as a correction factor to all measurements made without a tuner Under this condition the power indicated isthe power that would be delivered by the source to a load im pedance equal to Zo This measured power is called Zo available power 3 37 Calibration Factor correction ensures that a power measurement uncertainty range is centered on the Zo available power level instead of on the power delivered to the thermistor mount impedance Total measurement uncertainty limits for a given power measurement using Calibration Factor arethe sum of the uncertainties contributed by 1 Mismatch loss 2 Calibration Factor uncertainty and 3 Instrumen tation error 3 38 An example of power measurement uncertainty caused by source and thermistor mount mismatch is given in Paragraphs 3 23 through 3 25 Continuing the example will show the basic principle of Calibra tion Factor correction to a measurement of Zo avail able power Figure 3 3 shows the relationship and limits of error before correction A source SWR of 1 7 and a thermistor mount SWR of 1 3 result ina Zo available power uncertainty of 5 5 to 8 2 Assuming a thermistor mount Calibration Factor of 94 accuracy of 2 the Cal
10. LINE VOLTAGE Selects 115 or 230 volt line operation 13 Mechanically zeroes meter Refer to Figure 3 8 Figure 3 1 Front and Rear Panel Controls Connectors and Indicators 3 0 02316 4 Section 7 Model 431 Model 431C hp 4316 POWER METER Power CALIB FACTOR RANGE oS oc CALIBRATION INPUT DIGITAL VOLTMETER REFER TO PARAGRAPH 3 49 Connect the equipment shown above DC sub stitution is discussed in Paragraphs 3 44 through 3 52 Set the 431C Power Meter for normal oper ation using the procedure given in Figure 3 8 Set CALIBRATION FACTOR to agree with Calibration Factor or Effective Efficiency data on mount see Paragraph 3 32 Apply RF power to mount Note reading obtained on dig ital voltmeter Note If the 01 1 1 or 10 mW power meter ranges are used the digital voltmeter reads directly If the 03 3 or 3mW power meter ranges are used the digital voltmeter reading must be multiplied by 0316 316 or 3 16 respectively 4 5 Turn off or disconnect the RF source Turn on 8402B Calibrator by setting the FUNC TION switch to CURRENT OFF then apply a substitution current by setting FUNCTION switch to SUBSTITUTE Set CALIBRATION FACTOR to 100 Turn 8402B CURRENT CONTROL and VERNIER to duplicate reading obtained in step 3 Note 8402B VOLTMETER output DVM reading On 01 03 1 and 3 mW ranges reading is sub stituted current Ipc in mA On other ranges
11. line is presented in Application Note 64 The Application Note may be obtained from any Hewlett Packard Sales and Service Office 02316 5 Model 431C Section Paragraphs 3 1 to 3 13 SECTION III OPERATION 3 1 INTRODUCTION 3 2 This section presents the basic information re quired to operate the Model 431C Power Meter A dis cussion of microwave power measurement with empha sis on modern techniques accuracy considerations and sources of error is available in Application Note 64 available from any Hewlett Packard Sales and Service Office 3 3 The Model 431C is an automatic self balancing power measuring instrument employing dual bridge circuits The power meter is designed to operate with hp temperature compensated thermistor mounts such as the 8478B and 478A Coaxial and 486A Waveguide series Power may be measured with these mounts in 50 ohm coaxial systems from 10 MHz to 18 GHz and in waveguide systems from 2 6 GHz to 40 GHz Full scale power ranges are 10 microwatts to 10 milli watts and 20 dBm to 10 dBm Extended measure ments may be made to 1 microwatt and to 30 dBm The total measurement capacity of the instrument is divided into seven ranges selectable by a front panel RANGE switch 3 4 ZERO and VERNIER zero set controls zero the meter Zerocarry over from the most sensitive range to the other six less sensitive ranges is accurate to 1 Greater accuracy can be obtained by setting the zero point onthe p
12. multiply reading by 10 to obtain Ipc in mA Section IV Figure 3 9 hp84028 CALIBRATOR DIGITAL VOLTMETER REFER TO PARAGRAPH 3 48 4810 4 Disconnect thermistor mount from thermistor mount cable Connect thermistor mount cable between 431C Power Meter THERMISTOR MOUNT and 8402B Calibrator RESISTANCE STANDARD connectors Set 8402B Calibrator controls as follows a THERMISTOR RESISTANCE 0 5 b MOUNT RESISTANCE tocorrespond with resistance and type of thermistor mount used Set 431C Power Meter RANGE switchto NULL Rotate THERMISTOR RESISTANCE switch on 8402B Calibrator counterclockwise until the 431C Power Meter changes to a zero reading from a stable reading greater than zero The operating resistance of the detection ther mistor Rg is the nominal value indicated on the thermistor mount label plus or minus the correction indicated by the setting of the THER MISTOR RESISTANCE switch The percentage correction is a value in between the limits set by the two positions of the THERMISTOR RE SISTANCE switch that correspond to the zero reading and the stable meter reading obtained in step 10 If desired the average of these two values may be calculated and used as the correction value Calculate power in mW from the following expression Power mW Ra 1079 Where Ipc Substitution current inmA from step 7 Rg Operating resistance of the detection thermistor from step 11
13. of the micro wave band for which the mount is designed to operate For hp thermistor mounts this constant SWR is low thus the mismatch uncertainty is small Since the mount impedance and corresponding SWR deviate sig nificantly only at the high and low ends of a microwave band it is generally unnecessary to use atuner How ever a tuner or other effective means of reducing mismatch error is recommended when the source SWR is high or when high accuracy is required To mini mize mismatch betweenthe source and the thermistor mount without the use of a tuner a low SWR precision attenuator can be inserted in the transmission line to isolate the thermistor mount from the source Since atuner is not often used Calibration Factor is a more practical term than Effective Efficiency 3 34 CALIBRATION FACTOR Calibration Factor is the ratio of substituted audio or DC power in the thermistor mount to the microwave RF power incident upon the mount _ PDC Substituted Calibration Factor Pywave Incident Calibration Factor is a figure of merit assigned to a thermistor mount to correct for the following sources of error 1 RF reflected by the mount due to mismatch 2 RF loss caused by absorption within the mount but not in the thermistor element and 3 DC to micro wave substitution error 3 35 The CALIB FACTOR switch on the front panel allows rapid power measurements tobe made with im proved accuracy The switch is set to the Calibration
14. onthe microwave bandused refer to Table 1 2 The 2000 position is used with Model 478A thermistor mounts and the 2002 BAL position is used with a balanced thermistor mount such as the 8478B CAUTION To avoid severe damage to the thermistor mount be careful not to move the MOUNT RES svitch while operating the RANGE switch 3 8 Two output BNC type jacks are provided on the rear panel of the instrument labeled DVM and RE CORDER LEVELER The DVM jack provides a volt age linearly proportional to the meter current 1 volt equal to fullscale meter deflection A DVM conn ct d to the must have alr input impedance greater than 500 k ohms on the range used The RECORDER LEVELER jack furnishes a DC voltage of low source impedance necessary for isolation between a recorder or leveler amplifier and the metering circuit of the power meter The output voltage is proportional to the power measured and is offset 40 mV or less from its nominal value depending on the load impedance This output voltage allows the Model 431C to be used in a number of additional applications refer to Para graph 3 53 3 9 CONTROLS CONNECTORS AND INDICATORS 3 10 The front and rear panel controls connectors and indicators are explained in Figure 3 1 The des criptions are keyed to the corresponding items which are indicated on the figure Further information re garding the various settings and uses of the controls connectors andindicators is inclu
15. power measurement applications is available in Application Note 64 avail able from any Hewlett Packard Sales and Service office The RECORDER LEVELER output allows the 431C to be used in systems of greater capability than would be possible with a meter indication alone Im portant applications include 1 permanent recording of measurement data 2 output power leveling 3 in sertion loss or gain measurement and 4 control system monitoring These applications are discussed in the following paragraphs Other applications include readout of the level of a microwave RF power source at a remote location and using the ratio of two power meter DVM outputs to make precise measurements of small attenuations 3 55 OUTPUT POWER LEVELING Ablockdiagram of an output power leveling system is shown in Fig ure 3 5 The power meter is used as an element ina control circuit that maintains a constant power level at a particular point in the system The thermistor mount connected to the auxiliary arm of directional coupler senses a portion of the power incident upon the directional coupler The power meter RECORDER LEVELER output provides a DC voltage that is propor tional to the power measured at the thermistor mount This voltage canbe directly applied tothe power meter leveling input of one of the hp Model 690 Sweep Oscil lators or to the input of a leveler amplifier At the TEMPERATURE COMPENSATED THERMISTOR MOUNT
16. 0 213 00 RIOS 190 2 192 720 197 70 20820 256 00 258 00 261 40 1268 20 NULL 1 THERMAL CONDUCTING ADJUST j BLOCK L o i am mii an as es Se COAXIAL THERMISTOR MOUNT 478A PAGE 2 P O _ _ Figure 5 3 Power Meter Assembly lt 5 12 01370 2 Model 4318 ci20 47UF R152 3000 01370 2 OSCILLATOR AMPLIFIER FOLLOWER 2N383 2N388A AMPLIFIER IORC AMPLIFIER AMPLIFIER AMPLIFIER 8 SHORT BASE TO EMITTER OF QIO FOR THIS MEASUREMENT ST IORC AMPLIFIER 2NI370 POWER METER ASSEMBLY Section V FEEDBACK CURRENT GENERATOR 9107 1854 0003 15 UNREG ee Figure 5 3 P A S en i RI60 RIGT i i i 10 46 R173 1 194 22 82 09 57 46 i i i i i El 2130 1 8162 i i i ae ca NE I r i i Mie T 1 i El ge i 1 Riss 1 21384 i 13 i Los i XRI66 8171 252 177 I i i 1 i i 38 05 41 46R 2481 256 8 I i i i 103
17. AUXILIARY hp 418 486 COUPLER leveler amplifier the voltage is compared to an inter nal reference the difference voltage amplified and applied as negative feedback to the amplitude modu lation input of the source The feedback maintains a constant RF power level at the sampling point on the auxiliary arm of the directional coupler This control will hold the forward power at the main arm of the coupler at a constant level 3 56 INSERTION LOSS OR GAIN Figure 3 6 shows a block diagram of a system to determine insertion loss or gain as a function of frequency Initially the device to be tested is not connected into the system and the thermistor mount is connected directly to the sweep oscillator output Variations in power ampli tude are measured by the power meter as the frequency range of interest is swept by the sweep oscillator This is a reference measurement and is recorded by the X Y recorder The device to be tested is then inserted between the sweep oscillator and the thermistor mount Power amplitude versus frequency is again measured and recorded The difference between the second reading and the reference at any frequency is the insertion loss or gain of the device at that frequency 3 57 CONTROL SYSTEM MONITORING The ar rangement of a system to actuate alarm control cir cuits is shown in Figure 3 7 relay circuit can be connected directly to the RECORDER LEVELER out put This type of curcuit will pr
18. L BATTERY USE Whenthe Model 431C is to be battery operated for the first time perform the following steps a Set the POWER switch to the BATTERY TEST position and note meter pointer indication A meter pointer indication within the BAT CHARGED area indicates the internal battery is properly charged and ready for use A meter pointer indication to the left of the BAT CHARGED area means that the battery must be charged as described below Actual battery voltage canbe measuredonthe 0 3 mWscale Battery voltage is equal to 10 times meter scale reading b Connect the Model 431C to AC power source Set POWER switch to BATTERY CHARGE and charge the battery until a meter pointer indication within the CHARGED region can be obtained as in step a 3 15 BATTERY STORAGE Store the battery at or below room temperature Extended storage at high temperatures will reduce the cell charge but will not damage the battery if the temperature is below 140 F Charge the battery after removal from storage and before using the Model 431C for battery operation 3 16 OPERATING INSTRUCTIONS 3 17 Figure 3 8 Turn On and Nulling Procedure and Figure 3 9 DC Substitution present step by step instructions for operating the Model 431C Steps are numbered to correspond with the appropriate control connector or indicator on the power meter and or required auxiliary equipment 3 18 MAJOR SOURCES OF ERROR IN MICROWAVE POWER MEASUREMENT 3 19
19. able or 2 available 3 42 Conjugate available power is measured when the system consisting of the RF source transmission line tuner and thermistor mount is tuned for a maxi mum power level on the 431C In this application the system mount combination presents a conjugate match tothe source The power 15 the actual power that would be delivered by the source to aconjugate load 3 43 20 available power is measured when a tuner thermistor mount combination is tuned for minimum reflection caused by mount mismatch at the frequency of interest The tuner adjustment is made on re flectometer or slotted line system external to the measurement system used for power measurement After the tuner adjustment the tuner thermistor mount combination is connected to the transmission line and RF source on which a power measurement is made 3 44 HIGH ACCURACY OF POWER MEASUREMENT USING DC SUBSTITUTION 3 45 The instrumentation source of error canbe re duced by using DC substitution With precision instru ments used in a DC substitution set up and careful procedure instrument error can be reduced from 1 of full scale to 0 16 of reading or less The tech nique involves 1 applying the RF power to be meas ured to the thermistor mount and noting the power meter reading 2 removing the RF power from the thermistor mount and substituting a DC current from an external DC power source to precisely duplicate the meter re
20. ading obtainedin step1 and 3 calculating the power from the substituted DC current and ther mistor operating resistance 3 46 EQUIPMENT USED FOR DC SUBSTITUTION Figure 3 9 shows the instrument setup for a DC 3 5 Section Paragraphs 3 30 to 3 38 3 30 INSTRUMENTATION ERROR The degree of inability of the instrument to measure the true substi tution audio bias or DC power supplied to the thermis tor mount is called power meter accuracy or instru mentation error Instrumentation error of the Model 431C is 2 of full scale 20 C to 35 Instru mentation error can be reduced to 0 16 of reading or less by using DC substitution as described in Fig ure 3 9 3 31 CALIBRATION FACTOR AND EFFECTIVE EFFICIENCY 3 32 Calibration Factor and Effective Efficiency are two power ratios used as correction factors to improve overall accuracy of microwave power measurement The ratios are used under different measurement con ditions Calibration Factor is used when the ther mistor mount is coupled to the RF source without a tuner Calibration Factor corrects for both SWR and inefficiency of the thermistor mount Effective Effi ciency is used when a tuner matches the source to the thermistor mount Effective Efficiency corrects only for the inefficiency of the thermistor mount 3 33 Each thermistor mount has a particular imped ance This impedance and hence the mount SWR remain constant over the major portion
21. aphs 3 39 to 3 46 3 40 EFFECTIVE EFFICIENCY Effective Effi ciency is the ratio of substituted audio or DC power in the thermistor mount to the microwave RF power dissipated within the mount Ppc Substituted Puwave Dissipated This power ratio corrects for RF losses and DC to microwave substitution error inthe thermistor mount It is largely independent of the level of input RF power When a tuner is used to present either a conjugate or Zo match to the microwave RF source Effective Ef ficiency is to be applied as a correction factor to the power measurement because all of the power incident upon the mount is absorbed in the mount The use of a tuner and application of Effective Efficiency is the most accurate method of measuring power since source and thermistor mount power reflections are eliminated and thus measurement uncertainty due to mismatch is eliminated Tuner loss will generally be small However its effects on power measurement can be corrected for by dividing the indicated power by the tuner loss ratio power out power in Effective Efficiency 3 41 Effective Efficiency can be applied as rection factor to both conjugate available and Zo avail able power measurements The CALIB FACTOR switch is set to the Effective Efficiency value appro priate to the frequency under test imprinted on the thermistor mount label The type of application of the tuner determines if the power measured is conjugate avail
22. articular range tobe used When the RANGE switch is in the NULL position the meter indicates inherent metering bridge unbalance and a front panel NULL screwdriver adjustmentis provided for initial calibration 3 5 The CALIB FACTOR switch allows the intro duction of discrete amounts of compensationfor meas urement uncertainties related to SWR and measure menterrors caused by substitution error and thermis tor mount efficiency The appropriate selection of a Calibration Factor value permits direct meter reading of the RF power delivered to an impedance equal to the characteristic impedance 20 of the transmission line connecting the thermistor mount to the RF source Calibration Factor values are determined from the data marked on the label of each 8478B 478A or 486A thermistor mount 3 6 The Model 431C has a DC CALIBRATION jack on the rear panel that can be used for DC substitution method of power measurement DC substitution is an extension of the power measurement technique nor mally used Through the use of DC substitution in strument error can be reduced from a nominal value of 1 to 0 16 of reading or less depending on the care taken in procedure and accuracy of auxiliary equipment 02316 4 3 7 The MOUNT RES switch on the front panel per mits the use of three types of thermistor mounts with the 431C Model 486A waveguide mounts can be used by setting the MOUNT RES switch to the 1000 2000 position depending
23. ded inthe applicable procedures of this section 3 11 BATTERY OPERATION 3 12 The Model 431C option 01 can operate from bat tery instead of aconventional 115 or 230 volt primary power source A rechargeable Nickel Cadmium bat tery isfactory installedin Option 01 instruments The same battery can be ordered and later installed inthe basic instrument thereby modifying the power meter to the Option 01 configuration The rechargeable bat tery installation kit may be ordered by hp stock number 00415 606 Option 01 installationinstructions are given in Appendix I 3 13 OPTIMUM BATTERY USAGE It is recom mended that the Model 431C be operated by the battery for up to 8 hours followed by 16 hours of recharge If continuous battery operation is required for more than 8 hours the recharge time should be double the operating time Continuous battery operation is pos sible for up to 24 hours but this must be followed by a prolonged recharge period 3 1 Section Figure 3 1 POWER CALIB FACTOR RANGE 0 LINE y p BATTERY roca 3 TEST CHARGE LINE CHARGE Lamp lights POWER switch is inthe LINE ON or BATTERY CHARGE position POWER Determines connections to primary power sources and the battery charging circuit LINE OFF Instrument off LINE ON Instrument on Trickle charge ap plied to battery BATTERY ON Instrument on battery powered BATTERY TEST Meter indicates battery charge
24. e is 0 2 Corrections should be added tovolt meter readings since voltmeter impedance loading causes voltage measurements to decrease 3 49 POWER METER DVM OUTPUT MEASURE MENT A digital voltmeter can be connected to the 431C DVM jack to increase resolution of a power meter reading This feature provides convenience to the operator and allows an easy method of repeating a pre cise measurement readout value Measurement error corrections for voltmeter impedance loading must be made when using a voltmeter to measure the voltage output of the 431C Power Meter The DC voltage at the DVM jack on the rear panel is developed across a 1 ohm resistor Therefore a voltage measurement made with a digital voltmeter having an input imped ance of 500k ohms will introduce an error of 0 2 A digital voltmeter with an input impedance of 10 megohms will introduce a much smaller error of 0 01 Correc tion percentages should be added to voltmeter readings 3 50 DETECTION THERMISTOR RESISTANCE Steps 8 through 11 of Figure 3 9 list a procedure to determine the operating resistance of the RF detection bridge at balance and thus measure the operating re sistance of the detection thermistor element during a power measurement The actual operating resistance of detection thermistors may deviate as muchas 0 5 from their nominal values For this reason the actual operating resistance should be checked The true operating resistance must be kno
25. h junction may not be the same These differences cause an incomplete cancellation of the thermoelectric voltages resulting in a voltage that causes a thermoelectric effect error The magnitude of the error is important when making DC substitution 02316 2 measurements on the 0 1 mW 0 03 mW and 0 01 mW ranges On other ranges the effectis negligible For hp mounts maximum error introduced by thermoelec tric effect is about 0 3 uW and is typically 0 1 on the 01 mW range 3 29 THERMOELECTRIC EFFECT ERROR COR RECTION Use the following technique to correct for thermoelectric effect error a Measure power b Connect anhp Model 8402 Power Meter Calibra tor to the power meter DC CALIBRATION jack Note If a balanced thermistor mount is being used an 8402B Calibrator is required c Zero and null power meter d By DC Substitution see Figure 3 9 duplicate power measurement made in step a Calculate and record substituted power as P4 e Reverse connection polarity between the cali brator and power meter f Re zero and re null power meter if necessary g By DC Substitution duplicate power measure ment made in step a Calculate and record substituted power as h Calculate arithmetic mean of the two substitution powers and This mean power includes a cor rection for thermoelectric effect error Pit Power 3 3 Section Paragraphs 3 14 to 3 25 3 14 INITIA
26. ibration Factor uncer tainty is 6 42 or 4 to 8 The 431C Power Meter has an instrumentation error of 1 may be reduced by DC substitution Figure 3 9 The algebraic addition of Calibration Factor instrumen tation and Zo available power uncertainties determines the limits of error before Calibration Factor cor rection In this case the limits 2 5 to 17 2 4 0 TO 8 0 94 CALIBRATION FACTOR UNCERTAINTY Zo AVAILABLE POWER INSTRUMENTATION 5 5 TO 8 2 ERROR UNCERTAINTY Zo AVAILABLE POWER 4 2 5 TO 17 2 TOTAL LIMITS OF ERROR BEFORE CORRECTION 19 7 MAXIMUM POSSIBLE ERROR 17 2 STATISTICALLY MOST PROBABLE ERROR 7 35 43IC A II Figure 3 3 Limits of Error Before Correction 02316 4 Model 431C CONJUGATE MAXIMUM AVAILABLE POWER MAXIMUM 0 06 dB MIN POWER CONJUGATE LOSS ENTERING THERMISTOR MOUNT Zo AVAILABLE POWER 0 29 dB SOURCE 20 MISMATCH 0 07 dB 1 7 THERMISTOR Section III Paragraphs 3 26 to 3 29 0 23 dB TO 0 35 dB 5 5 TO 8 2 UNCERTAINTY ON Zo AVAILABLE POWER POWER Zo MISMATCH LOSS THERMISTOR MOUNT 0 64dB MAXIMUM 0 30 dB TO 0 28 dB CONJUGATE LOSS 72 6 5 UNCERTAINTY ON POWER ENTERING THERMISTOR MOUNT MINIMUM POWER ENTERING 0 64 dB THERMISTOR MOUNT SOURCE SWR 1 7 0 26 THERMISTOR MOUNT SWR 1 3 8 0 13
27. of 0 12 This uncertainty includes a 0 06 uncertainty of the thermistor resistance func tion of the calibrator steps 8 through 11 of Figure 3 9 However the output impedance of this voltage is finite 100 ohms on 1 0 mW through 10 mW ranges 1kohms on lower ranges This output impedance requires the use of a differential or high impedance voltmeter in order to obtain an accurate measurement of the cali brator output At null a differential voltmeter does not draw current from the calibrator voltage output circuitry For this reason a differential voltmeter will not introduce measurement error due to loading When using a voltmeter other than a differential type correction must be made for the measurement error that is introduced by the voltmeter input impedance For example a digital voltmeter with an input imped ance of 1 megohm will introduce a measurement error of 0 1 when used to measure calibrator output on ranges below 1 0 mW Substitution current measure ment error corrections must be doubled since the power measured is proportional to the square of the substituted current Twice the voltage uncertainty is the power uncertainty introduced by the voltmeter Therefore the correction to be applied in the above TEMPERATURE COMPENSATED THERMISTOR MOUNT AUXILIARY hp 418A486A ARM AMPLITUDE a POWER DIRECTIONAL ARM SOURCE COUPLER e CONTROL VOLTAGE NEGATIVE FEEDBACK Model 431C exampl
28. ossible error is 17 2 and the statistically most probable error is 7 35 3 39 Figure 3 4 shows the total power measurement uncertainty after Calibration Factor correction Note that the range of uncertainty 19 7 is the same as it was before correction However the measurement uncertainty range has shifted and is now more sym metrical about the 20 available power level The totaluncertainty after correction is the algebraic sum of the instrumentation error 2199 the accuracy to which Calibration Factor is determined 2 and the uncertainty on the power actually entering the ther mistor mount After correction the power measure ment uncertainty on the Zo available power is 10 2 to 9 5 The maximum possible error is 10 2 was 17 2 and the statistically most probably error is 0 35 was 7 35 This is a typical example showing how the use of Calibration Factor correction to a measurement of Zo available power not only re duces the maximum possible error but more impor tantly the magnitude of the statistically most probable error is reduced to very near the Zo available power level The relationship between indicated power onthe 431C and the 20 available power is given by the following equation p P indicated 1 PsPm 2 Calibration Factor Where Zo available power Ps source reflection coefficient Pm thermistor mount reflection coefficient _ SWR 1 SWR 1 02316 1 Section Paragr
29. ovide a control system operated by full scale magnitude power changes of the power meter Small magnitude power change control can be achieved through the use of a comparison ref erence level and a differential amplifier The differ ential amplifier output can be connected to the relay circuit to actuate the alarm or control circuits RECORDER ho MODEL LEVELER Aan di En DEVICES 431 6 Figure 3 7 Control System Monitoring 02316 1 3 7 Section 4318 Figure 5 3 WAVEGUIDE RF SHIELD THERMAL CONDUCTING d CABLE TO lt i IOXC BIAS RINAL P O ro _ EP fp I 2 3 DC BIAS 4 5 i DETECTION 1 COMPENSATION i THERMISTOR THERMISTOR R PEE IN ETE j WAVEGUIDE THERMISTOR MOUNT siol 4 486A 3 103 THERM STOR NT Sheed aaa RF SHIELD LIOI 92009 1 COMPENSATION THERMISTORS Wr Cl GU RV RA 1 METERING BRIDGE 81058 clos 200 THERMISTOR MOUNT CABLE 7 144 N 25 N gt a gt ED CED o de ES NOTES D CONNECTION BETWEEN PIN 5 ON AND TERM 4 ON ASSY 101 IN STANDARD 4318 8 OPTION 10 ON OTHER OPTIONS NO CONNECTION 10 1821 12822 13823 318 10 320 10 323 40 1529 81 fro on R103 202 5N 206 6
30. s is the Z available power Thethermistor mount Zo mismatch causes an additional power loss of 0 07 dB However on the thermistor mount Zo mis match loss is an uncertainty resulting from the un known phase relationships between the impedances of the source and thermistor mount This uncertainty is 0 30 dB to 0 28 dB and can be determined from the Mismatch Loss Limits charts in Application Note 64 3 24 The result of the total mismatch loss uncer tainty on the available power level is determined by algebraically adding the thermistor mount loss to the uncertainty caused by source and thermistor mount Zo mismatch SWR Thus the Zo available power un certainty is 0 07 dB 0 30 dB and 0 07 dB 0 28 dB equal to a range of 0 23 dB to 0 35 dB or 5 5 to 8 2 The power delivered by the source to a Zo load with source and thermistor mount mis match as in this example would be somewhere be tween 0 23 dB 5 5 below the maximum power and 0 35 dB 8 2 above the minimum power actually entering the thermistor mount 3 25 Power measurement uncertainty caused by mis match loss is one source of error to consider when measuring Z available power without a tuner con tinuation of this example is given in Paragraphs 3 38 through 3 39 to discuss the basic principle of Cali bration Factor correction to a measurement of Zo available power Detailed analysis of accuracy degradation due to SWR in the transmission
31. t screw clockwise until pointer approaches zero mark from the left Continue rotating clockwise until pointer coincides with zero mark If pointer over shoots continue rotating adjustment screw clockwise until pointeronce again ap proaches zero mark from the left Rotate adjustment screw about three degrees counterclockwise to disengage screw adjust ment from meter suspension Note When using an hp Model 478A or other 200 ohm un balanced coaxial thermistor mount the power meter should be zeroed and nulled with the RF power source turned off and connected to the thermistor mount If the RF power source cannot be turned off the power meter must be zeroed and nulled while the RF input connection of the thermistor mount is terminated in the same 10 kHz impedance as that presented bythe power source short open or 50 ohm These precautions are not necessary when waveguide mounts such as the hp Model 486A series or balanced 200 ohm coaxial mounts are used ak Set MOUNT RES switch to correspond to the operating resistance and type of thermistor mount used Model 431C RANGE 08m 431 2 CAUTION To avoid severe damage to the thermistor mount be carefulnot tomove the MOUNT RES switch while operating the RANGE switch 3 Set RANGE to 01 mW 4 Set POWER to LINE ON If instrument is to be battery operated rotate POWER to BAT TERY ON Adjust ZERO control for 25 to 75 of full scale on meter
32. thermistor mount without the use of atuner The amount of mismatch loss in any measurement depends on the total SWR present The impedance that the source sees is determined by the actual thermistor mount impedance the electrical length of the line and the characteristic impedance of the line Zo 3 22 In general neither the source nor the thermis tor mount has Zo impedance and the actual impedances are known only as reflection coefficients mismatch losses SWR These forms of information lack phase information data As a result the power delivered to the thermistor mount and hence the mismatch loss can only be described as being somewhere between two limits The uncertainty of power measurement due to mismatch loss increases with SWR Limits of mis match loss are generally determined by means of a chart such as the Mismatch Loss Limits charts in Application Note 64 3 23 An example may explain how imperfect match affects the uncertainty of power measurement A typical Zp available power measurement situation can involve a source with an SWR of 1 7 ps 0 26 and a thermistor mount with an SWR of 1 3 pm 0 13 Figure 3 2 shows a plot of power levels and mismatch power uncertainties that result from source and ther mistor mount mismatch The source mismatch results in a power loss of 0 29 dB from the maximum power that would be delivered by the source to a con jugate match The power level that results from this los
33. wn in order to accu rately calculate substituted DC power in a DC substi tution measurement 3 51 The hp Model 84028 Calibrator provides a con venient method of determining the detection thermistor operating resistance The thermistor mount cable is connected between the 431C Power Meter THERMIS TOR MOUNT and 8402B Calibrator RESISTANCE STANDARD connectors Bythe THERMISTOR RESIS TANCE switch the 8402B Calibrator substitutes pre cision resistance values in place of the thermistor elements normally in the 431C bridge circuits The switched resistances provide a method of determining a oscillation no oscillation state of the 431C Power Meter 3 52 Withthe 431C RANGE switch at NULL a stable reading greater than zero indicates an audio bias os cillation state While changing the substituted resis tances the operator can determine when oscillations RECORDER LEVELER J2 hp MODEL 431C POWER METER hp 01 8401 LEVELER AMPLIFIER OUTPUT 43iC A 4 Figure 3 5 Output Power Leveling 02316 2 Model 431C 47 2 TO 6 5 UNCERTAINTY ON POWER ENTERING THERMISTOR MOUNT 20 AVAILABLE POWER INSTRUMENTATION ERROR 12 CALIBRATION FACTOR ACCURACY 10 2 TO 9 5 TOTAL UNCERTAINTY aa MEASUREMENT MAXIMUM POSSIBLE ERROR 10 2 STATISTICALLY MOST PROBABLE ERROR 0 35 431 A 12 Figure 3 4 Total Uncertainty After Correction Before correction the maximum p
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