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1. Set Calibration Use Cal or Fact Cal Is there any frequency limited device in the signal Want to see beyond path filter those devices duplexer Set DTF by Limit Bandwidth Set DTF by Cover Distance Run Save results Measurement gt Figure 7 Making DTF measurements Basics of Using the YBA250 gt Application Note Conclusion The YBA250 performs swept reflection measurements in the form of return loss and VSWR in frequency domain and distance domain Return loss VSWR in the frequency domain is the primary tool to diagnose the health of an antenna system If the diagnostics suggest there is a fault DIF measurements are taken to determine where the fault is Reflection measurements are usually calibrated to improve accuracy The quality of the standards used during the calibration determines the accuracy of the measurements Since the calibration process in the YBA250 is performed across its entire frequency range changing the frequency parameters of a measurement does not require re calibration Other unique features of the YBA250 are the factory calibration set and the DIF setup The former allows making cali brated measurement when the standards are not available The latter allows the user to set up the DIF by distance or frequency and view their actual values before performing measurements These unique features translate into time savings when testing antenna systems Recommended Literature
2. 1 Application Note Accurate Antenna Measurements and the NetTek YBA250 Literature Number 2GW 15651 0 URL htto www tek com Measurement App_Notes 2G_15651 eng 2GW_15651_0 pdf 2 Application Note Accurate Reflection Measurements and the NetTek YBA250 Literature Number 2GW 15653 0 URL htto www tek com Measurement App_Notes 15653 eng 2GW_15653_0 pdf 3 YBA250 User Manual Part Number 071 1047 01 www tektronix com wireless 7 8 www tektronix com wireless Contact Tektronix ASEAN Australasia Pakistan 65 6356 3900 Austria 43 2236 8092 262 Belgium 32 2 715 89 70 Brazil amp South America 55 11 3741 8360 Canada 1 800 661 5625 Central Europe amp Greece 43 2236 8092 301 Denmark 45 44 850 700 Finland 358 9 4783 400 France amp North Africa 33 0 1 69 86 80 34 Germany 49 221 94 77 400 Hong Kong 852 2585 6688 India 91 80 2275577 Italy 39 02 25086 1 Japan 81 3 3448 3010 Mexico Central America amp Caribbean 52 55 56666 333 The Netherlands 31 0 23 569 5555 Norway 47 22 07 07 00 People s Republic of China 86 10 6235 1230 Poland 48 0 22 521 53 40 Republic of Korea 82 2 528 5299 Russia CIS amp The Baltics 358 9 4783 400 South Africa 27 11 254 8360 Spain 34 91 372 6055 Sweden 46 8 477 6503 4 Taiwan 886 2 2722 9622 United Kingdom amp Eire 44 0 1344 392400 USA 1 800 426 2200 USA Export Sales 1 503 627 1916 For other areas contact T
3. a certain frequency range Ideally an antenna would radiate all the power that is sent gt Figure 1 Antenna reflection characteristics to it by the transmitter In practice however some of that power separates the incident signal from the reflected signal for each is reflected back as seen in Figure 1 frequency The ratio of the reflected signal and the incident signal The YBA250 generates RF signals and applies it to the antenna known as reflection coefficient is computed corrected using cali The RF signal is generated in steps point by point from a start bration data and displayed in the form of return loss or VSWR to a stop frequency The core of the YBA250 is a test set that 1 www tektronix com wireless Tektronix Enabling Innovation Basics of Using the YBA250 gt Application Note Figure 2 shows an example of a return loss measurement of a GSM antenna that was connected directly to the test port of the YBA250 The frequency band of this antenna is from 1850 MHz to 1990 MHz The Blue line in Figure 2 shows the measured return loss of the antenna The Green line is a user defined mask entered into the YBA250 that shows the manufacturer s specifications When the antenna meets the specification the line is Green The line turns Red when it does not Return loss indicates how far down from the incident signal at 0 dB the reflected signal is A larger value of return loss farther down indicates a smaller reflec
4. gt Application Note oO o m p Basics of Using the Netlek YBA250 SNOILVJINAWNOJ gt Properly Test Antennae and Locate Faults Use the NetTek YBA250 for determining the health of base station antenna systems identifying transmission line trouble and easily locating faults The YBA250 performs swept reflection measurements over a frequency range from 25 MHz to 2500 MHz This range covers Antenna the frequency bands for 2G and 3G cellular standards It is also able to locate problems in antenna systems by measuring the Distance to Fault DTF of components in the system that cause Magnitude of the Reflection Coefficient Vietlected p large reflections V ncident Antenna systems constitute an essential part of a cell They could Return Loss degrade their performance over time or have sudden failures which RL 2010g p 2010g i may cause poor voice quality loss of coverage and dropped calls a Testing antenna systems regularly increases the reliability of a cn ei cell site nee 1 p This application note explains the basics of using the YBA250 to properly test antenna systems using calibrated reflection measure a a Incident signal ments in the form of Return Loss and VSWR The YBA250 is used to diagnose the health of an antenna system including the antenna Reflected signal itself and the feed line components attached to it An antenna is a device that radiates or absorbs radio waves in
5. m1 M2 2 667 Max 47 799 Min 20 026 dB 1920 00 MHz ARI Aser nee E EET gt Figure 5 Return Loss of a GSM antenna Blue Factory Cal Yellow User Cal at end of cable Other Devices in the Signal Path In some systems a duplexer or filter may be found in the signal path between the test port of the YBA250 and the antenna s port A duplexer is a device that isolates the receiver and the transmitter paths allowing the use of a common antenna reducing the numbers of antennae and feed lines in the system A filter is a device used to pass frequencies within its frequency band with very little loss while attenuating all signals outside its band Both components pass signals over a limited range of frequency and reflect those outside that range Measuring return loss or VSWR with them in the signal path will indicate the quality of the entire system attached to the base station Filters and duplexers present in the signal path will to some degree mask the antenna s performance For more accurate results about the antenna system only those components pertinent to the antenna system should be in the signal path Making Measurements in the Frequency Domain Return Loss or VSWR in the frequency domain is the primary measurement tool to diagnose the health of an antenna system gt Calibration Set Choose the most appropriate calibration data set see Calibration gt Frequency Setup There are two ways to select how wide
6. 6 www tektronix com wireless Limit Bandwidth is recommended when frequency limited devices such as filters or duplexers are in the signal path and one would like to see beyond these devices The user enters the frequency parameters by choosing the central frequency and bandwidth The YBA250 then finds the distance related parameters The actual bandwidth used in the measurement will be less than or equal to the value entered by the user This guarantees that the signal stays within the specified range Notice that according to Equation c fixing the frequency bandwidth also fixes the distance resolution When the bandwidth is fixed a change in the number frequency points changes the maximum distance as indicated by Equation b In this case the methods are Fast Normal and Long Distance If a fault is located beyond these devices the distance to the fault may be displayed as being farther away than it actually is This occurs because these devices add delay to the RF signal traveling forwards and backwards through them If the DIF setup contains frequencies outside the specifications of a frequency limited device the DIF results might be meaningful only at distances less than or equal to the distance where the device is and will never be good beyond it In general the results may be poor or completely useless This occurs because the device will reflect the RF signal at frequencies outside its bandpass and the effects of measureme
7. easurement test setups requiring the use of a jumper cable will be affected by the characteristics of the jumper cable When performing measurements using the Factory Cal it is recommended to use a precision jumper cable such as the Tektronix part number 012 1619 00 The errors introduced by this cable are mainly due to its loss and not its reflection characteristics These errors can be removed from the return loss results by subtracting two times the value of the cable loss from the return loss results equivalent to raising the return loss waveform by twice the value of its cable loss Figure 5 shows the return loss results of a GSM antenna system when using a precision cable of 1 4 dB loss The Blue line is the measured return loss using Factory Cal The Yellow line is the meas ured return loss using User Cal at the end of the precision jumper cable Notice that the effects of the precision jumper cable are pri marily to increase the actual return loss by about 2 8 dB In other words it makes the overall return loss measurement looks better farther down by about 2 8 dB To obtain more accurate results we recommend calibrating the YBA250 at the end of the jumper cable 4 www tektronix com wireless Measurement acto Calibration Rean Loss Fast Factory Trace 2 Saved gsm1900 p 200364 4 1871 300 o 1871 300 lt 0 0 000 A 1915 400 Y 2003 600 MHz Mi _22 877 M2 20 209
8. ektronix Inc at 1 503 627 7111 Updated 20 September 2002 For Further Information Tektronix maintains a comprehensive constantly expanding collec tion of application notes technical briefs and other resources to help engineers working on the cutting edge of technology Please visit www tektronix com Copyright 2003 Tektronix Inc All rights reserved Tektronix products are covered by U S and foreign patents issued and pending Information in this publication supersedes that in all previously published material Specification and price change privileges reserved TEKTRONIX and TEK are registered trademarks of Tektronix Inc All other trade names referenced are the service marks trademarks or registered trademarks of their respective companies 05 03 HB SFI 2EW 16667 0 Tektronix Enabling Innovation
9. hanging the frequency parameters of a measurement does not require re calibration The YBA250 offers two types of calibration Factory Calibration and User Calibration The Factory Calibration Factory Cal is done during manufacturing and the results are stored in non volatile memory so that it can be used at any time A User Calibration User Cal is 2 www tektronix com wireless 7 Running KO EEE PA User AA Trace 1 Normal J ob GSM 1900 Measurement Return Loss Fast Auto Scale Start MHz 1836 12 1836 12 4 1920 00 MHz 2003 88 4 1913 300 o 1919 950 M1 Mask 12 800 A 1923 800 V 1866 400 MHz M1 26 500 mM2 28 838 M2 Mask 14 838 Max 34 489 Min 1 Astart_ fa vB4250 gt Figure 2 Return Loss of a GSM antenna Blue line performed before taking measurements and it also compensates for changes in temperature and component aging to ensure the most accurate measurement A User Cal can be stored in the YBA250 and it can be recalled at another time When using a non precision cable to connect the YBA250 to the device under test we recommend performing the calibration at the end of the cable to remove the effect of the jumper cable over the measurement results The Factory Cal Can be Used When gt The user does not need the best accuracy for his her measurements A User Cal Should be Used When gt The user wants to obtain the most accurate measurement
10. nt at each point in the sweep are combined at each distance point of the DIF measurement In other words reflections off the device s input at some frequencies will affect the compound result at each distance point in the distance domain A fault that does not exist may be shown at the device s input and a fault beyond the device may not be seen by the instrument DTF could be used to determine the return loss of an antenna by taking the amplitude of the DIF at the distance where the antenna is This method however is not nearly as accurate as the return loss vs frequency measurement described in Making Measurements in the Frequency Domain If used however one should set up the DIF by Limit Bandwidth and be careful to make sure that the sweep is not wider than the antenna s bandwidth The return loss or VSWR shown at the distance where the antenna is will be roughly correct but will be affected by the transmission line loss vs frequency selected in the DIF setup The principal goal of the DIF measurement is to locate the fault therefore a smaller distance resolution is desired which requires measurements over a wide frequency range Limiting the frequency range to the bandpass of the in line device will result in poor distance resolution Figure 7 shows a flow diagram of the main steps when doing a DIF measurement The YBA250 User Manual 3 contains a detailed description of this process Set output power if necessary
11. oints Connectors PREN Test Port YBA250 BTS House Note This graphic shows only one sector for simplicity gt Figure 4 Antenna testing in the field The YBA250 makes measurements in two domains frequency domain and distance domain Tests using return loss or VSWR versus frequency are performed to determine the health of the antenna system In the event that the antenna system fails these tests distance domain diagnosis Known as Distance to Fault DTF is then performed in order to locate the fault The conversion from frequency to distance is achieved by processing the reflection coefficient data using digital signal processing algorithms www tektronix com wireless 3 Basics of Using the YBA250 gt Application Note Using a Jumper Cable Another practical issue for antenna testing in the field is that the test points are usually in places that may make it difficult to directly connect the YBA250 test port to them see Figure 4 Thus a jumper cable is normally used between the test port of the YBA250 and the test points The jumper cable s reflection characteristics and loss cause errors in the reflection measurement Both of these errors can be eliminated by calibrating the instrument at the end of the jumper cable lf a standard calibration kit is not available the YBA250 can still make calibrated measurements using its Factory Calibration Factory Cal Since this calibration is performed at the test port m
12. possible gt The calibration is done at the end of a jumper cable a cable between the YBA250 test port and the device under test To Perform a Quality Calibration We Recommend gt Use of a high precision calibration kit such as the YBA250C1 gt Properly tightened standard to ensure a good connection A loose connection will produce poor calibration data gt Minimal cable movement when calibrating at the end of the cable More detailed information on calibration and accurate measurements can be found at 1 and 2 see page 7 Performing a New Use Calibration Set output power if necessary Select where to perform calibration Test port end of cable Calibrate using OPEN SHORT and LOAD in any order Cancel Automatically saved as the current cal data set OK gt Figure 3 User calibration flowchart Figure 3 shows a flowchart for the calibration process A more detailed flowchart can be found in the YBA250 User Manual 3 see page 7 Basics of Using the YBA250 gt Application Note Antenna Testing Performed in the Field Antenna tests are usually performed at ground level as shown in Figure 4 The measured return loss of the antenna will be affected by the connector s jumper cable s and transmission line feed line or feeder that are in the path between the YBA250 calibrated output and the antenna port Antena Transmission line feeder Possible test points to test p
13. riations in frequency The length of the coaxial cable s and the group delays of the devices in the sig nal path affect the change in phase Each distance point in the DIF measurement is affected by all frequencies in the sweep Therefore one must be careful when setting the DIF in order to obtain meaningful results The basic relationships between frequency and distance are given by V C 1 Dmax a 2 Af Af BW _ Fstop 7 F start b N 1 N 1 Dia Vp C 1 Ad _ xk C N 1 2 BW where Diis is the maximum measurable distance A is the velocity of propagation of the coax cable with respect to the speed of light C is the speed of light N is the number of frequency points in the sweep BW is the frequency bandwidth Af is the frequency step of the sweep Ad is the distance resolution www tektronix com wireless 5 Basics of Using the YBA250 gt Application Note Notice that Dmax is mainly determined by the A as indicated by Equation a When a measurement with wide frequency span is possible more frequency points can be acquired This gives a better resolution in distance as indicated by Equation c The YBA250 can acquire up to 4096 frequency points This capability is used to offer several levels of distance resolution and maximum distance according to the method used to setup the DIF as explained below gt Calibration Set Choose the most appropriate calibration data set
14. see Calibration gt Distance Setup The YBA250 offers a unique configurable tool that makes it easier for the user to set up the DIF On previous products the DIF mode has been difficult to adjust to trade off the start and stop frequency and the number of measurement points to get the total range and distance resolution desired The YBA250 is designed to allow the user to set the controls in terms of performance required The user can select between two setup modes by Cover Distance and by Limit Bandwidth Cover Distance is recommended when no frequency limited devices such as filters or duplexers are in the signal path In this mode the user enters the desired total distance and the YBA250 automatically adjusts values to make sure that the actual measured distance covers the user s desired distance Thus the actual covered distance may be greater than or equal to the user s desired distance but will not be less In this mode a change in the distance resolu tion relates to a change in the number of frequency points The methods for this mode are Fast Normal or High Resolution Fast is used if a high update rate is desired but high resolution is not required Normal gives a somewhat slower update rate but better distance resolution The High Resolution method gives the highest distance resolution making it possible to identify which end of a jumper at the top of a tower needs repair This method requires longer measurement time
15. ted signal meaning a better antenna In other words more power is radiated and less power is reflected All reflection measurement test sets introduce errors to the actual measurement results They are removed by a process called calibra tion In general reflection measurements are usually made as a comparison of the device being measured to a known standard This standard is assumed to be perfect The process of comparing is called calibrating the measurement equipment Calibration Calibration is a process that compensates for errors introduced by the reflection measurement test set This process uses high precision measurement standards whose characteristics are precisely known The YBA250 calculates the difference between the measurement results from the calibration standards and the ideal results to create correction data This data is used to remove deterministic errors from the measurement results The YBA250 uses a calibration kit that contains three precision standards an Open a Short and a Load such as the YBA250C1 or equivalent The accuracy of the measurements is determined by the quality of the standards primarily Load A measurement may indicate a return loss of 50 dB This reading is meaningless however if the standard load used to make the calibration was specified to be only 40 dB The calibration process in the YBA250 provides a unique feature It is performed across its entire frequency range Therefore c
16. to sweep select the uplink downlink button of the cellular standard used or dial the Start and Stop frequencies gt Saving Results We recommend saving the results to monitor a possible degradation in the antenna system over time Figure 6 shows a flow diagram of the main steps when doing a frequency domain measurement The YBA250 User Manual 3 see page 7 has a detailed description of this process Set output power if necessary Set Calibration Use Cal or Fact Cal Set start and stop frequencies Set mask Run Measurement Save results B Others gt Figure 6 Making frequency domain measurements Basics of Using the YBA250 gt Application Note Making Measurements in the Distance Domain DTF The distance domain is return loss or VSWR measured as a function of distance along the coaxial cable connected to the YBA250 This secondary measurement tool is best known as Distance to Fault DTF It is used to locate faults when the measurements in the frequency domain indicate a problem in other words when the fre quency domain return loss results are out of spec When performing DIF measurements the YBA250 still sweeps the system under test in frequency and the calibrated reflection coefficients are computed Then these results are transformed to the distance domain using digital signal processing algorithms The distance information is contained in how much the phase of the reflected signal changes with va

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