Theory of High Power Loadpull Characterization
Contents
1. Gmm 10 log 4 51 where T and T are the source and load reflection coefficients respectively looking back into each tuner Figure 15 shows a typical response of an entire cascade incinding the quarter wave pre matching network A transducer gain response boundary of 0 1 dB is typical and 0 2 should be considered the maximum Summary Load pull isa valuable tool for evaluating high power RF and microwave transistors designing power amplifiers and verifying large signal model performance and validity domains To enhance the reliability of the data thata load pull system provides itis essential that high performance VNA calibration techniques be adopted Further as emphasized in the present section treating each section of the 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved 89 loadpull separately is useful from a measurement perspective and from a problem resolution perspective In the former case it was shown that measuring quarter wave pre matching networks and tuners separately reduces the uncertainty of the calibration In the latter case it was shown that characterization of each section individually allows its performance to be verified prior to integrating it within the entire system The central theme of this section has been the VNA and its associated calibra2. June 1995 J Sevic R Baeten G Simpson and M Steer Automated Large Signal Loadpull Characterization of Adjacent Channel Power Ration for Digital Wireless Communication System Proceedings of the 45th ARFTG Conference pps 64 70 November 1995 J Sevic K Burger and M Steer A Novel Envelope Termination Loadpull Method for the ACPR Optimization of RF Microwave Power Amplifiers Digest of the IEEE International Microwave Symposium Digest pps 723 726 June 1998 G Simpson and M Majerus Measurement of Large Signal Input Impedance During Loadpull Proceedings of the 50th ARFTG Conference pps 101 106 December 1997 D Rytting ARFTG Short Course Analyzer Calibration Theory 1997 N etwork R Marks Formulation of the Basic Vector N etwork Analyzer Error Model Including Switch Terms Proceedings of the 50th ARFTG Conference pps 115 126 December 1997 R Marks and D Williams Characteristic Impedance Measurement Determination Using Propagation Measurement IEEE Microwave and Guided Wave Letters pps 141 143 June 1991 Ontario California 91764 4804 Fax 909 987 1112 http www maurymw com 18 March 2005 D G Engen and C Hoer Thru Reflect Line An Improved Technique for Calibrating the Dual Six port Automatic Network Analyzer IEEE Transaction on Microwave Theory and Techniques pps 987 993 December 1979 13 14 R Marks A Multi line M ethod of N etwork Analyzer Calibration
3. 12 L E is ee I ar SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 5 of 17 18 March 2005 PORT 1 PORT 2 Figure 4 Signal Flow Graph of the Forward Direction of a Typical VNA Figure 4 shows a signal flow graph of the forward direction of acommon VNA architecture where six systematic error terms are identified An identical flow graph exists for the reverse direction with six additional error terms Consider the situation where it is required to measure an impedance that exhibits a near total reflection such as a load tuner set for 1 Q Assuming a 50 Q reference impedance nearly all of the incident power is reflected back toward the VNA along with a phase shift of 180 Consider what happens when the reflected wave is sampled at the VNA denoted as b in Figure 4 If there is any reflection of the reflected wave incident at the VNA an error will occurin measuring the actual impedance of the load The ability of a VNA to minimize this reflected poweris characterized by its residual source match which isthe corrected sourceimpedance looking into the VNA The uncorrected source impedance looking into the VNA is characterized by the E term in the flow graph of Figure 4 Continuing with this example Figure 5 showsa plot of the upper bound on apparent load impedance versus the residual source match with respect to a reference impedance of 50 Q and an actual impedance of 1 Q For simplicity itis assume
4. Proper torquing of all connector interfaces is essential Since the tuner files usually consist of a small number of frequencies with respect to the number of frequencies presentin atypical VNA calibration itis appropriate to increase the number of averages to 128 or 256 It is generally most useful to characterize a tuner without any additional components attached such as a bias tee in orderto maintain maximum flexibility in the use of the tuner subsequent to the characterization For tuners that are being characterized for the first time it is recommended that they be fully evaluated for insertion loss minimum and maximum VSWR and frequency response to ensure they are compliant with the manufacturer s specifications After characterization the tuner file should be verified by setting the tuner for arbitrary impedances near the center and edge of the Smith chart over 27 radians The error should be less than 0 2 for magnitude and 0 1 for phase Anything worse than this may indicate a problem with either the calibration verify it again or the tuner 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com 18 March 2005 10 F 0 05 Forward Load Match dB a ddis d d yoyeyy aounos pseMio4 0 25 70 l l l 0 3 1 1 5 2 25 3 Frequency GHz Figure 7 Typical Response of a 3 5mm TRL Calibration Using an Offset Short and De
5. calibration used in the pre matching fixture described in References 16 and 18 This fixture was intended to present 0 1 Q at 2 GHz with extremely high accuracy From the verification data the resultant source match is better than 45 dB across the band and the resultant load match is better than 52 dB across the band Comparing these results with Figure 5 shows thatthe uncertainty is very low A significant advantage of using a transforming network to increase system VSWR whether it be a quarter wave line or an additional cascaded tuner is that the two port characterization of each elementis done at manageable impedance levels Char acterization of a tuner presenting a 50 1 VSWR in direct cascade of a quarter wave pre match network would resultin a significantincrease in measurement uncertainty since the VNA must resolve impedances near 0 1 Q Segregating the characterization process moves the impedances that must be resolved to the 1 Q to 2 Q range where the calibration uncertainty is considerably smaller The final step of the fixture verification process is to verify that the two tier calibration has provided the L E is ee I er SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 11 of 17 18 March 2005 D tH 7mm COAXIAL REFERENCE PLANET MICRO STRIP REFERENCE PLANE Z1 ss gt LOAD SIDE H FIXTURE HALF 7 7 T gt TEST SET TEST
6. delay line which if different from 50 Q must be appropriately re normalized back to 50 Q3 34415 TRL calibration can be done in a variety of media including APC 7 mm coaxial waveguide rectangular cylindrical 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com 18 March 2005 5 4 5 o So N a S o T 6 v oO N D s n p 2 3 F mH E E P e l E E oc o o E e E E N 2 o E E i a rrr 9 2 lt q N M 2 i 1 i i i i i i S i i i i i 2 i 1 f 2 i i 1 lt 1 i 1 0 H 5 80 70 60 50 40 30 20 Residual Source Match dB with respect to 50 9 Figure 5 The Influence of Residual Source Match on the Ability of a VNA to Resolve a 1 Q Impedancewith a 50 Q Reference Impedance The Calibration Performance Numbers are Typical for an HP 8510C with an 8514B Test Set Operating a 2 GHz 0 0 Forward Load Match dB ajddia d d yoyeyy B94n0S pseMioy Frequency GHz Figure 6 Typical Response of an APC7mm TRL Calibration Using an Offset Short and Delay Line to Extract Source Match and Load Match Respectively This Data was Taken from an HP 8510C with an HP 8514B Test Set 2900 Inland Empire Blvd Ontario California 91764 4804 technical data Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights
7. of 17 18 March 2005 Measured p nse n apnyiubey uoissiwsuedl pejoipaig pue painseayy Measured and Predicted Reflection Magnitude 0 0 1 1 5 2 2 5 3 Frequency GHz Figure 13 Forward Reflection and Transmission Magnitude Comparison of Measured and Cascaded Fixture Response The Error is so Small the Curves Sit on Top of Each Other 200 200 150 150 z T o 2 100 100 A s a a 5 v S 50 50 7 ar ee al Ss Xo 0 0 ee 35 aF 2 2 ap 0E 50 50 a 5 o S S 3 100 100 F E 2 e N o oO oS o Qa 150 150 200 1 i 1 200 Frequency GHz Figure 14 Forward Reflection and Transmission Phase Comparison of Measured and Cascaded Fixture Response The Error is so Small the Curves Sit on Top of Each Other 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com L E is ee I I ar SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 14 of 17 88 18 March 2005 0 1 Compensated Transducer Gain dB 0 1 l l 15 20 25 30 35 40 45 Available Source Power at DUT Reference Plane dBm Figure 15 Measured Transducer Gain under the Condition of Conjugate Match with Mismatch Loss Compensation Included measured loss must be compensated to arrive at actual loss To compensate for this the mismatch loss is computed as 2 2 Gach ce 2 1 2 rT f
8. quarter wave pre matching network capable of presenting 0 1 Qat2 GHz isdescribed 28 Itis based on a two tier calibration with thin film gold on alumina substrates quarter wave pre matching networks on soft substrates are not recommended due to substrate variations and repeatability issues over time The theory of quarter wave pre matching begins with the mismatch invariance property of lossless networks Consider the quarter wave line of characteristic impedance Z shown in Figure 9 This line is terminated in a mismatch of VSW R oad with an arbitrary phase The reference impedance of VSWR load is Z The mismatch invariance property of lossless networks shows that the input VSWR is identical to the load VSWR but it is with respect to the quarter wave transformed impedance of Z Thus the minimum achievable impedance which is real valued is the impedance looking into the quarter wave line when it is terminated in Z divided by VSWR load This is expressed as z a 4 48 inmin VSWR load INPUT VSWR LOAD VSWR WITH RESPECT L 2 4 Figure 9 Network to Describe the Mismatch Invariance Property of Lossless Networks technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 10 of 17 TO ZL 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Suppose it is desired to synthesize a minimum impedance of 0 1 Q
9. reserved SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE 81 Page 7 of 17 waveguide microstrip and stripline Calibration verification standards which must be used to extract the residual error terms described above are also easily fabricated Figure 6 shows the residual forward source and load match response of an APC 7mm calibration using an HP8510C with an HP8514B test set These were obtained with a 30 cm offset short airline and 30 cm delay line respectively 1 18 The effective source match is computed from the peak peak ripple using _P p ripple 1 10 2 Ey 10 log oe 4 47 1 10 2 where itis seen that better than 53 dB source match is obtained across the band Due to finite directivity 6 dB must be subtracted from the plot showing the delay line response indicating that better than 56 dB load match is obtained except near the low end of the band Calibration performance such as that obtained in Figure 6 is necessary for accurate tuner and fixture characterization and is easily achievable using standard TRL calibration For comparison purposes Figures 7 and 8 show forward source and load match for 3 5 mm TRL and SOLT calibration respectively Here it is observed that the source match of the 3 5 mm TRL calibration has significantly degraded with respect to the APC 7 mm TRL calibration and the 3 5 mm SOLT calibration has significantly degraded with respect to the 3 5 mm TRL calibration Proper VNA calibratio
10. the most useful for carrying out these tasks In addition load pull is also necessary for large signal model developmentand verification Load pull as a design tool is based on measuring the performance of a transistor at various source and or load impedances and fitting contours in the gamma domain to the resultant data measurements at various bias and frequency conditions may also be done Several parameters can be superimposed over each other on a Smith chart and trade offs in performance established From this analysis optimal source and load impedances are determined Load pull can be classified by the method in which source and load impedances are synthesized Since the 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 e Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved complex ratio of the reflected to incident wave on an arbitrary impedance completely characterizes the impedance along with a known reference impedance itisconvenientto classify load pull by how the reflected wave is generated The simplest method to synthesize an arbitrary impedance is to use a stub tuner In contrast to early load pull based on this method contemporary systems fully characterize the stub tuner a priori precluding the need for determining the impedance at each load pull state This results in a significant reduction in time and increases the reliab
11. which might be required for characterizing high power PCS and UMTS LDMOS transistors If a typical passive mechanical tuner is capable of conservatively generating a 40 1 VSWR then the input impedance of the quarter wave line must be approximately 4 Q requiring the characteristic impedance of the quarter wave line to be approximately 14 Q assuming a Z of 50 Q To the extent that the minimum impedance deviates from the ideal is directly related to fixture losses Thus the importance of using a low loss substrate and metal system is apparent Full two port characterization of each fixture side is necessary to reset the reference plane of each associated tuner Several methods are available to do this including analytical methods based on approximate closed form expressions full wave analysis using numerical techniques and employment of VNA error correction techniques 21 22 The first method is based on approximations that have built in uncertainty as does the second method in the form of material parameter uncertainty The third method is entirely measurement based and relies on well behaved TRL error correction mathematics to extract a two port characterization of each fixture half from a two tier calibration More importantly using verification standards it is possible to quantify the accuracy of the de embedding as described in the section on VNA calibration Using the error box formulation of the TRL calibration it is po
12. ERENCE PLANE LOAD BIAS TEE i LOAD SUBHARMONIC TUNER TO LOAD TUNER LOAD SIGNAL SAMPLE PORT DRAIN COLLECTER BIAS POWER SENSOR an Figure 3 Detail of the Load Portion of Figure 1 E R E ee I SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 4 of 17 78 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com typical load block The bias tee comes first Although remote sense can be sampled here in situations where significant currentis required the remote sense should be sampled directly on the DUT test fixture For a load pull system capable of 100 W average power the attenuator following the bias tee should be appropriately rated and exhibit at least 30 dB attenuation The load signal is sampled at a directional coupler after the high power pad A spectrum analyzer is often connected at this port and it may be useful to use alow coupling factor e g 30 dB to minimize the padding necessary in front of the spectrum analyzer This resultsin an optimal dynamic range of the system for measuring ACPR Following the directional coupler is a low pass filter to remove harmonics which is followed by another attenuator This attenuator is used to improve the return loss of the filter with respect to the power sensor As with the source block interface to the load tuner and power
13. IEEETransactions on Microwave Theory and Techniques pps 1205 1215 July 1990 15 MultiCal User s Manual v 1 0 National Institute of Standards and Technology 1997 J Sevic A Sub 1 Q Loadpull Quarter wave Pre matching Network Based on a Two tier TRL Calibration Proceedings of the 52nd ARFTG Conference pps 73 81 December 1998 D Balo Designing and Calibrating RF Fixtures for SMT Devices Hewlett Packard 1996 Device Test Seminar 1996 John Sevic A Sub 1 Q Loadpull Quarter wave Pre matching Network Based on a Two tier TRL Calibration Microwave Journal pps 122 132 March 1999 R Collin Foundations for Microwave Engineering McGraw Hill New York 1966 2 B Wadell Transmission Line Design Handbook Artech House Boston 1991 21 EM User s Manual v 6 0 Sonnet Software Inc Liverpool New York 1999 22 HP 8510C User s Manual Hewlett Packard Company 1992 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 e Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE 91 Page 17 of 17 technical data
14. MAURY MICROWAVE 18 March 2005 CORPORATION D Theory of High Power Loadpull Characterization for RF and Microwave Transistors Author John Sevic MSEE Director Device Characterization Maury Microwave Corporation This paper first appeared as Chapter 4 5 in The RF and Microwave H andbook Series Electrical Engineering Handbook Volume 22 by Mike Golio Motorola Tempe Arizona USA Publisher CRC Press 12 20 2000 ISBN 084938592X available on line at http www crcpress com shopping_cart products product_contents asp id amp parent_id amp sku 8592 amp pc Reprinted by permission Introduction In both portable and infrastructure wireless systems the power amplifier often represents the largest single source of power consumption in the radio While the implications of this are obvious for portable applications manifested as talk time it is also important for infrastructure applications due to thermal management locatability limitations and main power limitations Significant effort is devoted toward developing high performance RF and microwave transistors and circuits to improve power amplifier efficiency In the former case an accurate and repeatable characterization tool is necessary to evaluate the performance of the transistor In the latter case it is necessary to determine the source and load impedance for the best trade off in overall performance Load pull is presently the mostcommon technique and arguably
15. SET PORT 1 H PORT 2 E _ I Figure 10 Reference Plane Definitions for a Two tier Calibration used for Fixture Characterization The First Tier is based on a TRL APC7mm Calibration and the Second Tier is based on a Microstrip TRL Calibration Forward Load Match dB ajddi d d yoyey 21nos puemio4 70 0 5 Frequency GHz Figure 11 Microstrip TRL Calibration Using an Offset Short and Delay Line to Extract Source Match and Load Match Respectively This Data was taken from an HP 8510C with an HP 8514B Test Set 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 e Fax 909 987 1112 http www maurymw com technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 12 of 17 86 18 March 2005 as2 bs1 SOURCE P2 FIXTURE P1 lt bso as1 a2 bj1 p LOAD P2 FIXTURE P1 bi2 alq Figure 12 Port and Traveling Wave Definitions for Cascading the Source Fixture and Load Fixture to Examine the Accuracy of the Two tier Calibration Fixture Characterization correct two port s parameter description of each fixture half Figure 12 shows each fixture half cascaded using the port definitions adopted by NIST Multical 15 W ith microstrip an ideal thru can be approximated by butting each fixture half together and making top metal contact with a thin conductive film When thisis not possible itis necessary to extrac
16. ation in the future at new frequencies If possible each component of the source and load blocks should be individually characterized prior to integration into their respective block This is particularly so for circulators and high current bias tees which tend to have limited bandwidth The response of the source and load block should be stored for future reference and or troubleshooting Fixture Characterization to Increase System VSWR In the beginning of this section it was indicated that high power load pull may require source and load impedances in the neighborhood of 0 1 Q This does not mean that the DUT may require such an impedance as much as it is necessary for generating closed contours which are useful for evaluation of performance gradients in the gamma domain A very robust and simple method of synthesizing sub 1 Q impedances is to use a quarter wave pre matching Zo Zref D 18 March 2005 network characterized using numerically well defined two tier calibration methods To date use of quarter wave pre matching offers the lowest impedance though itis limited in flexibility due to bandwidth restrictions Recently commercially available passive mechanical systems cascading two tuners together have been made available offering octave bandwidths though they are not able to generate impedances as low as narrow band quarter wave pre matching In this sedion a robust methodology for designing and characterizing a
17. d that the residual source match is in phase with the reflected signal Also shown are typical residual source match performance numbers for an HP8510C using an L E E ee YI SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 6 of 17 HP8514B test set From this graph it is clear that use of low performance calibration techniques will result in latent errors in any characterization performed using a DUT with reflection VSWR near 50 1 Using a 3 5 mm SOLT calibration can result in nearly 20 uncertainty in measuring impedance N ote that TRL the calibration method available on low cost VNAs offers similar performance to 3 5 mm SOLT due to its inability to uniquely resolve the test set port impedances This limitation is due to the presence of only three samplers instead of four and does not allow switch terms to be measured directly For this reason it is recommended that three sampler architectures not be used for the characterization process Similar arguments can be made for the load reflection term of Figure 4 which is characterized by the residual load match error term Identical error terms exist for the reverse direction too so that there area total of four error terms that are significant for low impedance VNA calibration TRL calibration requires a thru line a reflect standard known only within 4 4 and adelay line The system reference impedances will assume the value of the characteristic impedance of the
18. enta high reflection a circulator to improve directivity separates each directional coupler the circulator also protects the reference PA from reflected power The circulator serves to present a power invariant termination for the source tuner the impedance of which is critical for sub 1 Q load pull The bias tee is the last element in the source block which is connected to the gate base bias source via _ CHARACTERIZATION REFERENCE PLANE D 18 March 2005 a low frequency tuner network for sub harmonic impedance control Since the current draw of the gate base is typically small remote sensing of the power supply can be done directly at the bias tee Although components within the source block may have type N or 3 5mm connectors interface to the source tuner is done with an adapter to an APC7mm connector Thisis done to provide arobustconnection and to aid in the VNA characterization of the source block Depending on the measurements that are to be made during load pull a variety of instruments may be connected to the incident and reflected sample ports including a power meter and VNA The former is required for real time leveling and the latter for measuring the input impedance to the DUT The load block of Figure 1 usually includes a port for sampling the load signal of the DUT and the padding and filtering necessary to interface the load signal to a power sensor Figure 3 shows the details of a CHARACTERIZATION REF
19. ental and harmonic power terms Wen the DUT is embedded into a matching network the matching network will usually attenuate the harmonics thus inclusion of the low pass filter more closely approximates the performance that will be observed in practice If the magnitude of the reflection coefficient approaches the residual directivity of the VNA calibration then errors may occur References 1 J M Cusak et al Automatic Loadpull Contour Mapping for Microwave Power Transistors IEEE Transactions on Microwave Theory and Techniques pps 1146 1152 December 1974 Automated Tuner System User s Manual v 1 9 Maury Microwave Corporation 1998 Computer Controlled Tuner System User s Manual v 6 0 Focus Microwave Corporation 1998 technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 16 of 17 D 2900 Inland Empire Blvd Tel 909 987 4715 e 18 March 2005 LP2 Automated Loadpull System User s M anual ATN Microwave Corporation 1997 F Larose F Ghannouchi and R Bosisio A New M ulti harmonic Loadpull Method for Non linear Device Characterization and M odeling Digest of the IEEE International Microwave Symposium Digest pps 443 446 June 1990 F Blache J Nebus P Bouysse and J Villotte A Novel Computerized Multi harmonic Loadpull System for the Optimization of High efficiency Operating Classes in Power Transistors IEEE International Microwave Symposium Digest pps 1037 1040
20. escribed with emphasis on calibration of the vector network analyzer and the characterization of the transistor test fixture Two tier calibration and impedance re normalization are considered for characterizing quarter wave pre matching test fixtures Vector Network Analyzer Calibration Theory Due to the extremely low impedances synthesized in high power loadpull the vector network analyzer VNA calibration is the single most important element of the characterization process Any errorsin the measurement or calibration use of low quality connectors e g SMA or type N or adoption of low performance calibration methods e g SO LT will resultin a significantreduction in accuracy and repeatability Only TRL calibration should be used particularly for tuner and fixture characterization Use of high performance connectors is preferred particularly APC7mm due to its repeatability power handling capability and the fact that it has a hermaphroditic interface simplifying the calibration process Vector network analysis derives its usefulness from its ability to characterize impedance based on ratio measurements instead of absolute power and phase measurements and from its ability to characterize and remove systematic errors due to nonidealities of the hardware For a complete review of VNA architecture and calibration theory the reader is encouraged to review notes from the annual ARFTG Short Course given in November of each year
21. harmonic and harmonic tuners are also included for characterization of out of band impedances The signal sample ports are used to measure the incident and reflected voltage waves at the source tuner interface and the incident voltage wave at the load The signals at each of these ports are applied to the equipment necessary to make the measurements the user desires Each of these blocks is described subsequently The source block of Figure 1 usually includes all of the components necessary for generating the signal leveling its power providing gate base bias for the device under test and providing robust sampling points for the measurement equipment Figure 2 shows the details of a typical source block For flexibility and expediency in applying arbitrarily modulated signals an arbitrary waveform generator and vector signal source are shown The signal is 2900 Inland Empire Blvd Tel 909 987 4715 e Ontario California 91764 4804 Fax 909 987 1112 http www maurymw com 18 March 2005 REFLECTED SIGNAL SAMPLE PORT LOAD SIGNAL SAMPLE PORT Zt zoe foo na wW Ba we oF Zz SOURCE TEST FIXTURE AND LOAD TUNER fo MULTIPLEXERS TUNER fo SUB HARMONIC SOURCE LOAD ji TUNER 2fo TUNER 2fo SOURCE TUNER 3fo LOAD SUB HARMONIC TUNER N lt a zZ q a a GATE BIAS Figure 1 Block Diagram of a Generalized High Power Loadpull System Illustrating the Source Tuners Test Fixtu
22. ility of the system This method of load pull is defined as passive mechanical Passive mechanical systems are capable of presenting approximately 50 1 VSWR with respect to 50 SI and are capable of working in very high power environments Repeatability is better than 60 dB Maury Microwave and Focus Microwave each develop passive mechanical load pull systems 3 For high power applications e g gt 100 W the primary limitation of passive mechanical systems is self heating of the transmission line within the tuner with the resultant thermally induced expansion perturbing the line impedance Solid state phase shifting and attenuator networks can also be used to control the magnitude and phase of a reflected wave thereby effecting an arbitrary impedance This approach has been pioneered by ATN Microwave These systems can be based on a lookup table approach similar to the passive L E i ee I a SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 1 of 17 mechanical systems or can use a vector network analyzer for real time measurement of tuner impedance Like all passive systems the maximum VSWR is limited by intrinsic losses of the tuner network Passive solid state systems such as the ATN typically exhibit a maximum VSWR of 20 1 with respect to 50 Q These systems are ideally suited for medium power applications and noise characterization due to the considerable speed advantage over other types of architectu
23. lay Line to Extract Source Match and Load Match Respectively This Data was Taken From an HP 8510C with an HP 8514B Test Set Forward Load Match dB ajddis d d yoyeyy aounos psemioy 70 i f j 1 Frequency GHz Figure 8 Typical Response of an 3 5mm SOLT Calibration Using an Offset Short and Delay Line to Extract Source Match and Load Match Respectively This Data was Taken from an HP 8510C with an HP 8514B Test Set 2900 Inland Empire Blvd Ontario California 91764 4804 or ie a Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE 83 Page 9 of 17 technical S parameter Characterization of System Components Characterization of system components consists of creating one port and two port s parameter files of the source block and load block as shown in Figures 1 and 2 respectively Each of these figures show suggested reference planes for characterization of the network Since the reflection coefficient of each port of the source and load blocks isin general small with respect to that exhibited by tuners the VNA3 calibration is not as critical as it is for tuner characterization Nevertheless it is recommended to use the same calibration as used for the tuner characterization and to sweep a broad range of frequencies to eliminate the possibility of characteriz
24. n is an essential first step in characterization of any component used for high power load pull characterization and is particularly important for tuner and fixture characterization All VNA calibrations should be based on TRL and must be followed by calibration verification to ensure that the calibration has been performed properly and is exhibiting acceptable performance using the results of Figure 6 as a benchmark Averaging should be set to at least 64 Smoothing should in general be turned off in order to observe any resonances that might technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 8 of 17 D 18 March 2005 otherwise be obscured Although APC 7 mm is recommended 3 5 mm is acceptable when used with a TRL calibration kit U nder no circumstances should type N or SMA connectors be used due to phase repeatability limitations and connector reliability limitations S parameter Characterization of Tuners Tuner characterization begins with proper calibration of the VNA as described in the previous section It is suggested at this point that any adapters on the tuner be serialized and alignment marks made to ensure that in the event of removal they can be replaced in their original positions Replacement of an adapter for any reason will require a new tuner characterization Tuners should be leveled using a bubble level and should be positioned such that the VNA test port cables are not flexed
25. quent to each power calibration Each of the methods will be described in this section Absolute power calibration is done by applying a signal to the source tuner via the source block of Figure 2 After appropriately padding a power sensor itis then connected to DUT side of the source tuner and with the tuners set for 1 1 transformation the resultant power is compared to what the overall cascaded response is expected to be This procedure is repeated for the load tuner except that the signal is injected at the DUT side of the load tuner and the power sensor is located as shown in Figure 3 Splitting this verification in two steps assists in isolating any issues with either the source or load side Itis also possible to vary the impedance of each tuner and calculate what the associated available gain or power gain is although this step is more easily implemented in the power gain verification Power gain verification starts with a two port characterization of a known mismatch standard The simplest way to implement this standard isto use one of the tuners and then set the other tuner for the conjugate of this mismatch In this case the mismatch standard is an ideal thru similar to the one used in fixture verification described in the previous section Since it is unlikely that both the source and load tuners would have identical impedance domains the technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 13
26. re and Load The Incident Reflected and Load Signals are Sampled at the Three Sampling Points Shown Also Shown Though not Necessary are Harmonic and Sub harmonic Tuners ARBITRARY COMPLEX ENVELOPE gt WAVE FORM VECTOR SIONAL GENERATOR p N x gt BIAS TEE TO SOURCE TUNER E CHARACTERIZATION _ REFERENCE PLANE Ze SOURCE 5S SUB HARMONIC arr TUNER Ba iri a RE 20 v 2 a ao Ss f Sk 2 no an a W Ee ka q O o We ira W ac Figure 2 Detail of the Source Portion of Figure 1 2900 Inland Empire Blvd Ontario California 91764 4804 or ie a Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE 77 Page 3 of 17 technical typically created using MATLAB and can represent not only digitally modulated signals but also the more conventional two tone signal The signal is applied to a reference PA which must be characterized to ensure that it remains transparent to the DUT for high power applications this is often a 50 W to 100 W PA Following the reference PA is a low pass filter to remove harmonics generated from the source and or reference PA Next are the sampling points for the incident and reflected waves which is done with two distinct directional couplers Since the source tuner may pres
27. res Tuner and fixture losses are the limiting factor in achieving a VSWR in excess of 50 1 with respect to 50 Q This would be necessary not only for characterization of high power transistors but also low powertransistors at millimeter wave frequencies where system losses can be significant In these instances itis possible to synthesize a reflected wave by sampling the wave generated by the transistor traveling toward the load amplifying it controlling its magnitude and phase and reinjecting it toward the transistor Systems based on thismethod are defined as active load pull Although in principle active load pull can be used to create very low impedance the power necessary usually limits the application of this method to millimeter wave applications 6 Because active load pull systems are capable of placing any reflection coefficient on the port being pulled including reflections greater than unity these systems can be very unstable and difficult to control Instability in ahigh powerload pull system can lead to catastrophic failure of the part being tested The present chapter is devoted to discussing the operation setup and verification of load pull systems used for characterization of high power transistors used in wireless applications W hile the presentation is general in that much of the discussion can be applied to any of the architectures described previously the emphasis is on passive mechanical systems There are t
28. sensor are done with APC7mm connectors to improve robustness and power handling capability The DUT test fixture is used to interface the source and load tuners to a package For cost and package de embedding reasons it is useful to standardize on two or three laboratory evaluation packages For hybrid circuit design it is useful to design a test fixture with feeds and manifolds identical to those used in hybrid to mitigate de embedding difficulties The collector drain side of the test fixture should also have a sampling port for remote sensing of the power supply After the load pull system has been assembled it is recommended that the maximum expected power be applied to the system and changes in impedance be measured due to tuner self heating This may be significant where average powers exceed 100 W or peak powers exceed several hundred watts Any impedance change will establish the upper power limit of the system with respect to impedance accuracy 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 e Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved D 18 March 2005 Characterization of System Components Each of the blocks described in the previous section must be characterized using s parameters in order for a load pull system to function properly In this section the characterization procedure for each of the sections of Figure 1 is d
29. ssible to extract the two port characteristics of an arbitrary element inserted between two reference planes of two different calibrations The first tier of the calibration is usually done at the test port cables of the VNA The second tier of the calibration is done in the media that matches the implementation of the test fixture which is usually microstrip Figure 10 illustrates the reference plane definitions thus described The second tier of the calibration will have its reference 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved D 18 March 2005 impedance set to the impedance of the delay standard which is the impedance of the quarter wave line Although there are many methods of determining the characteristic impedance of a transmission line methods based on estimating the capacitance per unit length and phase velocity are well suited for microstrip lines 15 The capacitance per unit length and phase velocity uniquely describe the quasi TEM characteristic impedance as o1 Q Yp 4 49 Once the characteristic impedance of the delay line is known the s parameters can be re normalized to 50 Q to make them compatible with the 50 Q reference impedance that most automated load pull systems use 3 15 Figure 11 shows the forward source and load match of the second tier microstrip
30. t atwo port characterization of the thru The cascaded transmission matrix is expressed as A Bp A B l 0 A Be C Dy cascade Cy Dy source 0 J thru Cy D load where the middle matrix of the right hand side transmission matrix of alossless zero phase shift thru network Converting the cascade transmission matrix back to s parameter form yields the predicted response of the cascaded test fixture which can then be compared to the measurements of the cascade provided by the VNA Figure 13 shows the measured and predicted cascade magnitude response of a typical PCS quarterwave pre matching fixture based on an 11 Q quarter wave line the phase is shown in Figure 141 8 The relative error across the band is less than 0 1 This type of fixture characterization performance is necessary to minimize error for synthesizing sub 1 Q impedances System Performance Verification Just as verification of VNA calibration is essential so too is verification of overall load pull system performance essential Performance verification can be done with respect to absolute power or with 2900 Inland Empire Blvd Ontario California 91764 4804 Tel 909 987 4715 Fax 909 987 1112 http www maurymw com Copyright 2005 Maury Microwave Inc all rights reserved respect to power gain The former is recommended only occasionally for example when the system is assembled or when a major change is made The latter is recommended subse
31. tion Due to the extremely low impedances synthesized in high power loadpull the VNA calibration is the single most important element of the characterization process Any errors or uncertainty encountered in the VNA calibration will be propragated directly into the loadpull characterization files and may result in erroneous data particularly if system performance verification is not performed To present the sub 1 Q impedances necessary for evaluation of high power transistors transforming networks are required These can be implemented using an impedance using an impedance transforming technical data SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 15 of 17 network such asa quarter wave line or by cascading two tuners together The former offers the highest VSWR atthe expense of narrow bandwidth while the latter isin general more flexible In either case high performance and reliable characterization methods are necessary to attain the best possible results for using loadpull as a verification and design tool Acknowledgments Kerry Burger Philips Mike M ajerus Motorola and Gary Simpson and Jon King M aury Microwave have in many ways influenced the content of this section Their support and friendship ishappily acknowledged Notes 1 Although afilter is not necessary characterization ofa DUT in significant compression will resultin the average power detected by the power sensor including fundam
32. wo reasons for limiting the scope The first reason is that passive solid state systems are usually limited in the maximum power incident on the tuners and to a lesser extent the maximum VSWR the tuners are capable of presenting The second reason is that currently there are oE is ee I ar SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 2 of 17 D 18 March 2005 no active load pull systems commercially available Further it is unlikely that an active load pull system would be capable of practically generating the sub 1 Q impedances necessary for characterization of high power transistors The architecture of the passive mechanical system is discussed first with a detailed description of the necessary components for advanced characterization of transistors such as measuring input impedance and ACPR Vector network analyzer calibration often overlooked and the most important element of tuner characterization is presented next Following this tuner source and load characterization methods are discussed Fixture characterization methods are also presented with emphasis on use of pre matching fixtures to increase tuner VSWR Finally system performance verification is considered System Architecture for High Power Loadpull Figure 1 shows a block diagram of a generalized high power automated load pull system although the architecture can describe any of the systems discussed in the previous section Sub
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