Anritsu HFE0303_Vondran 电路图.pdf

《Anritsu HFE0303_Vondran 电路图.pdf》由会员分享，可在线阅读，更多相关《Anritsu HFE0303_Vondran 电路图.pdf（5页珍藏版）》请在收音机爱好者资料库上搜索。

1、50High Frequency Electronics High Frequency Design PULSED MEASUREMENTS Techniques for Pulsed S-Parameter Measurements By David Vondran Anritsu Company M any devices, par- ticularly power devices, are not designed to operate con- tinuously or with CW sig- nals. This is especially true when devices ar

2、e being tested on-wafer, where the thermal resis- tance is greatly increased 1. In these cases, S-parameter measure- ments are best performed in a pulsed test environment. The details of pulsed measurement are greatly dependent on the pulse properties being studied. At one extreme is the realm of hi

3、gh pulse repetition frequencies (PRFs) and fairly narrow pulses, as is common in radar applications. At the other extreme is the com- munications arena where PRFs are quite low and pulse widths fairly wide (e.g., GSM 2). These two extremes exemplify two tech- niques, termed bandwidth limited and tri

4、g- gered, that are discussed in this note.To a cer- tain degree, the two approaches overlap in terms of allowed parameters, so most situa- tions can be covered by one if not both of them. The objective of this article is to provide an understanding of general S-parameter mea- surements performed wit

5、h a vector network analyzer (VNA), over a range of pulsed condi- tions for both RF and microwave/mm-wave measurement applications. Pulse profiling of the detailed transient response is not covered here although it is briefly discussed elsewhere with regard to triggered measurements 3. Certain rise/f

6、all time behaviors can be studied using time domain mode 4 but that will not be discussed either. The Spectra of Pulsed RF Signals As the reader may be aware, a pulsed RF signal will have a spectrum composed of a series of spectral lines with an envelope described by a sinc function 5.The spacing of

7、 the lines is set by the PRF while the envelope shape is fixed by the pulse width (assuming rise and fall times are small relative to the width). The relationship is shown in Figure 1. The size of this spectrum (occupied frequency range) with respect to the IF bandwidth (IFBW) of the network analyze

8、r determines the measurement mode. In the case of low PRF and wide pulses, the entire spectrum can fit within an IFBW. In this case, the measurement proceeds normally without a significant reduction in dynamic range. Some additional smoothing/averaging may be needed to reduce effects of the outlying

9、 Pulsed measurement is an essential tool for measuring the performance of power amplifiers under low duty cycle conditions, including on-wafer test applications and high peak-to-average modulation formats 1/T0 RF First null at offset = 1/T1 Figure 1. The spectrum of a pulsed RF signal is shown here.

10、 The center maximum is at the RF frequency, the line spacing is equal to one over the pulse period T0, and the first envelope null offset at one over the pulse width T1 (neglecting rise and fall times). From March 2003 High Frequency Electronics Copyright 2003, Summit Technical Media, LLC RadioFans.

11、CN 收音机爱 好者资料库 52High Frequency Electronics High Frequency Design PULSED MEASUREMENTS portions of the distribution in Figure 1. A requirement in this measure- ment is that the VNA measurement be aligned in time with the pulse, hence the term triggered. In the case of a high PRF, the line spacing can

12、be substantial relative to the IFBW so the analyzer can just pick off the center line (thus the term bandwidth limited). The measure- ment of just this line is sufficient to perform an S-parameter measure- mentsince it carries the magnitude and phase of the envelope at the cen- ter pointas long as a

13、 calibration is performed under those same condi- tions.However, since only a frac- tion of the total sig- nal energy is used, the dynamic range may be limited. Triggered Measurements Since the spec- trum fits entirely in an IFBW in this case,the depen- dence of the mea- surement on the pulse train

14、would appear simple. In the time domain sense, however, one wants the sampling to occur during the on period of the pulse in order to capture the desired information. This is accomplished by triggering the VNA to measure in the appropriate points in time. The details of this process (and its applica

15、tion to other measurement types) are covered in greater detail in 3 but will be summarized here. As shown in Figure 2, the idea is for the trigger pulse to arrive at the VNA sometime before the RF pulse in order to account for instrument latency although starting later is allowed. The sampling can b

16、egin sometime after the RF pulse has set- tled unless that process is of interest as well. The sampling can continue for a substantial portion of the pulse but should not continue beyond the end. As a gross limit, the IFBW must be greater than 1/T1 (T1=pulse width) to keep sampling from over- runnin

17、g the pulse (30 kHz with aver- aging is preferred in the MS462xx family; see the Appendix). Because of pulse settling, internal filtering and some other latency issues, some safe- ty margin is required. This will vary greatly depending on setup and may require some experimentation. If too small an I

18、FBW is used (or with too much averaging), the trace data will become very noisy. The pulse for the RF or the DUT control often comes from a pulse gen- erator. The external trigger pulse for the VNA can come from another channel of the same generator or it can be derived from the main pulse chain wit

19、h a small delay circuit of the users design. An example setup is shown in Figure 3. Some power level details for the various systems are Figure 2 An illustration of the timing in a triggered measurement: the VNA is triggered sometime near the start of the pulse so that data is sampled within the dur

20、ation of the pulse. Figure 3 An example setup for a triggered mea- surement is shown here. A dual channel pulse gen- erator is used to create the trigger pulse for the VNA as well as the RF pulse for the DUT. In other tests, a control line to the DUT may be pulsed instead of the RF itself. Scorpion

21、(MS462xx) and related systems CW mode: trigger rates up to 900 Hz NB swept: up to 900 Hz WB swept: up to 200 Hz Low frequency instruments may allow higher rates. Fast CW (available from GPIB in firmware versions 1.16 and later) allows higher rates. Lightning (37xxx) and related systems CW mode: up t

22、o 500 Hz NB swept: up to 150 Hz WB swept: up to 50 Hz for a 40 GHz instrument (less for 65 GHz and Panorama systems) Fast CW mode (available from GPIB) allows higher rates NB refers to a narrowband sweep that does not include bandswitch points. Consult the factory for more details. Trigger width mus

23、t generally be at least 50 s and is a TTL level signal. Table 1 Triggering limits for two Anritsu Company VNA instrument families. Margin for pulse settling Safety margin before end of burst RF Burst Time Ext. trigger pulse Data sampling Pulse Generator VNA Switch A B Ext. trigger in 1 2 DUT RadioFa

24、ns.CN 收音机爱 好者资料库 54High Frequency Electronics High Frequency Design PULSED MEASUREMENTS given in the Appendix. There are additional limitations on the trigger frequency that can be fed to the VNA. With lower PRFs as in GSM, for example, sampling can occur on every RF pulse. With higher PRFs, it may

25、be necessary to sample only on some pulses which may require a more elaborate timing setup. The maximum triggering fre- quency is dependent on the instru- ment, the mode, the IFBW, the fre- quency range, and user-intervention (button pushing or other interrupts) among other issues. To give the user

26、some idea of maximum possible trig- ger rates, some limits are listed by instrument in Table 1. These are not guaranteed and will vary with setup (all assume a 10 kHz IFBW, single S- parameter). As a very simple example, the gain of an amplifier was measured non-pulsed and under GSM condi- tions at

27、a pair of power levels. The IFBW was quite wide in these exam- ples (30 kHz) and only a frequency response (i.e., normalization) calibra- tion was performed. In the low power case, when the amplifier was far from compression, the pulsed and non- pulsed results agree rea- sonably well as shown in Fig

28、ure 4.When the amplifier is in compres- sion at the higher power level (0 dBm in), the results diverge as per- haps might be expected. In this measurement class, calibrations may be performed without pulsing. However, if a pulsing switch (as in Figure 3) is used, it should be present so its reflecti

29、ons and loss can be calibrated out. For full calibrations (other than just a normalization), any pulsing switch should precede the test coupler. Ideally, it should precede the reference coupler as well. In the 373xx family of VNAs, the external preamplifier loop allows the switch to precede the test

30、 coupler. In the MS462xx family, a version with direct receiver access (MS462xC models) is helpful for this. These models are used with the various power amplifi- er test systems (HATS and PATS). Bandwidth Limited Measurements In the case of bandwidth limited measurements, the spectrum of the pulsed

31、 RF signal is very wide with respect to an IFBW. The selected IFBW must be small enough that it does not capture significant energy from other lines than the center lobe. Clearly, this technique has the most utility for PRFs of 10 kHz or higher although lower rates are allowed if the user is willing

32、 to use IFBWs smaller than the typical few kHz. An example measurement where too wide an IFBW was used is shown in Figure 5. Here the PRF was about 30 kHz and a 30 kHz IFBW was used; although this might have worked if the IF filters had perfect stop bands, this is not true in practice. Note that nar

33、rower IFBWs also have the advan- tage of increased dynamic range. An important point about this technique is that a fair amount of the signal energy is thrown away by the IF filter. The signal reduction (SR) is given by Thus the dynamic range will become limited as the duty cycle shrinks. For duty c

34、ycles of 1 percent or more, the reduction is 40 dB or less. This is not a problem in the MS462xx family since 90-120 dB of dynamic range is usually available t begin with (at narrower IFBWs). In the microwave and mm-wave VNAs, SR (dB) period pulse width = 20 10 log Figure 4 A comparison of pulsed (t

35、riggered under GSM conditions) and non-pulsed measurements of an amplifier is shown here. At low power lev- els, the results basically agree (no match correction accounts for small dif- ferences). When the amplifier is under compression, the results start to diverge. Figure 5 A bandwidth limited mea

36、surement with a PRF of 30 kHz: The flat trace represents a mea- surement with an IFBW of 3 kHz and is correct; the noisy, incorrect trace is a measurement with a 30 kHz IFBW in which additional spectral lines are admitted to the IF system. Gain (dB) Gain (dB) Frequency (GHz) Frequency (GHz) RadioFan

37、s.CN 收音机爱 好者资料库 56High Frequency Electronics High Frequency Design PULSED MEASUREMENTS where less dynamic range is original- ly available, the reduction becomes more important for some measure- ments and duty cycles below 1 per- cent become problematic. Narrower IFBWs will enhance dynamic range in a

38、ll of these limited bandwidth cases since less integrated noise is sent to the receiver. This measurement also requires periodicity of the pulse.The triggered measurement could be more lax in this respect as long as the VNA was triggered at the right time.The band- width limited measurement makes as

39、sumptions about the spectral con- tent as discussed previously so peri- odicity is implied. These assumptions about spectral content are also considered in the method of generating the RF pulse. It is assumed that the on/off ratio is very high so if the switching is poor (or the DUT control is not t

40、oo effec- tive), there will be additional uncer- tainties. While the spectral lines will not move in frequency, some of the amplitudes may change and a quality measurement may require a smaller IFBW than would be obvious. An on/off ratio of 50 dB or better is preferred. As a simple measure- ment exa

41、mple, consider an amplifier to be pulsed with a PRF of 30 kHz and a duty cycle of 3 percent. The measurements will be made with an IFBW of 1 kHz and compared to non-pulsed measure- ments at a pair of power levels. A one-path two- port calibration was used to correct for input match effects. Differen

42、t calibra- tions were needed for the pulsed and non-pulsed cases since the calibra- tion must occur under the same signal condi- tions for bandwidth limited measure- ments. The results for pulsed and non-pulsed drives are about the same when the RF level is low as shown in Figure 6. At compressive l

43、evels, how- ever, the results diverge. This might be expected since the average signal seen by the amplifier is quite differ- ent in the two cases. In this measurement type, calibra- tions must be performed under the same pulsed conditions. As discussed in the previous section, any pulsing switches

44、should precede at least the test coupler for full calibrations (other than simple normalizations). In the above example, a MS462xC direct receiver access VNA was used togeth- er with a Handset Amplifier Test System (HATS) test set If simple normalization calibra- tions are all that are desired, then

45、 the pulsing switch may be located after the couplers. This was done for the example measurement in Figure 7. In this case the DUT was a simple W-band waveguide high pass filter and it was measured both pulsed and non-pulsed over approximately 71 to 80 GHz. The pulsed normalization was done with the

46、 pulsing switch in place and operating at a PRF of 10 kHz with a duty cycle of 10 percent. The curves agree quite wellany dif- ferences are primarily due to differ- ences in uncorrected match in the two setups. It is important to note the dynam- ic range well in excess of 40 dB evi- denced by this p

47、lot. Even though the pulsing reduces the dynamic range by 20 dB (per the earlier equation) and the pulsing switch in this case only has about 50 dB of on/off ratio, there is still sufficient range to make this measurement. Summary Two different techniques for pulsed S-parameter measurements have bee

48、n presented along with some details on how to make successful measurements. Table 2 is a summary of conditions when each of these tech- Figure 6 Bandwidth limited example measurements for an amplifier are shown here. NP denotes a non-pulsed measurement while P denotes a pulsed measurement. Pulsed an

49、d non-pulsed amplifier |S21| Base RF drive = 10 dBm Frequency (GHz) S21 (dB) S21 (dB) Frequency (GHz) Pulsed and non-pulsed amplifier |S21| Base RF drive = 0 dBm Figure 7 The results of a W-band filter measure- ment, pulsed and non-pulsed, are shown here. As expected for a passive device, the results agree well. Some differences are present since only a normalization calibration was used and mismatches are not corrected. March 200357 niques is most appropriate. The boxes highlighted in bold are optimal while the other parts of the parameter space are mea