Pulsed VNA Measurements:

Transcription

1 Pulsed VNA Measurements: The Need to Null! January 21, 2004 presented by: Loren Betts Copyright 2004 Agilent Technologies, Inc.

2 Agenda Pulsed RF Devices Pulsed Signal Domains VNA Spectral Nulling Measurement Types/Hardware Configurations Comparisons with the 8510

3 Why Test Under Pulsed Conditions? CW test signals would destroy DUT High-power amplifiers not designed for continuous operation On-wafer devices often lack adequate heat sinking Test-power levels same as actual operation Measuring behavior within pulse is critical to characterizing system operation

4 Radar and Electronic-Warfare Biggest market for pulsed-rf testing Most applications 20 GHz Devices include Amplifiers T/R modules Up/Down converters

5 Pulsed Antenna Test About 30% of antenna test involves pulsed-rf stimulus Test individual antennas or complete systems Radar Cross Section (RCS) measurements often require gating to avoid overloading receiver

6 Wireless Communications Systems TDMA-based systems often use burst mode transmission to save battery power (only transmit during assigned time slot) Measurement example: GSM handset power amplifiers are tested under pulsed conditions Most wireless communications applications 6 GHz

7 On-Wafer Amplifier Test and Modeling Most applications are at microwave frequencies Devices lack adequate heatsinking for CW testing, so pulsed-rf used as a test technique to extract S- parameters Arbitrary, stable temperature (isothermal state) set by adjusting duty cycle Duty cycles are typically < 1% Often requires synchronization of pulsed bias and pulsed RF stimulus

8 Agenda Pulsed RF Devices Pulsed Signal Domains VNA Spectral Nulling Measurement Types/Hardware Configurations Comparisons with the 8510

9 Pulsed RF in Time Domain y( t) = ( rect ( t) x( t)) shah 1 ( t) pw prf rect pw (t) x(t) t -1/2pw 0 1/2pw *... n δ(t-n(1/prf)) -2/prf -1/prf 0 1/prf 2/prf /prf 0 1/prf... t To visualize the pulsed signal in time, we take a windowed version of the continuous signal and convolve it with a shah function which periodizes the windowed signal to create a train of pulses.

10 Pulsed RF in Frequency Domain Fourier transform of time signal: Y ( s) = ( pw sinc( pw s) X ( s)) ( prf shah( prf s)) Y ( s) = ( pw sinc( pw s)) ( prf shah( prf s)) pw = pulse width, prf = pulse repetition frequency The frequency spectrum of the pulsed RF is a sampled sinc function with sample points (signal present) equal to the pulse repetition frequency and nulls (no signal present) equal to the 1/(pulse width).

11 Pulsed RF Spectrum E+6-1.5E+6-1.0E+6-5.0E+5 0.0E+0 5.0E+5 1.0E+6 1.5E+6 2.0E+6 Nulls at 1/PW Pulsed Reponse (db) Frequency Offset (Hz)

12 Pulsed RF Spectrum - Zoom Wanted frequency component First spectral tone at 1.69 khz = PRF Pulsed Response (db) Frequency Offset (Hz)

13 Agenda Pulsed RF Devices Pulsed Signal Domains VNA Spectral Nulling Measurement Types/Hardware Configurations Comparisons with the 8510

14 VNA Spectral Nulling Introduction Extract central spectral component only; measurement to VNA appears CW Spectral Nulling" technique achieves wider bandwidths and faster measurements No lower limit to pulse width, but dynamic range is function of duty cycle IF filter Time domain Frequency domain IF filter

15 Common Filtering Spectrum 0 Filter Passband Response (db) Filter Transition Region First spectral tone at 1.69 khz = PRF Wanted frequency component Filter Stopband Frequency Offset (Hz)

16 PNA Digital Filter in Frequency Domain Response of 500 Hz IF Filter Filter nulls Filter Response (db) Frequency Offset (Hz)

17 PNA Spectral Nulling Response of 500 Hz Digital IF Filter and Pulsed Spectrum Response (db) Frequency Offset (Hz) Wanted frequency component Filtered frequency components Nulling occurs at every 3 rd null in this case A narrower IF bandwidth would skip more nulls Trade off dynamic range and speed by varying IF BW

18 PNA Spectral Nulling Response of 166 Hz Digital IF Filter and Pulsed Spectrum Response (db) Frequency Offset (Hz) Wanted frequency component Filtered frequency components Nulling occurs at every 9 th null in this case

19 Filtered Output using Spectral Nulling X x 10 4 Pulsed spectrum Digital filter (with nulls aligned with PRF) x 10 4 Output With custom filters, number of taps (M) can be chosen to align filter nulls with pulsed spectral components With spectral nulling, IF bandwidth can be much higher compared to conventional IF filtering In practice, may have to slightly adjust PRF (as well as IF filter) to achieve proper nulling, or choose a narrower IF bandwidth

20 Finite Impulse Response (FIR) Filter x(n) Incoming sampled signal t M = number of filter taps (sections) = minimum number of ADC samples required for one data point h(n) = tap weightings BW 1 M Z -1 Z -1 Z -1 Z -1 h(0) x h(1) x h(2) x h(3) + x + + h(m-2) x + Unit delay h(m-1) x + Example FIR topology Sample decimator M y(m) The VNA can move the placement of the filter nulls by modifying the number of filter taps(m). Complex filtered output t

21 Elimination of Additional Interfering Signals Spectral nulling eliminates main pulse spectrum plus other undesired signals: Spectral components can wrap around DC and fold back into pulse spectrum Harmonics of "video feed-through" (leakage of baseband modulation signal) due to RF modulator and IF gates Receiver sensitive to 1 st and 2 nd LO images Aliased spectral components Video feedthrough DC freq

22 Spectral Nulling in the Time Domain 0 us 100 us Incoming pulsed signal... t Actual voltage waveform at ADC 0 us 100 us... t VNA samples t 60 us 100 us 8.33 MHz 41.7 khz IF gate Anti-alias filter ADC Digital FIR IF filter 1 st converter 2 nd converter

23 Agenda Pulsed RF Devices Pulsed Signal Domains VNA Spectral Nulling Measurement Types/Hardware Configurations Comparisons with the 8510

24 PNA Pulsed RF Measurements data point Average Pulse Magnitude and phase data averaged over duration of pulse Point-in-Pulse Data acquired only during specified gate width and position within pulse CW Pulse Profile Data acquired at uniformly spaced time positions across pulse

25 PNA Block Diagram with Internal Receiver Gates 8.33 MHz reference Offset LO Phase-locked loop Offset receiver A/D R1 A A/D φ YIG source (3-10 GHz) V tune Multipliers (2, 4, X 8) LO R2 A/D B A/D PNA point-in-pulse measurements achieved with internal IF gates Gates controlled with external pulse generators Typically, not all IF gates are used IF gates also used for pulse profiling Test port 1 Test port 2 IF gate (at 8.33 MHz 1 st IF) Minimum gate width = 20 ns

26 Receiver Gating Stimulus After Receiver Gating RF Gate RF Gate CW stimulus The Receiver Gates pulse the incoming signal from DUT at same PRF as stimulus therefore Spectral Nulling can be applied with gating to perform point-in-pulse and pulse-profiling. Pulsed stimulus

27 Pulsed Stimulus Configuration Example External pulse generator (e.g., 81110A/81111A) GPIB PNA (20, 40, 50, or 67 GHz) with: H11 IF access H08 Pulsed-RF measurement capability 10 MHz Ref PNA E8362B 20 GHz Pulse 2 drive to internal receiver gate B (for point-in-pulse measurements) Z5623A H GHz RF modulator DUT

28 Pulsed Bias Configuration Example Pulsed bias to amplifier External pulse generator Application software to set proper IF BW and PRF Point-in-pulse GPIB 10 MHz Ref Pulse2 drive to internal receiver gate B (for point-in-pulse) CW Pulsed-RF Power Supply Com

29 Duty Cycle Effect on Pulsed Dynamic Range The system dynamic range of the microwave PNA is much better than the 8510C, helping to overcome the limitations of narrowband detection 100 NO LOWER LIMIT ON THE PULSE WIDTH Dynamic Range (db) GSM Radar Narrowband Detection 8510 Narrowband Detection PNA Duty Cycle (%) Wideband Detection 8510 Isotherm.

30 Pulsed S-Parameter Measurements Duty Cycle Loss =20 log(.0135) =~ -37 db Pulsed CW Pulse Width = 300 ns (DC = 1.35%)

31 Pulsed S-Parameter Measurements Duty Cycle Loss =20*log(.0135) =~ -37 db Averaging =10*log(10 avgs) = +10dB Effective DC Loss = db = -27dB Pulsed CW Pulse Width = 300 ns (DC = 1.35%)

32 Pulsed S-Parameter Measurements Pulsed CW Duty Cycle Loss =20 log(.0135) =~ -37 db Pulse Width = 5 us (DC = 1.35%)

33 Agenda Pulsed RF Devices Pulsed Signal Domains VNA Spectral Nulling Measurement Types/Hardware Configurations Comparisons with the 8510

34 8510 Dynamic Range Receiver Dynamic Range (db) Dynamic Range vs. Pulse Width for Duty Cycles from 0.01% to 10% (90 ms per point) (using wideband detection, effective IF bandwidth = 3 MHz) E E E E E-05 Pulse Width (us) 10% 5% 1% 0.50% 0.10% 0.05% 0.01%

35 PNA Spectral Nulling Dynamic Range Receiver Dynamic Range (db) PNA Dynamic Range vs. Pulse Width for Duty Cycles from 0.01% to 10% (90 ms per point) (using 10 Hz bandwidth) E E E E E-05 Pulse Width (us) 10% 5% 1% 0.50% 0.10% 0.05% 0.01%

36 Comparing Dynamic Range and PNA Dynamic Range vs Duty Cycle (90 ms/point) Dynamic Range (db) PNA 20 us 10 us 5 us 2 us 1 us PNA % 0.10% 1.00% 10.00% Duty Cycle

37 Example PNA/8510 Comparison sweep time = 9.0 s PNA sweep time = 1.4 s S21 (db) x faster with ~15 db better dynamic range! PNA 8510 PW = 5 us PRF = 10 khz Duty cycle = 5% Points = : 3 MHz IFBW no avg PNA: no avg, BW=567 Hz Frequency (GHz) Calibration: 2-port

38 PNA Pulsed-RF Summary Measurements: Frequency coverage up to 67 GHz Average and point-in-pulse response (minimum gate width = 20 ns) Pulse profiling (availability from Agilent Spring 2004)

39 PNA Pulsed-RF Summary Setup: Pulsed RF created by one of following: user-supplied external switches/modulator Agilent RF modulator pulsing DC bias of DUT Option H11: internal receiver-gating switches (required for point-in-pulse) Option H08: software for unique spectral-nulling technique

40 Questions? Thank you!