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Pulsed-RF S-Parameter Measurements Using a VNA

Pulsed-RF S-Parameter Measurements Using a VNA. Agenda. Pulsed-RF Overview Pulsed-RF measurement techniques Wideband/synchronous Narrowband/asynchronous. Why Test Under Pulsed Conditions?. Device may behave differently between CW and pulsed stimuli

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Pulsed-RF S-Parameter Measurements Using a VNA

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  1. Pulsed-RF S-Parameter Measurements Using a VNA

  2. Agenda • Pulsed-RF Overview • Pulsed-RF measurement techniques • Wideband/synchronous • Narrowband/asynchronous

  3. Why Test Under Pulsed Conditions? • Device may behave differently between CW and pulsed stimuli • Bias changes during pulse might affect RF performance • Overshoot, ringing, droop may result from pulsed stimulus • Measuring behavior within pulse is often critical to characterizing system operation (radars for example) • CW test signals would destroy DUT • High-power amplifiers not designed for continuous operation • On-wafer devices often lack adequate heat sinking • Pulsed test-power levels can be same as actual operation

  4. Radar and Electronic-Warfare • Biggest market for pulsed-RF testing • Traditional applications £ 20 GHz • New applications in Ka band (26.5-40 GHz) • Devices include • amplifiers • T/R modules • up/down converters

  5. Wireless Communications Systems • TDMA-based systems often use burst mode transmission • Saves battery power • Minimizes probability of intercept • Power amplifiers often tested with pulsed bias • Most of wireless communications applications £ 6 GHz

  6. 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

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

  8. VNA Pulsed-RF Measurements VNA data display Magnitude and phase data averaged over duration of pulse data point Frequency domain Average Pulse Swept carrier Data acquired only during specified gate width and position within pulse Frequency domain Point-in-Pulse CW Data acquired at uniformly spaced time positions across pulse (requires a repetitive pulse stream) Magnitude Pulse Profile Time domain Phase Note: there may not be a one-to-one correlation between data points and the actual number of pulses that occur during the measurement

  9. Pulsed IF IF gate Anti-alias filter ADC Digital FIR IF filter Pulsed RF RF gate Anti-alias filter ADC Digital FIR IF filter Pulsed IF Data samples Defining the Acquisition Window acquisition window Point-in-Pulse Narrowband detection Broadband detection t Narrowband detection uses hardware switches (gates) in RF or IF path to define acquisition window Broadband detection uses sampling period to define acquisition window

  10. NA Demo: Point-in-Pulse, Pulse Profile

  11. Agenda • Pulsed-RF Overview • Pulsed-RF measurement techniques • Wideband/synchronous • Narrowband/asynchronous

  12. Pulsed-RF Network Analysis Terminology Measured S-parameters Pulse width (PW) Carrier frequency (fc) Time domain Pulse repetition period (PRP)Pulse repetition interval (PRI) Pulse repetition frequency (PRF = 1/PRI) Duty cycle = on time/(on+off time) = PW/PRI 1/PW Frequency domain fc

  13. Pulsed S-parameter Measurement Modes • Wideband/synchronous acquisition • Majority of pulse energy is contained within receiver bandwidth • Incoming pulses and analyzer sampling are synchronous(requires a pulse trigger) • Pulse is “on” for duration of data acquisition • No loss in dynamic range for small duty cycles (long PRI's), but there is a lower limit to pulse width Receiver BW Frequency domain Pulse trigger Time domain

  14. Pulsed S-parameter Measurement Modes • Narrowband/asynchronous acquisition • Extract central spectral component only; measurement appears CW • Data acquisition is not synchronized with incoming pulses (pulse trigger not required) • Sometimes called “high PRF” since normally, PRF >> IF bandwidth • “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 D/R degradation = 20*log[duty cycle] IF filter Frequency domain

  15. Wideband detection Narrowband detection Duty Cycle Effect on Pulsed Dynamic Range 100 Narrowband Detection Mixer Radar 80 Wideband Detection Wireless Dynamic Range (dB) 60 Isotherm. 40 Narrowband Detection Sampler 20 The system dynamic range of the microwave fundamental mixing is much better than samplers, helping to overcome the limitations of narrowband detection 0 Duty Cycle (%) 1.0 0.1 0.01 100 10.0

  16. Agenda • Pulsed-RF Overview • Pulsed-RF measurement techniques • Wideband/synchronous • Narrowband/asynchronous

  17. risetime (1/) = 300 ns fall time = 300 ns Pulsed I/Q Pulsed I/Q I I t t Q Q 0o 20 MHz Pulsed Signal Baseband pulsed I/Q 90o Analog Pulse Measurement Technique(Wideband Mode) I(t) Q(t) Pulse profile achieved by increasing delay of sample point A/D converter Fast sample/hold 20 MHz IF Sample delay Broadband, analog synchronous detector (BW  1.5 MHz) Pulse trigger

  18. 1 2 3 4 5 Digital Wideband Detection – Point-in-Pulse • Set delay of PNA sampling (relative to RF modulation) to establish desired position within pulse (controlled by pulse generator outputs) • Width of acquisition window is determined by IF bandwidth 20 us settling time Pulsed IF PNA Samples t Point-in-pulse delay Modulation trigger PNA sample trigger

  19. Agenda • Pulsed-RF Overview • Pulsed-RF measurement techniques • Wideband/synchronous • Narrowband/asynchronous

  20. Pulsed RF Spectrum of Measurement Example PRF = 1.7 kHz Pulse width = 7 us Duty cycle = 1.2% First null = 1/PW = 1/ (7 us) = 143 kHz

  21. Pulsed RF Spectrum (Zoomed In) Desired frequency component 3 dB bandwidth Practical filters First spectral sideband at 1.7 kHz ( = PRF) Ideal filter Higher-selectivity (smaller shape factor) filter

  22. NA’s IF Filters • Selectivity of the NA’s digital IF filters is not very high • They are optimized for speed Frequency nulls exist at regular spacing (determined by M) lin mag log mag Apparent filter selectivity

  23. Filtered Output Using Spectral Nulling X Output Pulsed spectrum Digital filter (with nulls aligned with PRF) • With “custom” filters, number of filter sections (M) can be chosen to align filter nulls with pulsed spectral components • With spectral nulling, reject unwanted spectral components with much higher IF bandwidths compared to using standard IF filters • Result: faster measurement speeds!

  24. Wanted frequency component Filtered frequency components Zoomed in View of Spectral Nulling Response of 500 Hz Digital IF Filter and 1.7 kHz Pulsed Spectrum 0 -20 -40 -60 -80 Response (dB) -100 -120 -140 -160 -180 -200 0 2000 3000 5000 4000 1000 -3000 -5000 -2000 -4000 -1000 Frequency Offset (Hz) • Nulling occurs at every 3rd null in this case (BW = 29% of PRF) • A narrower IF bandwidth would skip more nulls • Trade off dynamic range and speed by varying IF BW

  25. Delta Bandwidth Comparison IF bandwidth = 984 Hz sweep = 0.5 s IF bandwidth = 95 Hz sweep = 3.3 s Δnoise = 10*log[984/95] = 10.2 dB

  26. Elimination of Additional Interfering Signals • Spectral nulling eliminates main pulse spectrum plus other undesired signals • Sources of spectral contamination: • 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 Main spectral components Aliased spectral components Video feedthrough freq DC

  27. Duty Cycle Effects with Narrowband Detection (DUT = HPF) Pulse width = 3 ms (DC = 5.1%) Pulse width = 1 ms (DC = 1.7%) Pulse width = 100 ns (DC = 0.17%) Pulse width = 100 ns Dynamic range improved with averaging (101 avgs) Note: this is frequency domain data, not a pulse profile

  28. Calibrating Your Pulsed-RF System • Calibration is performed under pulsed conditions • Calibration methodology is identical to normal (swept sinusoid) mode • ECal or mechanical standards can be used • In general, each unique set of pulse and gating conditions requires a separate calibration

  29. Summary • Testing with pulsed-RF is very important for radar, EW, and wireless comms systems • Narrowband detection: • Spectral nulling technique improves measurement speed • For radar and wireless comms applications, offers superior dynamic range/speed • No lower limit to pulse widths

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