Pulsed-RF%20S-Parameter%20Measurements%20Using%20a%20VNA

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• Pulsed-RF S-Parameter Measurements Using a VNA

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Agenda

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

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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

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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

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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

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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 lt 1
• Often requires synchronization of pulsed bias
  and pulsed RF stimulus

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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

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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
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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
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NA Demo Point-in-Pulse, Pulse Profile
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Agenda

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

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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/(onoff time) PW/PRI
1/PW
Frequency domain
fc
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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
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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
  gtgt 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 20logduty cycle
IF filter
Frequency domain
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Duty Cycle Effect on Pulsed Dynamic Range
100
Narrowband Detection Mixer
Radar
80
Wideband Detection
Wireless
Dynamic Range (dB)
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Isotherm.
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Narrowband Detection Sampler
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The system dynamic range of the microwave
fundamental mixing is much better than samplers,
helping to overcome the limitations of narrowband
detection
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Duty Cycle ()
1.0
0.1
0.01
100
10.0
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Agenda

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

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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
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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
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Agenda

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

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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
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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
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NAs IF Filters

• Selectivity of the NAs 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
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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!

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Zoomed in View of Spectral Nulling
Response of 500 Hz Digital IF Filter and 1.7 kHz
Pulsed Spectrum
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-20
-40
-60
-80
Response (dB)
-100
-120
-140
-160
-180
-200
0
1000
2000
3000
4000
5000
-5000
-4000
-3000
-2000
-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

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Delta Bandwidth Comparison
IF bandwidth 984 Hz sweep 0.5 s
IF bandwidth 95 Hz sweep 3.3 s
?noise 10log984/95 10.2 dB
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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
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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
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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

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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