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 < 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

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

VNA data display

Frequency domain

Frequency domain

Time domain Pulse Profile

Data acquired at uniformly spaced time positions across pulse (requires a repetitive pulse stream) Magnitude

Phase

data point

Note: there may not be a one-to-one correlation between data points and the actual number of pulses that occur during the measurement

CWdB

deg

Swept carrier

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tData samples

Pulsed IF

Narrowband detection uses hardware switches (gates) in RF or IF path to define acquisition window

Broadband detection uses sampling period to define acquisition window

Point-in-Pulse

acquisition window

Narrowband detection

Broadband detection

Anti-alias filter

ADCIF gate Digital FIR IF filter

Pulsed IF

Anti-alias filter

ADCRF gate

Digital FIR IF filter

Pulsed RF

Defining the 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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fc

Pulse repetition frequency(PRF = 1/PRI)

1/PW

Time domain

Pulse width (PW)

Pulse repetition period (PRP)Pulse repetition interval (PRI)

Carrier frequency (fc)

Measured S-parameters

Pulsed-RF Network Analysis Terminology

Frequency domain

Duty cycle = on time/(on+off time)

= PW/PRI

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Pulsed S-parameter Measurement ModesWideband/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 widthReceiver BW

Pulse triggerTime domain

Frequency domain

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Pulsed S-parameter Measurement ModesNarrowband/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

IF filter

Time domain

Frequency domain

D/R degradation = 20*log[duty cycle]

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Duty Cycle Effect on Pulsed Dynamic Range

Wideband detection

Narrowband detection

Dyn

am

ic R

ang

e (d

B)

Duty Cycle (%)

100 10.0 1.0 0.1

100

80

60

40

20

0

WidebandDetection

Narrowband Detection Mixer

Narrowband Detection Sampler

WirelessRadar

Isotherm.

The system dynamic range of the microwave fundamental mixing is much better than samplers, helping to overcome the limitations of narrowband detection

0.01

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Agenda

• Pulsed-RF Overview• Pulsed-RF measurement techniques

• Wideband/synchronous • Narrowband/asynchronous

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I

Q

Pulsed I/Q

t

Broadband, analog synchronous detector

(BW 1.5 MHz)

Pulsed Signal Baseband pulsed I/QA/D

converter

I(t)

Q(t)

I

Q

Pulsed I/Q

t

Analog Pulse Measurement Technique(Wideband Mode)

risetime (1/) = 300 ns

fall time = 300 ns

20 MHz IF

Pulse trigger

Fast sample/hold

Pulse profile achieved by increasing delay of sample point

Sample delay

0o

90o20 MHz

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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

1 2 3 4 5

Pulsed IF

PNA Samples

t

Modulation trigger PNA sample trigger

Point-in-pulse delay

20 us settling time

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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

First null = 1/PW = 1/ (7 us) = 143 kHz

PRF = 1.7 kHzPulse width = 7 usDuty cycle = 1.2%

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Pulsed RF Spectrum (Zoomed In)

First spectral sideband at 1.7 kHz ( = PRF)

Ideal filter

Desired frequency component

Practical filters

3 dB bandwidth

Higher-selectivity (smaller shape factor) filter

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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)

log mag

lin mag

Apparent filter selectivity

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-3 -2 -1 0 1 2 3

x 104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Filtered Output Using Spectral Nulling

Pulsed spectrum Output

X

-3 -2 -1 0 1 2 3

x 104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

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

Frequency Offset (Hz)

Response of 500 Hz Digital IF Filter and 1.7 kHz Pulsed Spectrum

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

-500

0

-400

0

-300

0

-200

0

-100

0 0

1000

2000

3000

4000

5000

Re

sp

on

se

(d

B)

Wanted frequency component

Filtered frequency components

• 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 Hzsweep = 0.5 s

IF bandwidth = 95 Hzsweep = 3.3 s

Δnoise = 10*log[984/95] = 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

DCfreq

Aliased spectral componentsVideo

feedthrough

Main spectral components

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Duty Cycle Effects with Narrowband Detection (DUT = HPF)

Pulse width = 3 s (DC = 5.1%)

Pulse width = 1 s (DC = 1.7%)

Pulse width = 100 ns (DC = 0.17%)

Pulse width = 100 nsDynamic 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