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Agilent
PNA-X Series Microwave
Network Analyzers
Active-Device Characterization in Pulsed Operation
Using the PNA-X
Application Note
Table of Contents
Introduction ..........................................................................................................................3
Device Types ........................................................................................................................4
Pulsed-RF Measurement Types ................................................................................5
Pulsed-RF Detection Techniques..............................................................................6
Wideband detection ...........................................................................................................7
Narrowband detection .......................................................................................................8
Pulsed-RF S-parameter Measurements Using PNA-X.................................9
PNA-X pulse system ..........................................................................................................9
PNA-X hardware overview ...............................................................................................9
PNA-X IF paths.................................................................................................................10
Internal pulse generators ................................................................................................10
Internal pulse modulators ...............................................................................................11
Pulse I/O ..........................................................................................................................11
Pulse system delays ........................................................................................................12
Setting up measurements using Pulsed-RF measurement application ......................14
PNA-X wideband pulse measurements ......................................................................16
Wideband pulse data acquisition ...................................................................................16
Synchronizing pulsed-RF stimulus and measurements ...............................................19
Wideband pulse system dynamic range ........................................................................23
PNA-X narrowband pulse measurements ..................................................................24
Narrowband filter path with crystal filter ......................................................................24
Software gating ...............................................................................................................26
Digital filter nulling ..........................................................................................................28
Active-Device Measurements with Calibrated Stimulus..........................29
Power leveling modes ......................................................................................................29
Accurate pulsed stimulus using receiver leveling ....................................................30
Pulsed stimulus power calibration .................................................................................30
Receiver leveling with wideband detection...................................................................31
Receiver leveling with narrowband detection...............................................................33
Swept-power measurement examples ........................................................................35
Improving stimulus power level accuracy in pulse profile measurements.........36
Compression vs. Frequency Measurements in Pulse Mode ..................37
Two-tone IMD measurements in Pulse Mode ................................................39
Additional Resources ....................................................................................................42
Application note ................................................................................................................42
Web ......................................................................................................................................42
2
Introduction
Vector network analyzers (VNA) are the common tool for characterizing
RF and microwave components in both continuous-wave (CW) and pulsed
operations. Some external equipment may be used in conjunction with a VNA
to modulate the stimulus or DC bias, and to perform accurate S-parameter
measurements in pulsed operation. However, components that need to be
characterized in pulsed operation mode are often active devices such as
amplifiers or converters, and many active parameters are characterized in
addition to S-parameters. For amplifiers as an example, 1 dB compression (P1
dB), intermodulation distortion (IMD), and third-order intercept point (IP3) are
commonly measured, and many parameters such as noise figure, higher-order
distortion products, harmonics, etc. are characterized depending on their
intended application needs. These active parameters are power-dependent,
so additional factors must be considered for precise characterization.
To respond to such needs, Agilent's PNA-X Series, the most flexible VNA
that employs many capabilities designed for active-device characterization,
enables S-parameter and active parameter measurements with a single
set of connections. The PNA-X's four internal pulse generators and
pulse modulators, two internal sources with a combining network, and
active-application options provide fully integrated pulsed active-device
characterization. This application note discusses pulsed S-parameter
measurements using the PNA-X Series and measurement techniques that
enable power-dependent active-device characterization including compression
and distortion. It also provides a brief summary of pulsed-RF measurement
types, and two detection techniques (wideband and narrowband detection)
are explained specifically using PNA-X architecture and methodologies. Refer
to application note 1408-12 Pulsed-RF S-Parameter Measurements Using
Wideband and Narrowband Detection part number 5989-4839EN for further
details of measurement types and detection techniques.
3
Device Types
Figure 1 shows two types of pulse operation modes, pulsed-RF and pulsed-
bias. Pulsed-RF operation drives the device with a pulse-modulated RF
signal while the DC bias is always on. Amplifiers in receivers used in pulse-
modulated applications are typically tested under pulsed-RF operation. Testing
devices in pulsed-RF operation requires RF pulse modulators for the stimulus
as well as pulse generators to drive the RF modulator and to synchronize or
gate the VNA receivers to capture the modulated RF signals. The pulsed-bias
operation is when the DC bias is switched on and off to generate a pulse-
modulated signal while the input is mostly a CW signal and is always on.
Traveling-wave-tube (TWT) amplifiers are one example of this type and are
commonly used in radar transmitters. The RF pulse modulator is not required
for the stimulus in this mode, but pulse generators are needed to turn on and
off the DC bias and synchronize the VNA receivers to measure the output
signal when the device is on.
Pulsed-RF Pulsed-bias
Input: pulsed Input: CW
Output: pulsed Output: pulsed
DC bias: always on DC bias: pulsed
Figure 1. Pulsed-RF and pulsed-bias operation modes
4
Pulsed-RF Measurement Types
Figure 2 shows three major types of pulsed-RF measurements. The first two
are pulsed S-parameter measurements, where a single data point is acquired
for each carrier frequency. The data is displayed in the frequency domain
with magnitude and/or phase of transmission and/or reflection. Average
pulse measurements make no attempt to position the data point at a specific
position within the pulse. For each carrier frequency, the displayed S-parameter
represents the average value of the pulse. Point-in-pulse measurements result
from acquiring data only during a specified gate width and position (delay)
within the pulse. There are different ways to do this in hardware, depending
on the type of detection used, which will be covered later. Pulse profile
measurements display the magnitude and/or phase of the pulse versus time,
instead of frequency. The data is acquired at uniformly spaced time positions
across the pulse while the carrier frequency is fixed at some desired frequency.
Figure 2. Average, point-in-pulse and pulse profile measurements
5
Pulsed-RF Detection Techniques
Figure 3 shows an important measure of a pulsed RF signal and its relationship
between the time and frequency domain. When a signal is switched on and
off in the time domain (pulsed), the signal's spectrum in the frequency domain
has a sin(x)/x response. The width of the lobes is inversely related to the pulse
width (PW). This means that as the pulses get shorter in duration, the spectral
energy is spread across a wider bandwidth. The spacing between the various
spectral components is equal to the pulse repetition frequency (PRF). If the PRF
is 10 kHz, then the spacing of the spectral components is 10 kHz. In the time
domain, the repetition of pulses is expressed as pulse repetition interval (PRI)
or pulse repetition period (PRP), which are two terms with the same meaning.
Another important measure of a pulsed RF signal is its duty cycle. This is the
amount of time the pulse is on, compared to the period of the pulses. A duty
cycle of 1 (100%) would be a CW signal. A duty cycle of 0.1 (10%) means that
the pulse is on for one-tenth of the overall pulse period. For a fixed pulse width,
increasing the PRF will increase the duty cycle. For a fixed PRF, increasing the
pulse width increases the duty cycle. Duty cycle will become an important
pulse parameter when we look at narrowband detection.
Figure 3. Pulsed-RF network analysis terminologies
6
Wideband detection Wideband detection can be used when the majority of the pulsed-RF spectrum
is within the bandwidth of the receiver. In this case, the pulsed-RF signal will
be demodulated in the instrument, producing baseband pulses. With wideband
detection, the analyzer is synchronized with the pulse stream, and data
acquisition only occurs when the pulse is in the "on" state. This means that a
pulse trigger that is synchronized to the PRF must be present; for this reason,
this technique is also called synchronous acquisition mode. The advantage
of the wideband mode is that there is no loss in dynamic range when the
pulses have a low duty cycle (long time between pulses). The measurement
might take longer, but since the analyzer is always sampling when the pulse
is on, the signal-to-noise ratio is essentially constant versus duty cycle. The
disadvantage of this technique is that there is a lower limit to measurable
pulse widths. As shown in Figure 4, as the pulse width becomes narrower,
the spectral energy spreads out