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Keysight Technologies
PNA-X Series Microwave
Network Analyzers
Active-Device Characterization in
Pulsed Operation Using the PNA-X
Application Note
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, Keysight Technologies, Inc.'s PNA-X Series, the most
flexible VNA that employs many capabilities designed for active-device characteriza-
tion, 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 measure-
ment 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.
Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
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
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 appli-
cations 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 receiv-
ers 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) ampli-
fiers 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 repre-
sents 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 repeti-
tion 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 acquisi-
tion 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--once enough of the energy is outside the bandwidth of the receiver, the
instrument cannot detect the pulses properly.
Figure 4. Pulse width and receiver bandwidth with wideband detection in time and frequency domain
7
Narrowband detection Narrowband detection is used when most of the pulsed-RF spectrum is outside
the bandwidth of the receiver. In other words, the pulse width is narrower
than the minimum data acquisition period with the widest available receiver
bandwidth. With this technique, the entire pulse spectrum is removed by filtering
except the central frequency component, which represents the frequency of the
RF carrier. After filtering, the pulsed-RF signal appears as a sinusoid or CW signal.
With narrowband detection, the analyzer samples are not synchronized with the
incoming pulses (therefore no pulse trigger is required), so the technique is also
called asynchronous acquisition mode. Usually, the PRF is high compared to the
IF bandwidth of the receiver, so the technique is also sometimes called the
"high PRF" mode.
Keysight has developed a novel way of achieving narrowband detection using
wider IF bandwidths than normal, by using a unique "spectral nulling" and
"software gating" techniques that will be explained later. These techniques let
the user trade dynamic range for speed, with the result almost always yielding
faster measurements than those obtained by conventional filtering.
The advantage to narrowband detection is that there is no lower pulse-width
limit, regardless of how broad the pulse spectrum is, most of it is filtered away,
leaving only the central spectral component. The disadvantage to narrowband
detection is that measurement dynamic range is a function of duty cycle. As
the duty cycle of the pulses becomes smaller (longer time between pulses),
the power of the central spectral component becomes smaller, resulting in less
signal-to-noise ratio as shown in Figure 5. Using this method, measurement
dynamic range decreases as duty cycle decreases. This phenomenon is often
called "pulse desensitization" and can be expressed as 20*log (duty cycle) in dB.
The PNA-X employs a number of unique features to minimize this effect, resulting
in considerably less degradation in dynamic range. More details about these
features will be discussed later.
Figure 5. Duty cycle (time domain) versus signal-to-noise ratio of center spectrum (frequency domain)
8
Pulsed-RF S-parameter Measurements Using PNA-X
PNA-X pulse system This section discusses pulsed-RF S-parameter measurements using the PNA-X
with wideband detection and narrowband detection techniques.
PNA-X hardware overview
Figure 6 shows the PNA-X block diagram with two test ports, two internal
sources, source/receiver attenuators, internal combining network, and rear
access loops with mechanical path switches. Each source has two outputs;
Output 1 and 2 of Source 1 are routed to test port 1 and test port 2 and used
for basic S-parameter measurements. Output 1 and 2 of Source 2 are routed to
two front-panel source output ports on a 2-port configuration, and are routed
to test port 3 and test port 4 on a 4-port configuration (not shown). Output 1
(OUT 1) of each source is filtered to reduce harmonics, and can be switched
to the combining network for two-tone IMD measurements. Both outputs of
each source can be routed through the rear access loop for additional signal
conditioning or switched to the thru path to the test port. All source paths and
test-receiver paths have optional step attenuators to lower the source power for
high gain devices or to reduce the signal level to avoid receiver compression.
The source attenuators are sometimes used to improve the source match for
active measurements. All of these features make the PNA-X the most flexible
network analyzer, enabling accurate S-parameters and active-device measure-
ments with the best possible configuration. The PNA-X also integrates internal
pulse generators, and a pulse modulator on OUT 1 of each internal source.
Pulsed-RF measurements with pulsed stimulus in the forward direction only
requires one internal source with a pulse modulator. Adding the second internal
source and a pulse modulator enables bi-directional pulsed-RF measurements.
In this configuration, pulsed stimulus from the second internal source is routed
from J8 through J1 connectors of the rear panel to the port 2.
Figure 6. 13.5/26.5 GHz PNA-X block diagram with options 200, 219, 224, 020, 021, 022 and 025
9
PNA-X IF paths
Figure 7 shows the PNA-X receiver/IF path block diagram with internal pulse
generators and modulators. The narrowband filter path employs a crystal filter
with 30 kHz bandwidth centered at 10.7 MHz for better signal selectivity, and
also adds receiver gating capability for narrowband pulse measurements. Earlier
PNA-X models had 60 MSa/s analog-to-digital converters (ADC) with 60 MHz
system clock (DSP version 4), later versions have been upgraded to 100 MSa/s
ADC with 100 MHz system clock (DSP version 5).
Figure 7. PNA-X IF path block diagram
Internal pulse generators
The PNA-X with internal pulse generators (Option 025) adds four pulse genera-
tors (P1, P2, P3, and P4) and an additional pulse generator (P0) that is used for
synchronizing the data acquisition. These internal pulse generators can be trig-
gered internally or externally--all at the same time. They can have an individual
delay up to 35 s, and pulse width up to 10 s. When triggered internally, the pulse
generators output a continuous pulse train with specified periods up to 35 s.
The minimum timing resolution of all pulse generators is based on the clock,
16.7 ns with DSP version 4 and 10 ns with DSP version 5. P1 through P4 pulse
generators are used to drive internal pulse modulators, IF gates and/or external
devices, such as pulsed-bias switches. P0 pulse generator is used to trigger
the DSP for data acquisition. When P0 outputs a pulse, the DSP starts data
acquisition and continues until it completes the required number of samples per
specified IF bandwidth. If P0 outputs the next pulse before the DSP completes
the data acquisition, the P0 pulse is ignored. Approximate data acquisition time
is 1/(IF bandwidth).
10
Internal pulse modulators
The PNA-X with the internal pulse modulator options adds a pulse modulator
to the "OUT 1" of each internal source (Option 021 for Source 1 and Option 022
for Source 2). Both modulators are driven by a common pulse, which can be
selected from P1 through P4 internal pulse generators, or external pulse inputs
with minimum pulse width of system clock timing resolution.
Pulse I/O
The pulse input/output (I/O) port on the PNA-X rear panel adds accessibility
to internal pulse generators and modulators, and enables synchronous pulse
measurements with external pulse generators or a device under test (DUT). The
N1966A pulse I/O adapter converts the 15-pin D-sub connector to SMB connec-
tors for five PULSE IN (inputs for A, B, C/R1, D/R2, and R receiver gates), PULSE
SYNC IN (to trigger internal pulse generators for pulse modulation and data
acquisition), RF PULSE MOD IN (to modulate RF sources), and four PULSE OUT
(P1 through P4). The four PULSE OUT ports are hard-wired to the internal pulse
generators without switches. Figure 8 shows how the pulse I/O connectors map
to the IF block diagram.
Figure 8. Rear panel pulse I/O
11
Pulse system delays
From pulse trigger to internal pulse generators, to pulse modulators, and then to
ADC for data acquisition, there are some delays in the pulse system that need
to be taken into account. Figure 9 shows the timing diagram from the pulse
trigger at PULSE SYNC IN through the data acquisition with wideband detection
technique.
Figure 9. PNA-X pulse system delays with wideband data acquisition
12
Internal pulse generators start generating the pulses approximately 60 to 100 ns
after the pulse trigger inputs at the PULSE SYNC IN--(denoted as pulse trigger
delay). The jitter of this delay is the minimum time resolution of the system.
The exact pulse trigger delay can be measured with an oscilloscope between
PULSE SYNC IN and one of the PULSE OUT (P1 through P4). If the internal clock
is used to trigger the pulse generators, this pulse trigger delay is very small and
therefore it can be neglected. One of the P1 through P4 internal pulse generators
drives the internal pulse modulators to generate pulsed-RF signals. The actual
pulsed-RF signals have a delay of approximately 30 ns from the P1 through P4
modulation pulse at the test port with a 48 inch RF test port cable--(denoted as
pulse modulation delay). The pulse modulation delay with RF carrier frequencies
of 3.2 GHz and below is about 40 ns or larger (nearly 100 ns delay at the low-end
frequency of the analyzer). The combiner path on port 1 adds an additional 5 ns
delay compared to the thru path. The pulse modulators switch the RF sources on
and off with approximately 4 ns rise time and 10 ns fall time.
P0 pulse generator is also triggered by PULSE SYNC IN (or internal trigger) and
generates a data acquisition pulse with the same amount of pulse trigger delay
as other internal pulse generators (or zero if triggered internally). Although
the data acquisition process starts immediately with the P0 pulse, there is
approximately 250 ns data-processing delay, the time it takes for sampled data
with pulse on to become available in the buffer. The user-specified measurement
delay (the delay for P0) must take into account these data-acquisition and pulse-
modulation delays to align the pulsed stimulus and data-acquisition window.
Approximately 300 ns for the measurement delay accounts for pulse-modulation
delay and data-acquisition delay, as well as pulse-settling time. Additional
delay may be necessary depending on the frequency, the PNA-X's internal path
switches, and external cables and devices.
Setting up pulsed-RF measurements
The basis of pulsed-RF point-in-pulse measurements with wideband detection
method is to synchronize pulsed stimulus and data acquisition so that all receiv-
ers measure responses only within the RF pulses. The following three steps
must be completed for successful measurements.
Step 1 Setup pulse generators and modulators
Specify pulse width, delay and period for internal pulse generators with Pulse
Generator Setup dialog (Figure 10a). Note that Pulse0 width is determined by the
IF bandwidth chosen (in step 3) thus it is not editable on the pulse generator dia-
log. When internal pulse modulators are used, specify the drive source (typically
one of internal pulse generators). It is also required to disable automatic level
control (ALC) on the source port with pulse modulation enabled. Change the
leveling mode from "Internal" to "Open loop" on Power and attenuator dialog.
Otherwise the ALC will try to level the source with the detected power level with
pulse on and off, causing a source unleveled error.
13
Step 2 Synchronize data acquisition and pulses
With internal pulse generator option, pulse trigger feature is added to the PNA-X
trigger system (Figure 10b Pulse Trigger tab on Trigger dialog). Specify trigger
source--either internal or external (incoming pulses to PULSE SYNC IN) and
check "Synchronize receiver to pulse generator Pulse0". Note that measurement
trigger (MEAS TRIG IN port on PNA-X rear panel) is not used in typical setup.
Step 3 Adjust data acquisition time
Set IF bandwidth to be wider than at least 1/(pulse width), in order to complete
the data acquisition while pulse is on. For example, 10