Measurements, Testing, and Calibration Advanced Measurement Topics Informational

How do I perform pulsed S-parameter measurements on a power amplifier?

Pulsed S-parameter measurements on a power amplifier characterize the amplifier's RF behavior under pulsed operating conditions, where the bias is applied only during short duty-cycle pulses to replicate the actual operating environment (as in radar transmitters) or to prevent self-heating that would alter the device's performance characteristics. The measurement technique involves: configuring the VNA for pulsed mode (the VNA generates and receives RF signals that are synchronized with the bias pulse; the RF stimulus is applied only during the active portion of the pulse, and the VNA receiver gates its measurement window to capture data only during the stable portion of the pulse), synchronizing the pulse timing (the DUT bias pulse, the VNA RF stimulus pulse, and the VNA receiver measurement window must be precisely time-aligned; the bias pulse turns on first (typically 1-10 microseconds before the RF), the RF stimulus is applied during the bias-on interval, and the receiver captures data during the settled portion of the pulse, avoiding the leading and trailing transients), setting the pulse parameters (pulse width: typically 1-100 microseconds, matching the DUT's operational pulse width; duty cycle: typically 0.1-10%; pulse repetition interval: 10-1000 microseconds), and calibrating the pulsed measurement system (the VNA must be calibrated in pulsed mode using the same pulse parameters as the measurement; calibration standards are measured under pulsed conditions to account for the pulsed receiver's different noise floor and dynamic range compared to CW mode).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Pulsed S-Parameter Measurement Techniques

Pulsed S-parameter measurements are essential for characterizing high-power amplifiers that cannot operate continuously without thermal damage, and for capturing the device's true RF performance at the instantaneous junction temperature (before self-heating occurs).

Measurement Modes

Pulsed-RF, pulsed-bias: Both the RF stimulus and the DC bias are pulsed. This is the most realistic measurement for pulsed radar amplifiers. The device operates exactly as it would in the system
Pulsed-RF, CW-bias: The bias is continuous but the RF is pulsed. Used to study the RF transient response while the thermal state is at the CW operating temperature. Less common
Point-in-pulse profiling: The receiver measurement window is moved in time across the pulse to measure the S-parameters at different points within the pulse envelope. This reveals: thermal transients (the gain decreasing during the pulse as the die heats up), charge trapping effects (the gain changing in the first few microseconds due to trap filling in GaN devices), and bias settling effects

Pulsed S-Parameter Parameters

Pulse width: T_pulse = 1-100 microseconds (match operational conditions)
Duty cycle: DC = T_pulse / T_PRI = 0.1-10%
Measurement point: t_meas within settled portion of pulse
VNA IF bandwidth: BW_IF > 1/T_measurement_window for adequate averaging
Dynamic range penalty: ~10 log(1/DC) dB below CW measurement

Common Questions

Frequently Asked Questions

Why not just measure CW S-parameters?

CW measurements heat the device continuously, raising the junction temperature by 50-200 degrees C above the pulsed operating temperature. This temperature change alters: the gain (typically decreases by 0.01-0.03 dB per degree C), the output power (saturated power decreases at higher temperatures), the input match, and the output match. For a GaN PA operating at 100 W: the CW junction temperature may exceed 200 degrees C, while the pulsed junction temperature at 1% duty cycle is only 30-50 degrees C above ambient. The S-parameters are significantly different.

What VNA settings are critical?

IF bandwidth: must be wide enough to capture the pulse energy (BW_IF > 1/T_pulse), but wider BW increases noise. Typical: 10-100 kHz. Number of averages: pulsed measurements have higher noise (lower duty cycle = less energy per measurement); average 10-100 sweeps to achieve CW-like noise floor. Trigger: the VNA must trigger on the pulse timing signal (from a pulse generator) to synchronize the measurement. Receiver mode: most modern VNAs (Keysight PNA-X, R&S ZNA) have built-in pulse measurement modes that handle the timing and gating automatically.

What equipment do I need?

A modern VNA with pulsed measurement capability (Keysight PNA-X N5247B, R&S ZNA, or equivalent). A pulse generator (dual-channel: one for bias pulse, one for VNA trigger). A pulsed power supply (for the DUT bias, capable of fast switching with controlled rise time). A pre-amplifier/driver if the DUT requires high input drive under pulsed conditions. Calibration standards (the same standards used for CW, but measured under pulsed conditions).

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