Vector network analyzers are traditionally used to measure the continuous-wave (CW) S-parameter performance of components. Often, under these operating conditions, the analyzer is functioning as a narrowband measurement instrument. It transmits a known CW frequency to the component and measures the CW frequency response. If we were to look at the response of a single CW frequency, we would see a single spectral tone in the frequency domain. The analyzer has a built-in source and receivers that are designed to operate together in a synchronous manner, utilizing narrowband detection, to measure the frequency response of the component. Most analyzers can be configured to generate a frequency sweep over many frequency tones.

In some cases, the signal applied to the component must be pulsed (turned on and off) at a specific rate and duration. If we were to look at the frequency-domain response of a single pulsed tone, it would contain an infinite number of spectral tones making it challenging to utilize a standard narrowband VNA. This article describes how to configure and make accurate pulsed S-parameter measurements using a PNA vector network analyzer from Agilent Technologies (Santa Rosa, CA).

To see what the frequency-domain spectrum of a pulsed signal looks like, we first mathematically analyze the time-domain response. Equation 1 illustrates the time-domain relationship of a pulsed signal. This is generated by first creating a rectangular windowed version [rect(t)] of the signal with pulse width PW. A shah function is then realized consisting of a periodic train of impulses spaced 1/PRF apart where PRF is the pulse-repetition frequency. This can also be viewed as impulses at spacing equal to the pulse period. The windowed version of the signal is then convolved with the shah function to generate a periodic pulse train in time corresponding to the pulsed signal:

To look at this signal in the frequency domain, a Fourier transform is performed on the pulsed signal y(t):

Equation 2 shows that the frequency-domain spectrum of the pulsed signal is a sampled sinc function with sample points (signal present) equal to the pulse-repetition frequency (PRF).

The left-hand side of Fig. 1 shows what the pulsed spectrum would look like for a signal that has a PRF of 1.69 kHz and a pulse width of 7 µs. The right-hand side of Fig. 1 shows the same pulsed spectrum with a zoomed-in view of the pulsed fundamental frequency. The spectrum has components that are nPRF away from the fundamental, where n is the harmonic number. The fundamental tone contains the measurement information. The PRF tones are artifacts of pulsing the fundamental tone, with relatively large magnitudes for those spectral components close to the fundamental tone.

The PNA vector network analyzer operates by means of narrowband detection of microwave energy. It downconverts a received signal to an intermediate frequency (IF) that is then digitized (sampled at discrete intervals) and digitally filtered for display and analysis. There are two different methods for measuring the S-parameters of a pulsed signal with a microwave PNA: "synchronic pulse acquisition" and "spectral nulling." Synchronic pulse acquisition is analogous to the "full pulse characterization" mode of operation on an 8510 vector network analyzer. Spectral nulling is analogous to the "High PRF" mode of operation in the 8510 series except that point-in-pulse and pulse-profiling can be performed whereas they could not on the 8510 in "High PRF" mode.

The synchronic-pulse-acquisition method provides synchronic timing between the individual incoming pulses and the analyzers discrete sampling. If the pulse width exceeds the minimum time to synchronize and acquire one or more discrete data points then the measurement falls into the synchronic pulse-acquisition mode of operation (Fig. 2) and the receiver performs at its full CW sensitivity and dynamic range with no pulse desensitization. Pulse-to-pulse characterization can be measured in this mode with each displayed data point corresponding to one individual pulse. This measurement is configured by aligning the incoming pulses with the sampling intervals of the analyzer using trigger-on-point mode and applying an external trigger to measure each pulse. The analyzer must see 100 µs of pulsed signal before the acquisition period (less than the recommended 100 µs will result in reduced measurement performance). This accounts for PNA hardware filter settling. There is a 70-µs delay between the applied trigger and when the analyzer begins digitization of one discrete point. Therefore, a 30-µs delay should be applied between the incoming pulse and applied trigger to account for the 100 µs of pre-acquisition pulsed RF. The minimum acquisition time on the analyzer is roughly proportional to inverse of the intermediate-frequency (1/IF) bandwidth. As the IF bandwidth is decreased, the measurement acquisition time for each data point increases. The minimum acquisition time on the analyzer is 30 µs for an IF bandwidth setting of 35 kHz. This corresponds to a minimum measurable pulse width of 130 µs.

The synchronic mode of operation requires a pulse generator to supply the timing width and delays for the external triggering and the modulation. Modulation can be supplied by modulating the device-under-test (DUT) bias (Fig. 3) or modulating the source signal. A standard microwave PNA has both a trigger-in and trigger-out (ready for trigger) BNC connector that may be used to synchronize the trigger timing of the analyzer and pulse generator. In point mode, applying a trigger-in signal will cause the analyzer to acquire data for the first frequency point, move the source frequency to the next point, and then send a trigger-out signal to notify the pulse generator that it is ready to acquire the next data point. At this point, the pulse generator may send a trigger to the analyzer to acquire the next data point.