Burst signals are now commonly used in commercial communications systems, such as Global System for Mobile Communications (GSM) cellular networks. Such signals are essentially pulsed RF waveforms characterized by long pulse widths and long periods (typically hundreds of microseconds to a few milliseconds long). Many components, particularly power amplifiers (PAs), must be tested with such signals to evaluate them under actual operating conditions. Critical measurements include tests of S-parameters, harmonics, and intermodulation distortion (IMD), which can be challenging under burst-signal conditions. Fortunately, a vector network analyzer (VNA) can be a useful tool for rapidly extracting this information since it has fast-moving receivers (Rxs) and, assuming it has an appropriate per point analog triggering function, can make all of the measurements listed above among others. Some guidelines for performing burst measurements with a VNA follow.

A time-division-multiplexed communications system will normally have a pulsed RF waveform associated with it. The waveform for a single GSM channel (e.g., [1]) is shown in Fig. 1 and some other systems have parameters within an order of magnitude of those shown. Relative to pulsed radar systems, a measurement challenge from earlier years, the pulse widths for GSM and similar systems are large, duty cycles are fairly large and periods are long.

Many PAs used in such communications systems are heavily optimized for efficiency and spectral purity for a given output power in this pulsed format. As such, many do not operate correctly, if at all, with a CW waveform that would typically be used for a measurement. In addition, the amplifier (or other DUT) may exhibit unusual behavior at the start of the pulse or some other time subset. It may also be of interest to measure the DUT during a certain subinterval of the pulse (termed 'profiling').

Some PAs, particularly in handsets, can operate CW but they may be put in an inactive state (via a control voltage) during the off-periods of the cycle. How the amplifier responds, in a transient sense, at the beginning and end of the pulse period (being turned active and inactive) are very important and represent another example of profiling.

S-parameter and harmonic measurements are of somewhat obvious interest. Power sweeps of gain and output power, subsets of the S-parameter measurement class, are often of greatest interest for PAs. In terms of executing the burst measurement, however, there is little difference from the general case. Relative to the control-voltage profiling example, the power and frequency are usually constant; it is a pure transient response of the S-parameter or other quantity that is of interest.

Among other PA measurements is intermodulation distortion [IMD] (e.g., refs. 2 and 3) although its related cousin, adjacent-channel power rejection (ACPR) (e.g., refs. 4 and 5) is increasingly important. Both of these linearity measurements have the characteristic of some strong applied signal with a resulting distortion signal at a different, yet close, frequency. For a highly linear DUT, this is then a high-dynamic-range measurement with spot power levels below 70 dBc sometimes of interest.

One approach to doing this measurement would be with a spectrum analyzer and just consider the main lobe of the response generated by the pulse train. For IMD measurements with small separations, this may become impractical. There are also limitations in flexibility and amplitude accuracy, even with triggered measurements (similar to that discussed in the next section), with such a setup. For S-parameter measurements, the spectrum-analyzer approach cannot offer error correction, which further impairs accuracy. Previous techniques involving CW filtering of the main lobe of the pulse spectrum⁶ appear impractical due to the duty cycles and pulse widths involved in most relevant communications systems. This leaves direct data sampling during the pulse-on period as a potentially accurate and flexible approach.

In the case of examining transient amplifier behavior during a control pulse, direct data sampling is perhaps the only choice. Any kind of Rx can potentially make this measurement if it is scalar (e.g., gain) and the Rx can take samples fast enough. If error correction is required (S-parameters) or IMD measurements are needed, then a network-analyzer approach with direct data sampling is a valid option.

The measurement of a single signal using the direct-data-sampling technique is illustrated in Fig. 2 for both the (a) RF burst scenario and the (b) CW transient measurement. A trigger pulse is used to trigger the measurement and it must be synchronized with the RF burst (often by using a dual-pulse generator). The trigger pulse orders the VNA or vector-network measurement system [VNMS] (assuming the instrument has been set to accept external triggering) to perform the measurement and the instrument's analog-to-digital converters (ADCs) begin converting after a certain latency delay. The pulses must be arranged so that the burst is settled by the time the latency ends. For the setups to be discussed here, the pulse settling is very short (< 1 µs) but the DUT behavior early in the pulse may be of interest. The sampling must not continue on too long or it will overrun the burst and the resulting composite data will probably be corrupt. Since the measurement time is constrained by the GSM pulse definition (assuming no more than one burst is needed for a single tone measurement), reducing the averaging will not speed overall measurement throughput.

Figure 2 shows the measurement of one tone (as would be appropriate for an S-parameter or harmonic measurement) while a complete IMD measurement typically requires the measurement of four tones (one for each of the two main tones, one for the lower product and one for the upper product). While conceivably this could be done within one burst, there may not be enough time to get enough samples for the dynamic range required of the measurement. Most modern Rxs use some form of digital intermediate-frequency (IF) filtering so narrower IF bandwidths (and hence more dynamic range) require more samples. Thus the measurement configuration of Fig. 3 will be used where one tone is measured per burst. The picture shown would then repeat for the next frequency or power point (by this we mean both main tones move to new frequencies or new powers assuming both tones are sweeping³).

For a control-response-transient measurement (CW and constant power, a control voltage to the DUT is synchronized with the pulse-timing pattern), it is simpler to just continuously sample data. Each set of N samples (dependent on IFBW) represents one time point allowing a time-trace to be taken. This allows maximum time resolution of the DUT's response. In the case of IMD measurements, it is perhaps most accurate to measure one tone on each subsequent control pulse.