Anritsu HFE0303 Vondran

50 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
Techniques for Pulsed S-Parameter Measurements
By David Vondran Anritsu Company
M
devices, are not designed to operate con­tinuously or with CW sig­nals. This is especially true when devices are being tested on-wafer, where the thermal resis­tance is greatly increased
[1]. In these cases, S-parameter measure­ments are best performed in a pulsed test environment.
The details of pulsed measurement are greatly dependent on the pulse properties being studied. At one extreme is the realm of high pulse repetition frequencies (PRFs) and fairly narrow pulses, as is common in radar applications. At the other extreme is the com­munications arena where PRFs are quite low and pulse widths fairly wide (e.g., GSM [2]).
These two extremes exemplify two tech­niques, termed bandwidth limited and trig- gered, that are discussed in this note. To a cer­tain degree, the two approaches overlap in terms of allowed parameters, so most situa­tions can be covered by one if not both of them.
The objective of this article is to provide an understanding of general S-parameter mea­surements performed with a vector network analyzer (VNA), over a range of pulsed condi­tions for both RF and microwave/mm-wave measurement applications. Pulse profiling of the detailed transient response is not covered here although it is briefly discussed elsewhere with regard to triggered measurements [3]. Certain rise/fall time behaviors can be studied using time domain mode [4] but that will not be discussed either.
The Spectra of Pulsed RF Signals
As the reader may be aware, a pulsed RF signal will have a spectrum composed of a series of spectral lines with an envelope described by a sinc function [5]. The spacing of the lines is set by the PRF while the envelope shape is fixed by the pulse width (assuming rise and fall times are small relative to the width). The relationship is shown in Figure 1. The ‘size’ of this spectrum (occupied frequency range) with respect to the IF bandwidth (IFBW) of the network analyzer determines the measurement mode.
In the case of low PRF and wide pulses, the entire spectrum can fit within an IFBW. In this case, the measurement proceeds normally without a significant reduction in dynamic range. Some additional smoothing/averaging may be needed to reduce effects of the outlying
Pulsed measurement is an
essential tool for measuring
the performance of power
amplifiers under low duty
cycle conditions, including
on-wafer test applications
and high peak-to-average
modulation formats
1/T0
RF
First null at offset = 1/T1
Figure 1. The spectrum of a pulsed RF signal is shown here. The center maximum is at the RF frequency, the line spacing is equal to one over the pulse period T0, and the first envelope null offset at one over the pulse width T1 (neglecting rise and fall times).
From March 2003 High Frequency Electronics
Copyright © 2003, Summit Technical Media, LLC
52 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
portions of the distribution in Figure
1. A requirement in this measure­ment is that the VNA measurement be aligned in time with the pulse, hence the term triggered.
In the case of a high PRF, the line spacing can be substantial relative to the IFBW so the analyzer can just pick off the center line (thus the term bandwidth limited). The measure­ment of just this line is sufficient to perform an S-parameter measure­ment—since it carries the magnitude and phase of the envelope at the cen-
ter point—as long as a calibration is performed under those same condi­tions. However, since only a frac­tion of the total sig­nal energy is used, the dynamic range may be limited.
Triggered Measurements
Since the spec­trum fits entirely in an IFBW in this case, the depen­dence of the mea­surement on the
pulse train would appear simple. In the time domain sense, however, one wants the sampling to occur during the ‘on’ period of the pulse in order to capture the desired information. This is accomplished by triggering the VNA to measure in the appropriate points in time.
The details of this process (and its application to other measurement types) are covered in greater detail in [3] but will be summarized here.
As shown in Figure 2, the idea is for the trigger pulse to arrive at the
VNA sometime before the RF pulse in order to account for instrument latency although starting later is allowed. The sampling can begin sometime after the RF pulse has set­tled unless that process is of interest as well. The sampling can continue for a substantial portion of the pulse but should not continue beyond the end. As a gross limit, the IFBW must be greater than 1/T1 (T1=pulse width) to keep sampling from over­running the pulse (30 kHz with aver­aging is preferred in the MS462xx family; see the Appendix). Because of pulse settling, internal filtering and some other latency issues, some safe­ty margin is required. This will vary greatly depending on setup and may require some experimentation. If too small an IFBW is used (or with too much averaging), the trace data will become very noisy.
The pulse for the RF or the DUT control often comes from a pulse gen­erator. The external trigger pulse for the VNA can come from another channel of the same generator or it can be derived from the main pulse chain with a small delay circuit of the user’s design. An example setup is shown in Figure 3. Some power level details for the various systems are
Figure 2 · An illustration of the timing in a triggered measurement: the VNA is triggered sometime near the start of the pulse so that data is sampled within the duration of the pulse.
Figure 3 · An example setup for a triggered mea­surement is shown here. A dual channel pulse gen­erator is used to create the trigger pulse for the VNA as well as the RF pulse for the DUT. In other tests, a control line to the DUT may be pulsed instead of the RF itself.
Scorpion (MS462xx) and related systems
CW mode: trigger rates up to 900 Hz NB swept: up to 900 Hz WB swept: up to 200 Hz Low frequency instruments may allow higher rates. Fast CW (available from GPIB in firmware versions 1.16 and later) allows higher rates.
Lightning (37xxx) and related systems
CW mode: up to 500 Hz NB swept: up to 150 Hz WB swept: up to 50 Hz for a 40 GHz instrument (less for 65 GHz and Panorama systems) Fast CW mode (available from GPIB) allows higher rates
NB refers to a narrowband sweep that does not include bandswitch points. Consult the factory for more details. Trigger width must generally be at least 50 µs and is a TTL level signal.
Table 1 · Triggering limits for two Anritsu Company VNA instrument families.
Margin for
pulse settling
Safety margin before end of burst
RF Burst
Time
Ext. trigger pulse Data sampling
Pulse
Generator
VNA
Switch
A B
Ext. trigger in
1 2
DUT
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