Testing Radar Transmitter Amplitude and Timing Stability using the Agilent 8990B Peak Power Analyzer Application Note Introduction During the design, manufacturing, or even maintenance of a radar transmitter, testing and validating the transmitter s RF output stability is one of the key performance parameters. For a pulsed radar transmitter (see Figure 1), the transmitter must have high RF stability in order to meet the required signal processing requirements for its class or type of the radar operation. Pulse-topulse stability is one of the most important characteristics of a pulsed radar transmitter. It is usually characterized by pulse-to-pulse phase and amplitude variation between successive pulses of a burst or pulse train. The instability of a transmitter can be caused by things such as fluctuating or unwanted oscillation of sub-components in the transmitter system, saturation of the amplifier, or a mismatching issue at the different stages of the transmitter module. RF IN Pulse OUT Waveform generator Pulse power amplifier Modulator Power supply Figure 1. Simplified pulsed radar transmitter with RF pulse output This application note explains how the Agilent Technologies 8990B peak power analyzer (PPA) analyzes the pulse-to-pulse amplitude and timing stability using Multipulse mode, which is an exclusive feature of the 8990B. Using the multipulse function, the transmitted RF pulses are analyzed individually, measuring scalar pulse profiles such as peak power, top, overshot, width, repetition interval (PRI), rise time, and fall time. These pulse shape and timing measurements can be compared to check for the pulse-to-pulse stability. Instability is detected when comparisons show a droop across the pulse train, abnormal pulses, or even missing pulses as shown in Figure 2.
Introduction Radar pulse transmitter Pulse OUT GOOD Radar pulse transmitter Pulse OUT Pulse DROOP Radar pulse transmitter Pulse OUT ABNORMAL pulse Figure 2. Checking transmitter output pulse stability 8990B peak power analyzer multipulse feature The multipulse feature of the 8990B allows users to view, measure, and analyze continuous pulse trains from the transmitter or other power amplifier modules. It allows users to continually trigger and capture up to 512 frames or pulses by using a concept similar to the concept of segmented memory used in most digital oscilloscopes. In order for the 8990B to analyze all the pulses intended, each pulse must fit into each of the frames. Figure 3 illustrates the multipulse frame trigger and acquisition operation inside the 8990B. Once the scale adjustment and trigger settings are done, the 8990B acquires frames or pulses using a sampling rate of 100 MSa/s and stores each successful trigger into a designated frame. After the completion of the acquisition and storage, the frames can be recalled or played back one-by-one. Pulse characteristic measurements such as power, rise/fall time, top, and width can be done on each of the frames or pulses. Optimize the pulse(s) into the frame window by adjusting the timescale, trigger settings and trigger delay. Frame 1 Frame 2 Frame 3 Frame n Figure 3. The 8990B s multipulse frame acquisition Maximum 512 frames. There will be a time stamp on each trigger point on each frame. 2
To illustrate how this multipulse feature helps to test transmitter stability this section provides examples of real use cases. Testing long-pulse repetitive interval (PRI) pulses In certain radar operations such as the search and tracking radar system, the transmitter emits narrow pulses with very long PRIs, or off-time in between pulses. The PRI duration can be as long as several seconds. Conventionally, a digital oscilloscope (DSO) with a segmented memory feature is connected to a diode detector to convert the RF pulses to analog signal. This configuration is used to capture and analyze these pulses because the DSO is able to trigger and capture on the on-time pulses and discard the off-time pulses. As mentioned earlier, the multipulse function in the 8990B operates in a similar manner, thus enabling it to only capture the useful information when the pulses are emitted from the transmitter. The pulses can then be analyzed separately and the results retrieved from the 8990B PPA if desired. The test setup for capturing long PRI pluses using the 8990B PPA is shown in Figure 4. The goal is to capture and analyze 100 continuous long PRI pulses simulated from a signal generator. This is a 10 second total acquisition. The Agilent Technologies N5182B MXG RF vector signal generator emits a 10 µs pulse width with a 100 msec PRI pulse, which is equivalent to a 0.01% duty cycle pulse to the 8990B PPA. The amplitude is set at 0 dbm on the MXG. Agilent MXG Agilent 8990B PPA (with Multipulse Option) PW = 10 µsec PRI = 100 msec N1923A Power Sensor Figure 4. Long PRI pulse test set up The procedure for setting up the 8990B PPA for long PRI pulses using multipulse is as follows: 1. Set Trigger Source on Channel 1 to Sweep Triggered mode. Set the Timebase to 10 µs/div. Set the Vertical Scale to 5 db/div. A nice fitted pulse will be displayed on the PPA screen (refer to Figure 5a.) 3
Testing long-pulse repetitive interval (PRI) pulses (continued) Turn on Multipulse mode by going to the Acquisition menu and select Multipulse. The multipulse feature requires a license key to operate. However a 30-day trial license is automatically activated if no valid license is found. In the Multipulse Setup box (Figure 5b), enter 100 frames and turn on the Time Stamp. The Time Stamp will help you identify the sequence of the pulses under test. Select Acquire and the multipulse acquisition will start immediately. During the acquisition, there will be an incremental running number on the Available Frame column. 2. Once the acquisition is completed (see Figure 5c), select Playback to view the 100 pulses that have been captured and stored. This is feature is particularly useful when checking for irregularities or abnormal pulses. 3. Select Peak and Pulse Top on the left measurement panel. The pulse-to-pulse comparison panel and histogram distribution of the peak and pulse top measurement will be shown on the bottom of the 8990B s display (see Figure 5d). A comparison of pulse measurement results can be made between any two pulses from the captured frames. For example, a pulse top comparison of the last frame or pulse, to the first pulse provides the droop result. The histogram distribution charts provide a statistical view of the pulse measurements made. The results of the complete 14-point pulse measurements can be retrieved by saving it into a.csv file. From the Acquisition menu, choose Save Measurement and select Multipulse. The measurements can be viewed using Microsoft Excel. a. 10 µs pulse c. Multipulse acquisition completed b. Multipulse setup Figure 5. Long PRI pulses acquisition d. Pulse-to-pulse comparison and histogram 4
Measuring droop of a radar pulse burst or pulse train Sometimes the radar transmitter emits a series of pulse trains or bursts instead of a single pulse over a certain time duration. In this situation, the transmitter must sustain the amplitude of all the pulses inside the burst. Only a small amplitude drop or droop across the pulses in the burst is acceptable. In the droop test requirement, typically the amplitude of the last pulse is compared to the first pulse of the burst, as shown in Figure 6, although any two pulses can be compared. Besides identifying droop, multipulse can be used to detect any missing pulses by turning on the time stamp. It can also detect any abnormal pulses by playing back all the pulses captured in the allocated multipulse frames. N number of pulses inside the burst 1 st 2 nd 3 rd N 1 N Amplitude droop from the first pulse to the last pulse of the burst Figure 6. Droop of a pulse train/burst The test instrument setup is similar to the one shown in Figure 4. The MXG is set to emit a burst with 20 pulses inside it. The amplitude for the pulses have been reduced gradually from 0 dbm for the first pulse to 0.45 dbm for the last pulse. The pulse width of the pulse is also gradually increased. The procedure for setting up the 8990B PPA to measure droop over a pulse burst using multipulse is as follows: 1. Increase the 8990B timebase to have an overview of the burst as shown in Figure 7a and b. To ensure the trigger is stable, change the Trigger Holdoff to 300 µs. This will also ensure that when the multipulse is turned on, the first pulse will be captured and stored in the first frame, and the subsequent pulses will be stored in the subsequent frames. 2. Next, turn on Multipulse, enter 100 into the Frame column and turn on the Time Stamp. Once the multipulse acquisition is completed, turn on the pulse width, pulse top, and peak power measurements from the Measurement tab on the left. 3. Playback Frame 1 (Pulse 1) through Frame 20 (Pulse 20) one-by-one as shown in Figures 7c and d. Frame 20 is the first pulse of the second burst, consequently the Frame 41 is the first pulse of the third burst, and so on. Notice that the pulse measurements at the bottom change with the frame sequence. The pulse width, pulse top, and peak power represent the current pulse measurement. The time stamp is indicated on the triggering edge of each pulse. This time stamp is also useful when checking if there are any missing pulses inside the burst. 5
Measuring droop of a radar pulse burst or pulse train (continued) a. Overview of the pulse burst b. Zoom into the pulse burst (20 pulses) c. Multipulse capture: 1 st pulse d. Multipulse capture: 20 th pulse Figure 7. Multiples on pulse burst 6
Measuring droop of a radar pulse burst or pulse train (continued) 4. To measure the droop over the burst, enlarge the 8990B s Measurement panel. Go to the Pulse Top measurement as shown in Figure 8 and select Pulse 1 to Pulse 20. This will show the difference between the first pulse top and the pulse top of burst 20. The result shown in Figure 8 is 0.46 db, which is expected. Using the same method, the pulse width difference between pulses also can be obtained. Burst droop Pulse width difference Figure 8. Droop measurement in the burst Conclusion The multipulse feature of the 8990B simplifies the testing of the RF pulse stability of a radar transmitter or other forms of power amplifier modules. Since the 8990B s Multipulse mode is available on both of the RF channels, it provides the flexibility to simultaneously test two separate transmitter outputs. The RF power and timing measurements also provide best-in-class accuracy. It has the added benefit of minimizing test setup when used as an alternative to using both a scope and power meter to test transmitter stability. 7
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