Tuesday, January 11, 2011

A Low Cost Way to Capture Agile PRFs and PWs in Radar Test

In military radar applications the pulse repetition frequency (PRF) and sometimes pulse width (PW) parameters of a radar's pulsed RF/microwave signal may vary during operation. There is typically two reasons why PRF and PW may be varied:

  1. PRF and PW are tied to a measured target's range and resolution as well as the max range of the radar. A sophisticated radar system that can change modes from searching to tracking or from one search range to another must change its PRF and PW to match the radar's current mode. This is often referred to as mode changes.
  2. Military radars often employ PRF and PW that are constantly changing to help prevent an enemy from "spooking" the radar (creating a false target). These radar's employ complex highly secret algorithms for constantly varying the PRF and PW. This is often referred to as an agile pulse.
In this post I wanted to cover cover a fairly low cost solution for continuously capturing these parameters for verifying an agile PRF/PW algorithm, analyzing pulse noise from one mode to the next, or verifying tolerances from one pulse signal change to another. The solution is made up of a simple RF power detector and one or two gap-free sampling universal counters like the 53230A. The power detector is used to strip off the RF carrier and just output the pulse signal. The gap-free sampling counter or counters are then used to make the needed continuous timing measurements on the pulse signal. Why gap-free sampling counter?
  1. Counters provide high accuracy and high resolution timing measurements.
  2. Gap-free means no pulses will be skipped so you can get a complete and continuous picture of a group of pulses.
  3. Since counters are just making timing measurements with the edge event of a signal, you can capture a large amount of continuous pulse data with less memory and simpler post processing compared to an instrument that is digitizing the entire pulse signal. For instance the 53230A can store up to 1 million readings. There is not many scopes out there that can store 1 million digitized pulses in memory.
  4. Finally counters are low cost compared to RF / high speed digitizing instruments
As I mentioned above you may want to use one or two counters. You only need one if you are just interested in capturing one of the pulse parameters at once. The one counter based solution can be seen below.
The counter in the above figure is set for timestamped measurements which returns the time from positive signal edge to positive signal edge or negative signal edge to negative signal edge. Using the counter for timestamped measurements in this application returns the pulse repetition interval (PRF) from pulse to pulse. The PRF can them be obtained by inverting each reading. Below is an example measurement of an agile PRF signal using the setup above.
The below setup uses two counters to capture both PRF and PW simultaneously. One counter is used in timestamp mode for capturing the PRI and the other is making PW measurements. 
In the above setup, after the pulse parameter measurements are made on the desired set of pulses some simple software can be used to post process and combine the timing measurement data to give you a PRF, PRI, PW, and the duty cycle of each pulse in the set. 
Depending on the gap-free sampling counter you use, it may not measure the first couple of leading pulses from the set. This is because the counter may need to set the right input conditioning and edge leveling before the timing measurements can start. Also if you are using two different measurement modes in the counter, such as timestamps and PW, each mode may skip a different amount of leading pulses which you will need to know when using two counters in parallel to properly align your measurement. Finally, you want to be sure you use a high quality RF power detector for this type of test. Using a low quality detector can lead to carrier noise on the output pulse signal, which can really lower your measurement accuracy. 


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