November 14, 2013
In today’s radar world, there is an increased call for multi-use/function/mode adaptive systems that can be used in different applications. Whether you’re tracking high-speed targets or detecting slow, low, and small objects, better pulse measurement tools are needed.
Existing pulse test methodologies require tradeoffs to be made based on required duty cycle, pulse width, dynamic range and more. The question I pose is – if your radar system is being asked to do more without sacrificing performance, shouldn’t your test solution do the same?
Perhaps the best way to answer that question is to review the pros and cons of the most common measurement methods – band-limited, triggered, and “historic” wideband. I’ll also introduce a new method now available due to the design of the VectorStar MS4640B Vector Network Analyzers (VNAs).
Sometimes referred to as narrowband measurements, this method is based on the spectrum of a pulsed RF signal and capitalizes on the fact that the central spectral line carries the magnitude and phase information of the underlying RF signal, fc. After filtering everything but this frequency, only a CW signal remains and standard VNA processing may be applied.
Pros – One advantage of this technique is that narrow pulse widths can be measured. Also, the band-limited method only involves one frequency and is an asynchronous measurement, which means it does not require synchronization with the incoming pulse.
Cons – A major drawback is that the dynamic range becomes a function of the duty cycle. As the duty cycle becomes smaller, there is less power in the RF carrier signal, which affects the method’s dynamic range. Another consideration is the on-off ratios of the receive-side modulators, which increase measurement uncertainty, as shown in figure 1.
The triggered measurement method is mainly for slower repetition rate situations. The VNA measurement is triggered to make classical measurements on sequential pulse (or sync pulse) rising edges of the radar signal.
Pros - This method avoids the duty cycle dependence of the band-limited method and can allow for simpler profiling.
Cons - The main challenge is controlling the delay between the trigger event and actual measurement. If the delay is large, it can limit the usefulness of this method to a minimum pulse width of 10s or 100s of microseconds. VNA calibration is another challenge, as improper calibrations can lead to uncertainty between actual DUT transients versus transients from the test system itself.
Historic Wideband Measurements
I’m using the word “historic” because most VNAs have only offered bandwidth up to 5 MHz, limiting the wideband method to pulse widths of 200 ns or more. The wideband measurement method may be used when most of the RF pulse is within the receiver bandwidth.
Pros – For larger pulse widths, the wideband method has the advantage that there is no loss in dynamic range when the pulses have a low duty cycle.
Cons – The major disadvantage has been the lower limit of measurable pulse widths. Another challenge is that the data acquisition rate of many VNAs is not adequate enough for today’s radar-related measurements.
High-speed Digitizer Method
A new method in VectorStar incorporates a high-speed digitizer and it represents a major technology advancement over prior test methods, especially in radar system design. Similar to the “historic” wideband method, this method is based on direct acquisition - but at a much higher data rate. As a result, resolutions are enhanced and time referencing becomes more accurate.
We will discuss the advantages of this method in much more detail in our next post. If you can’t wait until then, a white paper on the topic is also available.