December 23, 2013
In our last post, we discussed various test methods using a vector network analyzer (VNA) when measuring complex radar signals. One of those approaches – using a high-speed digitizer – is new to radar testing circles and can bring some real benefits to engineers who must have confidence in their cutting-edge radar designs. Because it has only been available for a few months, many people are probably wondering exactly what it is.
The high-speed digitizer measurement method represents a major technology advancement over all prior test methods. While similar to the conventional wideband method, this technique is based on direct acquisition - but at a much higher data rate than was previously available. For example, the Anritsu VectorStar™ MS4640B VNA (figure 1) with PulseView™ offers a digital IF acquisition system with more than 200 MHz of bandwidth. As a result, resolutions are enhanced and time referencing becomes much more accurate.
VNA Architecture
Before discussing the advantages of the method, it’s important to understand how it works, specifically the acquisition system. IF signals are generated by the down converters in the VNA. When the VNA is equipped with the appropriate hardware and software, the standard IF system is bypassed and signals are routed to a special high-speed digitizing IF board. This board consists of analog processing, with a much wider bandwidth than the standard IF system, so it can measure much narrower pulses. This board also houses fast analog-to-digital converters, pulse generators, and digital processing components. Deep memory of 4 GB is necessary to store the data coming in from the converters. As a result, the VNA can acquire long time records of more than 0.5 seconds with full resolution.
Acquired data is keyed with T0 sync information. For the pulse measurement processing, the data of interest is selected (relative to T0) and run through the appropriate conversions to the frequency domain. S-parameters are created and the appropriate calibrations applied. Since the processing is relative to these time markers and not any of the pulses per se, there is considerable flexibility on where in time an engineer can look at the results. This becomes particularly valuable in complex pulse situations, where many sub-pulses are present or when multiple external generators are being used.
Design Advantages
Now that you know how it works, we can explain why it’s a big deal. By using a high-speed digitizer and performing the alignment with pulse data in a post-processing sense, engineers avoid the triggering latency issues associated with triggered measurements and potential jitter/inconsistency problems with that triggering. The resolution is set mainly by the acquisition rate instead. For example, the VectorStar MS4640B VNA with PulseView offers a 400 MS/s data acquisition rate, the fastest available in a VNA. As a result, the time resolution is on the order of 2.5 ns.
Another benefit is that since no energy is discarded in the high-speed digitizer pulse measurement method, there is no duty-cycle dependence. Even for full correction, no receive-side modulators are required and the on-off ratio, bandwidth, rise-time and video limitations of those structures are no longer an issue.
As you can see, the high-speed digitizer method offers a higher level of resolution and timing accuracy when measuring radar designs. It eliminates trade-offs between dynamic range and duty cycle, and ensures that engineers don’t miss behaviors masked by lower-resolution alternatives.
You can download a white paper that details the advantages of the high-speed digitizer method, which has recently been published. It can help in your next radar design.
