Toggle Menu
Application Notes
The radar series
This application note is the fourth in a series that delves into radar systems and the associated measurement challenges and solutions. Across the series, our goal is to provide a mix of timeless fundamentals and emerging ideas.
As derived in Part 1 of this series, the radar range equation captures the essential variables that define the maximum distance at which a given radar system can detect objects of interest. Because those variables relate directly to the major sections of the system block diagram, they provide a powerful framework for the essential process of understanding, characterizing, and verifying the actual performance of any radar system.
Parts 2 and 3 of the series defined the pulsed radar signal, described ways to measure the power in those signals and presented readily available ways to measure the essential characteristics of pulsed signals: frequency, timing, power, and spectrum.
In our ongoing discussion of practical test methods, the next step is to examine the use of vector signal analyzers (VSAs) and wideband instruments—signal analyzers and oscilloscopes—to measure the frequency and phase in today’s increasingly complex radar signals. Compared to traditional approaches that combine multiple instruments, these integrated tools provide enhanced ease of use that makes it easier to produce accurate, repeatable measurement results.
Table of contents
VSA: Understanding the Inner Workings
Unlike a spectrum analyzer, a vector signal analyzer captures the magnitude and phase information within a measured signal. As a result, a vector signal analyzer can perform advanced analysis including demodulation of highly complex signals. It can also display measurements and results in the time, frequency, and modulation domains. A short diversion to explain the inner workings of these analyzers will make this clearer and also provide hints that will ensure useful results when measuring pulsed radar signals.
In simple terms, a vector signal analyzer calculates a frequency spectrum by sampling the incoming signal, storing a finite-sized block of the sampled time-domain data (i.e., the time record), and performing an FFT on the data block. If the selected combination of center frequency and span moves the start frequency away from zero (or above the analyzer’s minimum frequency), then the analyzer uses mixing to translate the incoming signal down to an intermediate frequency (IF) section where the band-limited signal is sampled by an analog-to-digital converter (ADC). The results are stored as a time record that is used to calculate results such as an FFT spectrum.
Resolution in the frequency domain is proportional to the size of the time record. For example, using a 2,048-point time record and applying the relevant Nyquist factor typically provides 800 to 1,000 lines of resolution in the frequency spectrum. In most implementations, the size of the time record is a power of two and other common points/lines pairs range from 64/30 to 16,384/8,000.
To ensure accurate results, the analyzer must calculate the frequency spectra from a representative time-domain sample of the waveform. If the time record contains only a fraction of the signal relative to its period, then the displayed level of the spectrum may reflect neither the correct spectral characteristics nor the true power level of the signal. This can be resolved by either increasing the FFT measurement time to include several pulse periods or setting the RBW to satisfy the line-spectrum criterion (e.g., 0.3 x PRF).
Using time-gated analysis
In a vector signal analyzer, time-gated analysis computes the FFT using only the samples collected during a specific time interval. Similar to an oscilloscope, a vector signal analyzer can perform single-shot time-gated measurements. With the 89600 VSA software, the process is as easy as dragging and dropping the gate onto the time-domain display of the waveform. The time-gated analysis makes it possible to examine the spectral characteristics of an individual pulse, a portion of a pulse, or a transient.
Viewing dynamic behavior
A display called the spectrogram simplifies the visual recognition of major signal characteristics by plotting frequency and amplitude versus time. Each one-pixel line of the spectrogram is an overhead view of a single spectrum measurement with amplitude values encoded in color. Depending on the vertical time scale, a spectrogram can contain hundreds or even thousands of spectrum measurements.
Measuring and analyzing captured data
The 89600 VSA software has a record/playback capability that enables gap-free measurements on acquired data. A signal recorded (i.e., captured) into memory can be played back and analyzed at the same center frequency and span as the acquisition or because all data is available, at different center frequencies and spans within the originally captured span without capturing new data.
Performing wideband signal analysis
Until recently, there were very few commercial, off-the-shelf (COTS) solutions for wideband measurements at high frequencies. The typical approach has been to insert a block downconverter into the signal path in front of a digitizer, which could be a signal analyzer, oscilloscope or modular instrument (e.g., PXIe-based VSA). Today, the growing availability of microwave-capable oscilloscopes and signal analyzers with wider analysis bandwidths has opened the door to accurate, repeatable measurements of wideband radar systems in a single instrument.
×
Please have a salesperson contact me.
*Indicates required field
Thank you.
A sales representative will contact you soon.
