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18 November 2015

Analyzing RADAR Signals with Demodulation

An example of a radar signal with 1-GHz RF carrier
Figure 1: An example of a radar
signal with 1-GHz RF carrier
In the electronic warfare milieu, one of the most common RF applications is that of radar systems. Radar, which uses RF energy to determine the range, angle, and/or velocity of objects, can be used for detection of aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain, among other things. An oscilloscope's demodulation math function is very helpful in analysis of radar signals, so let's look at a couple of examples of how to approach such measurements.

First, we'll consider a radar pulse signal with an RF carrier frequency of 1 GHz (Figure 1). In this example, several math operators are in use. One is the demodulation of the input RF signal (math trace F1), which produces an envelope of the amplitude modulation on the input. Another math operator is demodulation applied to the zoom trace, seen as the pink math trace (F2) superimposed over zoom trace Z1. Trace Z1 shows a single RF pulse from the input waveform. Finally, at bottom of Figure 1 is a fast-Fourier transform (FFT) of the zoom trace (F3 in blue), showing an isolated view of the frequency content of this pulse.

This Barker-coded radar signal uses phase modulation rather than amplitude modulation
Figure 2: This Barker-coded radar
signal uses phase modulation rather
than amplitude modulation
Across the bottom of Figure 1 below the display grids is an array of measurements taken on the demodulated pulse, including the carrier and modulation frequencies, width, duty cycle, rise/fall times, overshoot, and amplitude data. This is representative of the bountiful information that can be extracted from an RF signal once it's been demodulated.

Another example of a radar signal is a Barker-coded, phase-modulated waveform (Figure 2). It is similar in shape to the previous example of an AM radar signal, but in this case, we use the demodulation math operator to derive the envelope of a phase-modulated signal. The demodulated envelope clearly shows the Barker encoding. At bottom, the blue F3 FFT trace displays the signal's frequency spectrum.

A final example is of a radar signal with linear frequency-modulation chirp (Figure 3). The demodulated RF envelope (trace F1) reveals the chirp in the signal, indicating that frequency is being modulated throughout the pulse. The frequency rises in a linear ramp. In this case, the demodulation is from the frequency perspective to extract the modulation shape. The frequency demodulation (trace F2 in pink) shows frequency as a function of time, while the FFT (trace F3 in blue) shows frequency vs. magnitude.

Hopefully, we've shown the usefulness of your oscilloscope's demodulation math function with regard to examination of radar signals. We will delve into other facets of electronic warfare-related RF measurements in future posts.

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