Saturday, July 28, 2007

Physics of a Radar System

If you ever wanted to know how a military radar transmission works, there's a pretty good overview that you can find in this Tektronix Application Note on pages 2-6 It talks about what a radar pulse looks like and why you might modulate the pulse in order to get better performance.

They skipped the most basic type of radar system but Mattel has utilized it to make a really neat and cheap radar gun.

You can use RADAR (technically it's an acronym so it should be capitalized) to tell a number of things about object it's aimed at. The Mattel radar gun gives you the most basic information, speed.  It transmits a microwave signal and observes the reflections (if any) that come back from the object.  If the object is moving toward or away from the gun, the reflected signal will have a Doppler shift.  The amount of frequency shift is proportional to the speed. 

Now if you remember your music theory, when you mix two waves of slightly different frequencies together, you get a signal with both frequencies as well as a "beat". The frequency of the beat is the frequency difference between the signals. 

Taking advantage of this, the Mattel gun combines the signal it sends and the signal it receives to get the resultant signal, filters out the microwave signals and is left with the beat signal.  Get the frequency of the beat signal and it's some simple math to get the velocity of the object.

All this for $30 retail.

Now, notice that the gun doesn't give you position. Radar systems figure out how far the target is away by measuring how long it takes for the signal to reach the target and come back. To do this, they send a pulse and wait for the return and then time the return.  The Mattel gun sends a continuous signal rather than a pulse so it has no way of measuring time of flight.  That processing would probably also cost a bit more to implement.

Why did I even bother to read this article?  Well, I'm seeing some of our customers testing radar systems by simulating return pulses with hardware such as the R-series plug in boards or our Vector Signal Generators and wanted to know a little more about what they were really doing.

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