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[DPRG] [Fwd: Re: Radar Gun]
Subject: [DPRG] [Fwd: Re: Radar Gun]
From: Ed Okerson
ed at okerson.com
Date: Sat Dec 30 22:03:01 CST 2006
Forgot to hit reply-all.
Ed,
> I finally got around to breadboarding a circuit for the TX/RX module
> of the toy radar gun. I ran the signal through a simple
> common-emitter amplifer (2N2222) and then to a LM339 comparator. I
> added some hysteresis, and got nice clean edges.
Sounds like a good start.
> The only problem is that I can't seem to find a good compromise for
> the amount of hysteresis. I can't seem to make it sensitive enough
> without having it be _too_ sensitive.
>
> Should I amplify the signal more before going into the comparator? I'm
> sort of afraid of adding too much more noise. Right now I'm looking
> at a gain of about 100 v/v.
>
> Should I look at some sort of way to measure the average signal
> strength and adjust the hysteresis accordingly? I can think of a few
> different ways to go, but I thought I'd ask the group since this seems
> like a pretty common sort of problem.
This is similar to what the Polaroid sonar modules do, after the ping is
sent the amplifier starts out very low gain, but ramps up with time until
the return is registered, or a timeout occurs.
> I've also through about doing everything in software on a
> microcontroller. The frequency of interest is quite low, so I could
> probably do some DSP on something like an Atmel.
Are you trying to get a speed reading from the Doppler signal, or trying
to find a way to get distance? If you want distance, you should probably
be pulse modulating the signal. At the short distances this is capable of
sending, time of flight is going to be very short. You would probably
have better luck measuring phase difference in a modulated signal than the
raw 10.525 GHz carrier. Pulse it with a psuedo random code so you can
distiguish which receive pulses go with which transmitted pulses.
> Also, it seems like you can get rough distance information from the
> amplitude of the output from the sensor.
How rough? Another approach might be to start transmitting at very low
power and ramp up until you get a return. The target rf reflectivity will
have quite a bit of influence over the power required to get a return
signal.
Ed Okerson
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