Characteristics of traffic radar

I was thinking about radar speed guns the other day (No, I didn't get a ticket). My understanding of these is that they produce an IF frequency between the transmitted carrier and the Doppler shifted reflection in the receiver diode. This is AC coupled(?) to an amplifier with a passband in the audio region. Most traffic radar operates with Doppler shifts in the audio band and, as a result, the signal may be monitored through a speaker in addition to using a frequency counter.

What does the passband of the IF section look like in terms of its lower and upper limits, dB/octave slopes, etc.? The initial AC coupling and positive gain vs freq. would appear to create a bias toward selecting the faster target over the larger one. However, at some point, there must be a high frequency roll-off where the opposite is true.

Also, how would one characterize the rejection of amplitude modulation in the receiver/mixer (if any)? The common technique for 'calibrating' speed radar seems to be to use a tuning fork whose frequency corresponds to some known speed Doppler shift. Unless radar guns are susceptible to AM interference, it would seem that a tuning fork would be useless. Unless one threw it, the average 'speed' of its tines would be zero whereas the peak would depend on the amplitude of its vibration. Using a tuning fork might verify the accuracy of the device's counter, but it would also indicate that could register something other than a Doppler shifted signal.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Paul Hovnanian P.E.
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Why do you ask? Looking for a 'calibrator'?

BTW

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Reply to
Homer J Simpson

You can't be serious! Audio IF? Tuning forks? "positive gain"? "reflection"? "receiver diode"??? Hey!- where the hell is the cat whisker "crystal" rectifier? LOL- throw that TAB Books library of yours into the trash where it belongs. Your post is absolutely demented.

Reply to
Fred Bloggs

On a sunny day (Sat, 10 Feb 2007 14:24:44 GMT) it happened Fred Bloggs wrote in :

No it is not, reflected RF will mix with the local osc and produce a difference in the audio range. I have designed doppler but not for car speed measurements. A tuning fork? use a freq counter and oscillator.

Reply to
Jan Panteltje

Huh? Why even use an LO if it's that close to the carrier?

Reply to
Fred Bloggs

On a sunny day (Sat, 10 Feb 2007 15:13:08 GMT) it happened Fred Bloggs wrote in :

Say you transmit at 100 kHz with one acoustic transducer, and receive with a second transducer the reflected 100kHz. If the object moves away or towards you, you can mix the reflected with the transmit frequency, and the difference represents the speed. If the object's distance does not change you get DC :-)

Reply to
Jan Panteltje

The LO is used for both transmit, and mixed with the received signal in the receiver to create the audio signal in simple police RADAR guns.

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Reply to
Michael A. Terrell

The *return" may be offset from the xmit by an audio frequency offset, but not the LO, was my point.

Reply to
Fred Bloggs

If they use acoustic transducer, it would be a cannon rather than a gun. Modern radar gun use RF in GHz. The Doppler shift is in KHz range.

Reply to
linnix

On a sunny day (10 Feb 2007 11:00:44 -0800) it happened "linnix" wrote in :

Not sure what you mean, size right, and in air? Remember 100kHz at 330 m /s gives a wavelength of: 330 / 100 000 = 3.3 cm And 10 GHz electromagnetic at 300 000 000 000 m/s is a wavelength of 3 cm. So the wavelength are about the same. Therefore for example a dish for 10GHz will work just as well for 100kHz sound.

Same for audio :-) And I have done that.

Reply to
Jan Panteltje

So, Fred, how are you going to recover that Audio signal without the original L.O. signal? The simplest and most reliable is to use the one oscillatior for both functions. I've seen it in use from the '60s with tube equipment, and a 1N23 series microwave diode for the mixer.

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Reply to
Michael A. Terrell

Traffic radar (in its simplest form) is a CW microwave source feeding a

3 port circulator. The outgoing CW signal is directed out the receive/transmit horn antenna. The reflected wave (Doppler shifted) returns via the horn and is directed via the circulator to a cavity and receiver/mixer diode. A small amount of the CW source is fed into the receiver cavity (the circulator's leakage may be sufficient).

For a 24 GHz CW source (the local oscillator), the Doppler shift is approximately 35 Hz per mile per hour. So a vehicle traveling at 30 MPH will produce a 1.05 kHz IF signal at the mixer.

So much for the basics review. This much, even I know about Doppler radar and I rarely fiddle with anything more than 60 Hz. I think this went over a few people's (Fred's) head. So now I'm wondering if I'm going to get answers to my more involved questions.

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Paul Hovnanian P.E.

a

frequency

lower

corresponds

to

Using a

So, just sample the IF at 10 Khz and FFT it. What's so difficult about the filter?

Reply to
linnix

On a sunny day (Sat, 10 Feb 2007 19:42:51 GMT) it happened Jan Panteltje wrote in :

Actually that is not correct, Xcuse my math, I am but a neural net ;-),

330 / 100 000 = 3.3 mm

So with a that small wavelength, doing it acoustic will allow a smaller horn or dish. But not very usable for traffic, a 50 km/h wind would screw up things badly :-)

Great for detecting motion (what I used it for) say alarm systems,. But very sensitive, will detect a fly in a room.

Reply to
Jan Panteltje

Your questions were way to open-ended and no one feels like giving you a treatise on the subject. Go look here:

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Reply to
Fred Bloggs

That's what I meant. You need a huge cannon (lower freq, higher power) to track a moving car. We gave up on this idea decades ago.

Reply to
linnix

On a sunny day (11 Feb 2007 08:52:08 -0800) it happened "linnix" wrote in :

:-)

No, higher power and or lower frequency, will not help for wind speed (say medium moving) I think? There is also air pressure to keep into account.

Reply to
Jan Panteltje

:-)

Yes, RF is the way to go. 1.5GHz can track moving satellites as well as moving cars. If you build you receiver right, perhaps you can track the reflective gps signal off the moving car.

Reply to
linnix

On a sunny day (11 Feb 2007 09:53:19 -0800) it happened "linnix" wrote in :

I should not write this, because somebody may make it a requirement,and start implementing it, but if you gave each car say a precise 1GHz transmitter, and modulated it with an ID (say license plate number or chassis number), all you would need is sit next to the road and register any ID that appears in the forbidden 1GHz + max speed band.... Could be fully automatic receivers at every road. Would pay for itself too.

Reply to
Jan Panteltje

I would do it differently. We don't need to transmit at 1GHz for every car. Every car can install a $5 GPS receiver and transmit their ID and location with less than 100 MHz.

Reply to
linnix

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