Re: Characteristics of traffic radar

On Feb 10, 5:09 pm, "Paul Hovnanian P.E." <p...@xxxxxxxxxxxxx> wrote:
"Michael A. Terrell" wrote:

Fred Bloggs wrote:

Michael A. Terrell wrote:
Fred Bloggs wrote:

Jan Panteltje wrote:

On a sunny day (Sat, 10 Feb 2007 14:24:44 GMT) it happened Fred Bloggs
<nos...@xxxxxxxxxx> wrote in <45CDD5A9.5050...@xxxxxxxxxx>:

Paul Hovnanian P.E. wrote:

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.

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.

No it is not, reflected RF will mix with the local osc and produce
a difference in the audio range.

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

I have designed doppler but not for car speed measurements.
A tuning fork? use a freq counter and oscillator.

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.

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

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.

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.


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

.

Relevant Pages

  • Re: Characteristics of traffic radar
    ... passband in the audio region. ... Most traffic radar operates with Doppler ... in the receiver to create the audio signal in simple police RADAR guns. ...
    (sci.electronics.design)
  • Re: Characteristics of traffic radar
    ... between the transmitted carrier and the Doppler shifted reflection in ... Most traffic radar operates with Doppler ... in the receiver to create the audio signal in simple police RADAR guns. ...
    (sci.electronics.design)
  • Re: Pioneer 10 test of light speed
    ... >>trajectory that predicts accurately the received radiation. ... the Doppler shift of the known transmission frequency. ... This signal is immediately returned by the spacecraft's ranging ... the position of the spacecraft and in fact Doppler corrected receiver ...
    (sci.astro)
  • Re: Pioneer 10 test of light speed
    ... >>trajectory that predicts accurately the received radiation. ... the Doppler shift of the known transmission frequency. ... This signal is immediately returned by the spacecraft's ranging ... the position of the spacecraft and in fact Doppler corrected receiver ...
    (sci.astro)
  • Re: Characteristics of traffic radar
    ... between the transmitted carrier and the Doppler shifted reflection in ... the receiver diode. ... Most traffic radar operates with Doppler ... it would seem that a tuning fork would be useless. ...
    (sci.electronics.design)
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