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Characteristics of traffic radar

Discussion in 'Electronic Design' started by Paul Hovnanian P.E., Feb 10, 2007.

  1. 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.
     
  2. Fred Bloggs

    Fred Bloggs Guest

    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.
     
  3. 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.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Huh? Why even use an LO if it's that close to the carrier?
     
  5. 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 :)
     

  6. 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.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  7. Fred Bloggs

    Fred Bloggs Guest

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

    linnix Guest

    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.
     
  9. 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.
     

  10. 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.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  11. 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.
     
  12. linnix

    linnix Guest

    So, just sample the IF at 10 Khz and FFT it.
    What's so difficult about the filter?
     
  13. 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.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    Your questions were way to open-ended and no one feels like giving you a
    treatise on the subject. Go look here:
    http://www.copradar.com/preview/content.html
     
  15. linnix

    linnix Guest

    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.
     
  16. 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.
     
  17. linnix

    linnix Guest


    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.
     
  18. 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.
     
  19. linnix

    linnix Guest

    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.
     
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