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Characteristics of traffic radar

  • Thread starter Paul Hovnanian P.E.
  • Start date
P

Paul Hovnanian P.E.

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.
 
F

Fred Bloggs

Paul said:
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.
 
J

Jan Panteltje

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

Fred Bloggs

Jan said:
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?
 
J

Jan Panteltje

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

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 :)
 
M

Michael A. Terrell

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


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
 
F

Fred Bloggs

Michael said:
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.
 
L

linnix

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 :)

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.
 
J

Jan Panteltje

If they use acoustic transducer, it would be a cannon rather than a
gun.

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.


Modern radar gun use RF in GHz. The Doppler shift is in KHz range.

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

Michael A. Terrell

Fred said:
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.


--
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
 
P

Paul Hovnanian P.E.

Michael A. Terrell said:
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.
 
L

linnix

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?
 
J

Jan Panteltje

Remember 100kHz at 330 m /s gives a wavelength of:
330 / 100 000 = 3.3 cm

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.
 
F

Fred Bloggs

Paul said:
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.

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
 
L

linnix

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 :)

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.
 
J

Jan Panteltje

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.

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.
 
L

linnix

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.


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.
 
J

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.

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.
 
L

linnix

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.

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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