Radar Gun Fundamentals

[[email protected]]

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

1. The doppler shift from a vehicle moving a few feet per second is
only a few tens or hundreds of Hertz.

2. Typical path loss would be 50dB on the forward path and 50dB on the
return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

 

[Jim Yanik]

[email protected] wrote in

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

1. The doppler shift from a vehicle moving a few feet per second is
only a few tens or hundreds of Hertz.

25MPH translated (by my math) to 36.7 FPS,45MPH to 66 FPS,65 MPH to 94 FPS.
A uP freq counter circuit could count those easily.
ISTR that some radar guns have an audio output so the officer can listen to
detect intentional interference(jamming).

2. Typical path loss would be 50dB on the forward path and 50dB on the
return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

I believe they use the Gunn diode oscillator as the detector,too.(TX/RX)
They read the freq. difference off the bias voltage,I believe.

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

Maybe they ignore the phase noise? I dunno.

 

[martin griffith]

On 24 Oct 2006 09:55:41 -0700, in sci.electronics.design

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

1. The doppler shift from a vehicle moving a few feet per second is
only a few tens or hundreds of Hertz.

2. Typical path loss would be 50dB on the forward path and 50dB on the
return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

http://www.newscientisttech.com/article.ns?id=dn10356&feedId=tech_rss20

or dont drive as fast


martin

 

[Jamie]

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

1. The doppler shift from a vehicle moving a few feet per second is
only a few tens or hundreds of Hertz.

2. Typical path loss would be 50dB on the forward path and 50dB on the
return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

i watched a video not to long ago, they were testing different sport
cars to see if excessive speed could cause the radar gun to fail.
I don't remember which car it was that was able to do it how ever,
the speed was around 140+ MPH if memory serves? the radar gun didn't
register anything going by it.
so put it to the metal and go like hell.

 

[Joel Kolstad]

[email protected] wrote:
I don't remember which car it was that was able to do it how ever,
the speed was around 140+ MPH if memory serves? the radar gun didn't
register anything going by it.
so put it to the metal and go like hell.

I once heard -- what's probably an urban legend -- that in some state the
penalties for speeding were tabulated, e.g., 0-4MPH, $xx, 5-10MPH, $yy, etc.,
but that it stopped at something like 50MPH over so technically it you went,
e.g., 120MPH you weren't breaking any laws.



Of course, in reality, even if that part could be shown in court, you'd still
be ticketed for reckless driving!

 

[GregS]

I once heard -- what's probably an urban legend -- that in some state the
penalties for speeding were tabulated, e.g., 0-4MPH, $xx, 5-10MPH, $yy, etc.,
but that it stopped at something like 50MPH over so technically it you went,
e.g., 120MPH you weren't breaking any laws.



Of course, in reality, even if that part could be shown in court, you'd still
be ticketed for reckless driving!

i might have said it here before, i once got a ticket for going over
70 in Nevada, when the limit was 55. The fine was $15.

greg

 

[Ancient_Hacker]

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

It's been done since at least 1948, with lousy klystrons and 1N21 diode
mixers.

All you do is take reasonable precautions to keep the transmitting horn
from leaking in to the receiving horn. If you can get 60db of
isolation that's plenty. The phase noise isnt very significant as
you're mixing with the same signal, so the noise mostly cancels out.

IIRC the inventor of Radar, Watson-Watt, was stopped for speeding by a
radar gun once. Hoisted by your own petard, as they say.

 

[[email protected]]

on the 10 and 24 ghz ones, the detector is in the waveguide with a iris
between it and the gunn oscillator, so its a superhetrodyne. Door
opener modules are simpler, and use teh same construction, so if you
want a close up look at one go to www.shfmicro.com and look at the
"Gunn Modules"

BTW a door opener Gunn module, a simple FM radio for a 88 mhz FM IF
and a current modulator will get you a FM set with a range of 10 or
more miles on the 10.5 GHZ ham band, same for the 24 ghz band. Much
farther if you install a higher power diode and use a dish or bigger
horn instead of the factory horn.

Steve Roberts

 

[Kevin White]

After being "illuminated" by police radar a few times, I started ....

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Don't forget that you need some of the transmitter to get into the
receiver to mix with the received signal and create the difference
signal.

About 40 years ago I set up a simple microwave oscillator with an diode
adjacent receiver and intentionally leaked some of the transmitted
signal into the receiver horn by means of a small reflector. I then
amplified the output of the detector with an audo amplifier - it would
easily detect people walking around over a range of 40-50 feet.

kevin

 

[Rich Grise]

Don't forget that you need some of the transmitter to get into the
receiver to mix with the received signal and create the difference
signal.

About 40 years ago I set up a simple microwave oscillator with an diode
adjacent receiver and intentionally leaked some of the transmitted
signal into the receiver horn by means of a small reflector. I then
amplified the output of the detector with an audo amplifier - it would
easily detect people walking around over a range of 40-50 feet.

Was the sound higher-pitched when they were moving faster? When there
were multiple targets, did you get multiple tones? _was_ it running CW?
(which it kind of sounds like)

Please, tell us everything about your experiences with that - it sounds
fascinating - I had no idea such a thing could be so conceptually simple.

Thanks!
Rich

 

[Kevin White]

On Tue, 24 Oct 2006 13:11:31 -0700, Kevin White wrote: ...
Please, tell us everything about your experiences with that - it sounds
fascinating - I had no idea such a thing could be so conceptually simple.

Thanks!
Rich

Rich,

This was when I was at Grammar school, they had a couple of pieces of
microwave demonstration equipment in the Physics lab that I was playing
with.

One was a reflex klystron transmitter with a horn (producing a few
milliwatts of power at about 10GHz I guess) and another was a horn with
a "crystal" detector (germanium diode) - no amplification or anything.
A general purpose audio amplifier with speaker was connected to the
receiver. A microammeter was useful for setting up.

I placed the two units adjacent and pointing in the same direction then
put a small metallic object (a metal can or something I seem to
remember) close to both of them and just in view of both horns so that
a small amount of the transmitter signal was reflected into the
receiver - this acted as the reference signal. I adjusted the position
until I had a few microamps of current in the receiver.

If I put my hand in front of the horns the current would increase or
decrease. Depending upon the relative phase of the reflected signal to
the reference summation or cancellation would occur.

When fed into the audio amplifier I could hear a signal whose frequency
was dependent on the speed of my hand movement - the frequency was
pretty low and also pretty distorted so it was more a series of thumps
than a tone. It would also detect people walking a few tens of feet
away (the other end of the lab) with the frequency dependent on the
speed of walking or running.

Modern microwave door openers are pretty much the same using Gunn diode
microwave oscillators.

kevin

 

[[email protected]]

I once heard -- what's probably an urban legend -- that in some state the
penalties for speeding were tabulated, e.g., 0-4MPH, $xx, 5-10MPH, $yy, etc.,
but that it stopped at something like 50MPH over so technically it you went,
e.g., 120MPH you weren't breaking any laws.

You heard wrong. I have had a lot of speeding experience and every
state I have dealt with had a law that said if you were "in excess of"
30 over (or a similar number) that was a criminal speeding violation
that you had to post collateral for to get out of jail.

 

[[email protected]]

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

1. The doppler shift from a vehicle moving a few feet per second is
only a few tens or hundreds of Hertz.

2. Typical path loss would be 50dB on the forward path and 50dB on the

return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase
noise) out of the receiver?

My guess is you simply can't do that - does that mean you need
incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

You use a mixer to get the difference frequency.

 

[Homer J Simpson]

After being "illuminated" by police radar a few times, I started
thinking about some of the basics a bit, I am hoping for some
discussion of the fundamentals.

Radar is now becoming obsolete - most cops use lasers.

 

[[email protected]]

Radar is now becoming obsolete - most cops use lasers.

That is not true here. The laser requires the cop's undivided
attention from a speed trap. They can set the radar and forget it. The
comic alarm wakes them up. They can also use the radar while they are
driving.
Thre laser is good for selective enforcement where they are working
neighborhood streets from a hiding place. Most of our speed
enforcementr is done while running other calls.

 

[Eeyore]

Radar is now becoming obsolete - most cops use lasers.

Not here.

Then again we don't have many cops doing speed checks. It's all
'automated' now.

Graham

 

[JoeBloe]

Radar is now becoming obsolete - most cops use lasers.

Get yer story straight. Radar is in diminished use in Law
Enforcement organizations. It is still quite heavily used elsewhere.

 

[Homer J Simpson]

That is not true here. The laser requires the cop's undivided
attention from a speed trap. They can set the radar and forget it. The
comic alarm wakes them up. They can also use the radar while they are
driving.
Thre laser is good for selective enforcement where they are working
neighborhood streets from a hiding place. Most of our speed
enforcementr is done while running other calls.

They banned radar speed (not intersection) cameras here to get elected, so
the cops use laser and stand there with it.

 

[[email protected]]

They banned radar speed (not intersection) cameras here to get elected, so
the cops use laser and stand there with it.

Most of the radar in SW Florida will be shot from a moving deputy
cruiser while he is running other calls. They shoot you coming at them
and come get you if a speeding ticket is more important than his next
call.

 

[Jim Yanik]

[email protected] wrote in

Most of the radar in SW Florida will be shot from a moving deputy
cruiser while he is running other calls. They shoot you coming at them
and come get you if a speeding ticket is more important than his next
call.

Are you claiming that SW Florida (local PDs) does not run speed "traps" on
urban streets,but only uses moving radar during ordinary patrol?
That they don't monitor school zones or local city streets?
That is very hard to believe.

In Central Florida,the local PD squad cars often sit in local street
medians clocking oncoming traffic,they sit motorcycles up on sidewalks
under shady trees in residential areas(25MPH zones),they hide cops with
radar guns in cardboard boxes next to roads,or dress them as construction
workers,panhandlers,etc.
Moving radar is rare unless you're on a Interstate or beltway.