measuring distance between two cars using infrared circuits

[Dave VanHorn]

How do those supermarket door openers work?
(not the footpad - the ones with the little box over the door)

Same thing, doppler microwave.
There may be some PIR units out there, but most are X or K band microwave.

 

[Mac]

[snip]

It looks like it doesn't really matter, anyway. The Fourier transform is
just a sum of two sinc() functions, one shifted right and one shifted left
by the carrier frequency. The pulse duration controls the magnitude of the
FT.

Sure. I'm looking at launching a ~2GHz (wherever the FCC allows) CW pulse
and measuring its time in flight. At a ns/ft that's 6"/ns round-trip.
Some tricks should be able to get this down significantly less than this.
A ns is a long time these days.

I believe the total bandwidth is infinite, but any finite signal
has infinite bandwidth, so that doesn't really help us.

Sure. I don't see a few kHz on either side of 2GHz to be a big deal
though. It might be a challenge to gate an uwave tranmsitter on in a
millisecond, but...

Unfortunately, I'm not sure I know how to answer the question myself.

I'll try to remember to ask some people who might know tomorrow and get
back to you. (It also might pay to ask in the radar/sonar newsgroup.)

RADAR was my primary interest here. Measuring ns delays is rather trivial
these days. ...and that gets us to 6" distance resolution. Put enough of
these together with a (very) little computation and we get velocity. I
don't see how the mechanics of a couple of cars will exceed the physics or
computational needs.

But the more you constrain the bandwidth, the more difficult it will be
to identify exactly where the pulse starts or stops. So for precise
ranging, you need more BW, regardless of pulse duration.

Ok. We can measure more points of the envelope. The question is where is
the bandwidth limitation. I suspect it will be in the transmitter,
though I don't know. Again, a few kHz isn't a lot of bandwidth.

I talked to one of my co-workers today, and he said that as a very rough
order of magnitude estimate, the receive bandwidth needs to be about 1/T,
where T is the pulse duration. So if you want a 10 ns pulse, you will need
on the order of 100 MHz of receive bandwidth.

The situation is somewhat analogous to sending a digital pulse through a
bandwidth-constrained channel (filter). Depending on the nature of the
filter, it may ring or just ramp up slowly.

If the bandwidth is too narrow, you may not see the pulse at all.

There are other practical problems to overcome.Some of the other practical
problems with this system are that unless the beam width is kept narrow,
strong returns from objects on the side of the road will swamp the
receiver.

Anyway, it is fun to think about it.

--Mac

 

[Paul Hovnanian P.E.]

microwave is indeed more reliable
using a bursting microwave gives an indication of absolute distance and
speed between objects

timing between start of burst and start of reception of it is a measure for
absolute distance
doppler frequency gives relative speed

Modulate the microwave pulse frequency. There are modulation schemes (a
quick ramp up in frequency, or chirp for example) that will allow the
detection of both range and speed from the reflected signal. The DSP
might get a little more expensive than what is needed for Doppler alone.

 

[Dave VanHorn]

Modulate the microwave pulse frequency. There are modulation schemes (a
quick ramp up in frequency, or chirp for example) that will allow the
detection of both range and speed from the reflected signal. The DSP
might get a little more expensive than what is needed for Doppler alone.

'swhat I suggested earlier.
You don't need DSP though, WWII radar fused bombs used this technique, and
they did NOT have dsp!

 

[Keith Williams]

On Sun, 23 Jan 2005 14:07:55 -0500, keith wrote:

On Sun, 23 Jan 2005 17:51:45 +0000, Mac wrote:

[snip]

Ok, what's the bandwidth of a kHz modulated ~2GHz carrier (wherever there
is some free bandwidth). It should be trivial to measure the round-trip
delay to withing a nS, which is about six inches. At a kHz,
that gives us a distance measuremnt every millisecond, which should be
enough for distance and differentiate to give a relative velocity
number.

Are you talking about on/off modulation of a 2GHz carrier at a 1KHz
rate? How long is the "on" time?

Yes, pick your poision.

It looks like it doesn't really matter, anyway. The Fourier transform is
just a sum of two sinc() functions, one shifted right and one shifted left
by the carrier frequency. The pulse duration controls the magnitude of the
FT.

Sure. I'm looking at launching a ~2GHz (wherever the FCC allows) CW pulse
and measuring its time in flight. At a ns/ft that's 6"/ns round-trip.
Some tricks should be able to get this down significantly less than this.
A ns is a long time these days.

I believe the total bandwidth is infinite, but any finite signal
has infinite bandwidth, so that doesn't really help us.

Sure. I don't see a few kHz on either side of 2GHz to be a big deal
though. It might be a challenge to gate an uwave tranmsitter on in a
millisecond, but...

Unfortunately, I'm not sure I know how to answer the question myself.

I'll try to remember to ask some people who might know tomorrow and get
back to you. (It also might pay to ask in the radar/sonar newsgroup.)

RADAR was my primary interest here. Measuring ns delays is rather trivial
these days. ...and that gets us to 6" distance resolution. Put enough of
these together with a (very) little computation and we get velocity. I
don't see how the mechanics of a couple of cars will exceed the physics or
computational needs.

But the more you constrain the bandwidth, the more difficult it will be
to identify exactly where the pulse starts or stops. So for precise
ranging, you need more BW, regardless of pulse duration.

Ok. We can measure more points of the envelope. The question is where is
the bandwidth limitation. I suspect it will be in the transmitter,
though I don't know. Again, a few kHz isn't a lot of bandwidth.

I talked to one of my co-workers today, and he said that as a very rough
order of magnitude estimate, the receive bandwidth needs to be about 1/T,
where T is the pulse duration. So if you want a 10 ns pulse, you will need
on the order of 100 MHz of receive bandwidth.

Makes sense to me.

The situation is somewhat analogous to sending a digital pulse through a
bandwidth-constrained channel (filter). Depending on the nature of the
filter, it may ring or just ramp up slowly.

Sure, filters have non-zero response time. Again, makes sense. So
with a 10ns pulse one should be able to measure down to 5' without too
much trouble. One ns resolution shouldn't be all that difficult (gate
delays on the order of 10-20pS aren't all that big of a deal).

If the bandwidth is too narrow, you may not see the pulse at all.

There are other practical problems to overcome.Some of the other practical
problems with this system are that unless the beam width is kept narrow,
strong returns from objects on the side of the road will swamp the
receiver.

Look for the first return. That's the object that has the highest
probability of hitting you the soonest, thus the most "interesting".
;-).

Anyway, it is fun to think about it.

Sure. There are other issues, such as "you aren't the only one on the
road", but that's all a simple matter of engineering.

 

[Paul Hovnanian P.E.]

Same thing, doppler microwave.
There may be some PIR units out there, but most are X or K band microwave.

Yes. Anyone with a radar detector can confirm this.

 

[Mac]

Doppler or direct time of flight, is doing it the hard way.

Chirp the transmitter at some number of MHz/uS
The reflection, even at a few nS delay, will be your carrier frequency of
that number of nS ago (can't be anything else!) so, the mix frequency
product at the receiver, will be proportional the the distance.

Smooth the output a bit to eliminate jitter, and you're there.

What you are describing is a linear FM homodyne radar. I believe this is
how the actual radars now available on some cars work. For a large variety
of reasons, I agree with you that this is the best way to do it.

Because you can transmit almost continually, you can get by with transmit
power in the milliwatt range.

--Mac

 

[Mac]

Modulate the microwave pulse frequency. There are modulation schemes (a
quick ramp up in frequency, or chirp for example) that will allow the
detection of both range and speed from the reflected signal. The DSP
might get a little more expensive than what is needed for Doppler alone.

AIUI, the reflection from a single sweep can't give you Doppler. You would
like to keep the single sweep duration short enough so that the object
doesn't move much during the sweep.

Doppler would manifest itself over the course of several chirps as a
gradual phase shift in the IF.

If your chirp repetition rate is fast enough, you could still get very
frequent (100's of Hz) Doppler updates.

--Mac


--Mac

 

[Dave VanHorn]

AIUI, the reflection from a single sweep can't give you Doppler. You would
like to keep the single sweep duration short enough so that the object
doesn't move much during the sweep.

Chirp dosen't do doppler, it measures the distance.
So if you want rate of change of distance, you'll have to compare distances
over time.

 

[Paul Hovnanian P.E.]

Chirp dosen't do doppler, it measures the distance.
So if you want rate of change of distance, you'll have to compare distances
over time.

Correct. There are some more complex modulation schemes where both
Doppler (target velocity) and distance can be derived. That's what got
me thinking about the complex DSP. But distance is what the OP wanted
anyway.

The chirp modulation ramp is selected to give a difference frequency
that is much higher than the Doppler shift created by target motion for
any reasonable distance measurement precision.

I didn't see your original post, Dave. It was in a different branch of
the thread. But great minds think alike.

 

[Mac]

Chirp dosen't do doppler, it measures the distance.
So if you want rate of change of distance, you'll have to compare distances
over time.

You cut a lot of relevant information from my post, and you dropped the
attribution.

As I said, a single chirp (AFAIK) can't give you any information about
target velocity, but if you look at the IF of a target in motion from
chirp to chirp, you can see that it shifts over time. This phase shift
between chirps can give you information equivalent to Doppler.

There are FFT-based techniques for estimating this "Doppler" accurately.

You may choose to not call it Doppler, but it is called Doppler by many
people.


--Mac