Re: measuring distance between two cars using infrared circuits
From: Keith Williams (krw_at_att.bizzzz)
Date: 01/26/05
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Date: Wed, 26 Jan 2005 12:36:48 -0500
In article <pan.2005.01.26.04.29.45.81430@bar.net>, foo@bar.net says...
> On Sun, 23 Jan 2005 22:59:46 -0500, keith wrote:
>
> > On Mon, 24 Jan 2005 00:09:10 +0000, Mac wrote:
> >
> >> On Sun, 23 Jan 2005 14:07:55 -0500, keith wrote:
> >>
> >>> On Sun, 23 Jan 2005 17:51:45 +0000, Mac wrote:
> >>>
> >>>> On Sat, 22 Jan 2005 23:41:45 -0500, keith 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.
-- Keith
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