Re: measuring distance between two cars using infrared circuits
From: Mac (foo_at_bar.net)
Date: 01/26/05
- Next message: Richard H.: "Re: Weller WTCPN Problems"
- Previous message: Noah Roberts: "Re: Peterson's Death Sentence"
- In reply to: keith: "Re: measuring distance between two cars using infrared circuits"
- Next in thread: Keith Williams: "Re: measuring distance between two cars using infrared circuits"
- Reply: Keith Williams: "Re: measuring distance between two cars using infrared circuits"
- Messages sorted by: [ date ] [ thread ]
Date: Wed, 26 Jan 2005 04:27:48 GMT
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.
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
- Next message: Richard H.: "Re: Weller WTCPN Problems"
- Previous message: Noah Roberts: "Re: Peterson's Death Sentence"
- In reply to: keith: "Re: measuring distance between two cars using infrared circuits"
- Next in thread: Keith Williams: "Re: measuring distance between two cars using infrared circuits"
- Reply: Keith Williams: "Re: measuring distance between two cars using infrared circuits"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
