I'm trying to fabricate a simple radar system. I was wondering if a pic
can react fast enough to measure the span between the transmit pulse and a
recieve detection pulse? both signals would be ttl and processed by analog
hardware front end. I know it can do it with ultrasonic, but I was
wondering about radio and or laser/IR.

On Wed, 24 Jun 1998, Tom Handley wrote:

>    Mark, it sounds like you are describing CHIRP. A long time ago I use to
> work with this in airborne RADARs. It was used in a side-looking RADAR on an
> RF-4C. Secret `stuff' at the time... That would be one solution and a good
> one. In anycase, Dominic is not going to be able to do this in current
> digital `main-stream' technology for a 1ns resolution. He's going to have to
> use analog with the PIC used for control, processing, driving a display,
> and/or communicating with a host platform. Tjaart mentioned charging a cap
> but that brings a whole new set of problems. I don't know if Dominic works
> for a company with the design tools and the ability to manufacture products
> in the 1+ GHZ range.
>
>    If Dominic could provide more details about the target environment,
> background `clutter', and why he needs 1ns resolution, maybe we can offer
> more help.
>
> BTW, I think you and I have `crossed paths' before. I live just north of you
> in Portland and I created and operated the Northwest Amiga Group (NAG) BBS for
> 6 years. If so, please forgive my memory. I'm lousy with names but I never
> forget eyes. I can almost hear Andy Warren saying; "See! If I was that old I
> would'nt have any memory!". Just kidding Andy ;-)
>
>    - Tom
>
> At 10:08 PM 6/18/98 -0700, Mark G. Forbes wrote:
> >[Dominic explains....]
> >"Okay here is why I am looking to get a 1nanosecond clock pulse. My applicati
on
> >is to measure the distance between 2 unknown points ( at most being 500-600
> >yards
> >apart) using a two-way (round trip) ranging principle based on point source
> >radio
> >systems. This is basically how I want it to work. Transceiver 1 transmits a
> >signal(pulses), the signal is received at transceiver 2, and after a fixed
> known
> >delay , it is re-transmitted back to transceiver 1, is received by  transceiv
er
> >1`s receiver and input to a ranging circuit. The ranging circuit measures
> >the time
> >difference between the original transmission time and the time of reception
> >(less
> >the known fixed delay) , which would be a direct measure of the two-way
> distance
> >when multiplied by the speed of light.This is the problem, the speed of light
> >travels at 186,000 miles per second.That translates into roughly 98,208,000
> feet
> >per second. For an accurate measurement I would need an oscillator( I guess
> >out of
> >the question with a pic) that would supply my ranging circuit with a time
> period
> >that would translate into approximately 1nanosecond per foot. I would like
> >to use
> >a PIC to do all the conversions( time to feet and/or yards) and drive an LCD
> >display. Any help will be greatly appreciated."
> >
> >Ok...everybody together now....."You're doing it all wrong!"
> >
> >That's a great idea, in principle, but as you've already observed,
> >it's hard to put into practice because of the annoyingly fast rate
> >that light moves about. Galileo had the same problem, *way* back when.
> >
> >This problem has already been solved, and you don't even need two radio
> >transmitters to solve it. One will do.
> >
> >Create a radio signal of some reasonable frequency, say 902MHz. That's the
> >ISM band in the US, a fairly un-regulated area to work in. Apply a modulating
> >signal such that the carrrier frequency is swept up and down over a range.
> >This is technology you can get off-the-shelf. Point your signal at the
> >distant target, where you've installed an antenna, configured as a reflector.
> >You don't actually *have* to use a reflecting antenna, but it'll make
> >the signal discrimination a lot easier. At 900MHZ, these things don't have
> >to be very large.
> >
> >The transmitting antenna sends out a signal, and it sweeps up in frequency. O
ne
> >round-trip-time later, the receiving antenna begins to pick up the signal
> >that's been reflected back from the reflecting antenna at the other end. But
> >right now, the transmitter is sending a signal that's higher in frequency
> >than it was when it started, and so there's a difference in frequency between
> >the transmitted and received signals. And that difference is proportional to
> >the round-trip-time, and hence the distance. Using a mixing circuit, combine
> >a little of the transmitter signal with the received signal. This will genera
te
> >sum and difference signals, and you filter out the carrier frequency and the
> >sum, and keep the difference. At zero distance, there's no difference between
> >the two, and the difference frequency goes to zero (DC). As the distance
> >increases, the difference frequency goes up. The sweep rate of the modulation
> >provides the scale factor, and now all you have to sense is a signal that's
> >somewhere in the audio range of frequencies. That's a *much* easier problem!
> >
> >Note that you can also determine speed this way; set the sweep to zero for a
> >fixed frequency, and the difference tells you whether the target is moving
> >or not, and how fast. Alternate sweeping with fixed frequency, and you can
> >also tell which way it's going! This is some of the science behind radar guns
.
> >
> >This works with light, too, and it's the principle behind things like laser
> >rangefinders. Even cameras have things like this, these days, so it's not
> >an expensive problem to solve.
> >
> >The ARRL handbook will have an assortment of useful information on RF design,
> >which is where you need to be looking. A PIC could probably do the sweep
> >control and the signal detection, but you've got other things to figure out
> >that dwarf simple things like the control circuit.
> >
> >Mark G. Forbes, R & D Engineer  |  Acres Gaming, Inc.    (541) 766-2515
> >KC7LZD                          |  815 NW 9th Street     (541) 753-7524 fax
> >forbesm@peak.org                |  Corvallis, OR 97330
> >http://www.peak.org/~forbesm
> >mforbes@acresgaming.com
> >
> >"There has been an alarming increase in the number of things I know nothing
> >about."
> >---Anomalous
> >
> >
>