>I'm building a unit that needs to head away from its point of origin
>until it gets to a preset distance. At the moment the distance calc is
>based on time elapsed. This doesn't work very well because speed varies.

>I like the idea of building a UHF transponder/beacon set. The unit send
>a unique pulse train then listens for a reply. The beacon receives and
>decodes the pulse train, waits a preset time, then replys. The unit
>receives the reply and calculates distance from the signal's time of
>flight. Jittering the PRF and range gating would improve signal to
>noise.

>Max distance is around 1500m so a 2km range would be desireable. I'd
>like transmit status information in the pulse. Basically what I'm after
>is a low power UHF (~400MHz) version of the air traffic secondary
>surveilance radar.

>Trouble is I don't know where to start :

>I'm comfortable making my own PCBs and programming PICs but don't know
>much at all about RF circuits. Would someone care to point me to a good
>starting point? A website or a readily available book would be nice. A
>similar project I could examine would be even nicer :)

>TIA
>Rob

You are going to have a couple major problems in this approach. Most
importantly relates to bandwidth. You're proposing to implement a system
that will have relatively narrow pulses (you didn't say what range
resolution you require, but I'll assume is on the order of 10 meters.)  This
means that you must measure the pulse timing within 30 ns. To do this, one
would like a pulse that has a rise time of less than 30 ns, but let's say we
can make it work with a 30ns pulse. That implies we need a 30MHz transmitted
and received bandwidth, using the rule of thumb of 1/pulse width for
bandwidth requirements.

I'm familiar with the ITU's international radio bandplan, which almost all
countries follow to some degree or another. You're in NZ, and I think you
may well find that licensing a 30 MHz bandwidth pulse system in the 400 MHz
band is somewhere between difficult and impossible. There are military
radars that use this wide bandwidth in the 420-450 MHz band in the US (the
band is shared amateur radio and military radiolocation) so perhaps that's
your best bet for a wideband system.

This also means that you are going to have to build all the equipment, as I
can't think of a non-military pulse system operating in that frequency range
anywhere in the world.

But, there are other approaches that can be implemented in a narrow band
system, say 30KHz bandwidth, using off-the-shelf FM transceivers. Let me
describe one and you can judge whether it meets your requirements or not.

The radios I have in mind are conventional FM low power devices, operating
in the UHF band. There's a good chance NZ allows these on an unlicensed
basis, as long as the power is modest. In the US, the Family Radio System
(460MHz) comes to mind.

Anyway, configure one radio as a repeater, to simultaneously receive
frequency F1 and retransmit the received audio on a second frequency F2. In
most places in the world, the standard split between F1 and F2 for systems
in the 400-500 MHz range is either 5MHz or 10MHz. In the US, it's 5MHz, but
I seem to recall some UK systems operating with 10MHz splits.

Depending on the power level, you may be able to get by with separate TX and
RX antennas. Or, you may need a cavity duplexer or at least a couple cavity
filters to prevent the receiver from being desensitized by the transmitter.

At the other end of the link, you have the reverse arrangement, transmit on
F1 and receive on F2.

You transmit a audio sine wave, at as high a frequency as you can get
through your radios. For a typical voice radio, without modifications, it's
going to be around 3 KHz. If you can modify the radio (which may void the
regulatory approval to use it) you might be able to get this up to 5 or 6
KHz.

So, what we then have is a, say 3 KHz sine wave going out and the same
signal returning, but delayed by (a) the transponder delay in the repeater
and (b) the distance delay caused by the speed of light.

So, here we are with these two signals. We then phase compare the outgoing
sine wave with the incoming, repeated sine wave. The difference in phase
gives the distance to the repeater, plus some fixed offset for the delay
through the repeater. The fixed delay can be calibrated out.

This system, by the way, was implemented on analog cellular telephones to
give ranging. The real problem is how accurately you can make the phase
comparison, and that is in large part governed by the S/N of the repeated
signal. If you can measure the phase to an accuracy of 1 part in 1000, your
total range is accurate to 100 meters, or the one-way range to 50 meters,
assuming 3KHz as the modulating signal.  If you can get it up to 6KHz, and
still maintain 1/1000 phase comparison, the one way range gets cut in half.


Jack Smith

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body