Re: Propagation/Timing Delay Question
From: Tim Shoppa (shoppa_at_trailing-edge.com)
Date: 10/06/04
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Date: 6 Oct 2004 10:33:36 -0700
George W <look_below@my_signature.com> wrote in message news:<Omj8d.2738$M05.872@newsread3.news.pas.earthlink.net>...
> I'm developing a product based on ordinary low-cost handheld VHF FM
> transceiver equipment with modifications.
>
> The transceiver will receive a short carrier burst containing an audio
> tone pulse (1 kHz or so) that serves as a timing reference. It must
> then turn on its transmitter and transmit an audio tone pulse a precise
> amount of time following the timing reference pulse. Transceiver
> operation will be simplex mode.
>
> The amount of time between the two pulses is not important. It can be
> 200 msec or more, as needed to let the transmitter and receiver settle
> after turning on and off. The only requirement is that the time between
> pulses be kept to a close tolerance of 50 nanoseconds if possible.
50 nsec is going to be impossible with a 1kHz audio tone burst. Well,
theoretically it is possible with an insanely high signal-to-noise
ratio or a long enough integrating time but you won't ever get there.
Going to more bandwidth will make 50ns possible. As an example, GPS
receivers do timing to within a few tens of nanoseconds (and in fact
they measure the timing for multiple satellites simultaneously thanks
to clever correlator coding). Look up how a GPS correlator works.
You will have to send and receive signals much more complicated than
a simple tone burst but the technology to do all this is well
developed and consumerized.
Doing all this with a common VHF handie-talkie probably won't work,
group delay through the filters is completely uncontrolled. Unlike
a GPS, where you need to know the time difference between received
signals and thus are largely insensitive to slow-changing delays in your
receiver, you are very sensitive to any delays. The
"modern" way of doing this is all at the IF frequency with DSP's.
Tim.
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