Re: Propagation/Timing Delay Question

From: ChrisGibboGibson (chrisgibbogibson_at_aol.com)
Date: 10/04/04 

Date: 04 Oct 2004 23:01:40 GMT

John Larkin wrote:

>On Mon, 04 Oct 2004 21:35:42 GMT, George W
><look_below@my_signature.com> wrote:
>
>>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.
>>
>>The transceiver filters and logic would contribute to the tolerance.
>>Can anyone give me an educated guess as to:
>>
>>How uncertain would the timing of the control logic be? That is, the
>>time between when the incoming audio reference burst is detected and the
>>time when the following burst is sent? I assume this is a clocking
>>stability question. The circuit would utilize a common low-cost
>>clocking reference and components.
>>
>>How much variation can be expected in propagation time through the RF
>>interstage filters? Would this vary much from unit to unit and over
>>temperature? If so, calibrating each transceiver during manufacture is
>>a possibility.
>>
>>Thanks for any input.
>>
>>George
>
>
>I'd prefer not to use the word "impossible", but it's just about
>there. Timing the receive event to 50 ns is extreme, and generating a
>200 ms delay accurate to 50 ns is a delay accurate to 0.25 PPM, hardly
>"common low-cost clock" turf. Temperature and noise will be nasty
>problems, too.
>
>John
>

Trying to detect the exact time that a 1KHz sinewave was switched on after
having passed it through a few 3KHz bandwidth (assuming communication
equipment) channels is, I believe, impossible to the accuracy required by OP.

Now let's watch someone tell us exactly how to do it :)

Gibbo

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