Re: Propagation/Timing Delay Question

From: Joerg (notthisjoergsch_at_removethispacbell.net)
Date: 10/04/04 

Date: Mon, 04 Oct 2004 23:47:28 GMT

Hi Chris,

>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 :)
>
>
It can theoretically be done if you know all parameters of the chain.
For example prop delay in the IF filter, whether it is 3KHz bandwidth or
more. There will also be the ramp time of the detector that detects the
presence of the 1KHz tone. The behavior of that AM detector that sits
after the FM demodulator needs to be known precisely. If you don't know
all of this there would be the method of testing the total delay and
ramping pattern but that would be venturing onto thin ice. Since we are
talking 50nsec here about 1/50th of a degree of phase error in the 1KHz
signal would throw everything off kilter already. Not a whole lot.

One low cost way to do this in the real world is to detect the umpteenth
zero crossing of the 1KHz signal. Just pick one and stick to that. It
has to be the exact zero crossing, not a few millivolts up or down. That
could be achieved by a differential measurement. This is the way it used
to be done in Doppler ultrasound where you sometimes have to measure
minute movements of blood or vessel walls. This strategy would get rid
of the uncertainty, drift and what not of an additional AM detector.

Then, as John said, the whole circuitry must contain a good timing
reference. I would roll my own crystal oscillator in a case like this.
An off the shelf oscillator in a can isn't likely going to cut the
mustard. After all, a gourmet dinner doesn't come in a can either. And
watch out for that FM detector. The slightest DC offset (by frequency
drift etc.) and the precision is toast. The same goes for the modulator
in the transmitter.

Regards, Joerg

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