Re: Propagation/Timing Delay Question

From: Joerg (notthisjoergsch_at_removethispacbell.net)
Date: 10/06/04 

Date: Wed, 06 Oct 2004 17:15:38 GMT

Hi George,

>>Another idea: Why don't you do all that at the RF level? Fire up the
>>transmitter, gently modulate it with something unique, wait until
>>stabilized, then close an RF gate with good isolation and synchronized
>>to the RF signal.
>>
>>
>
>OK, if the receiver has a DSP up front at the VHF input and the transmit
>carrier is gated on cleanly at the source after it stabilizes then we should
>be able to tell within a few RF cycles when the carrier arrives if S/N is
>reasonable.
>
>
You don't have to do this with a DSP. It can also be done with a
detector. The trick is to provide a narrow enough filter to avoid false
triggers and there are many ways to do that. Directly at the RF level it
may be cumbersome because large resonators are bulky and expensive. I'd
probably down-convert to something reasonable, maybe in the 50MHz range
where you can achieve enough precision but can obtain or build filters
at reasonable expense. This has been a long time ago but TV sets contain
IF filters of about 5MHz bandwidth and because they are made by the
gazillion should be cheap. They are also carefully designed for a nice
flat group delay.

Of course the filter will shallow the slope of the onset. The lower the
bandwidth the more difficult it will be to obtain 50nsec precision. You
would need to accomodate for variances in RF signal strength which can
be done by registering the RF level once the detector has ramped up and
then adjust the measured slope position to compensate if the RF level
changed because of obstacles or whatever may be in the path.

I don't know the application but if it is short range and you can use an
ISM band with plenty of bandwidth there should not be too many issues
with interference.

>It's been a few years since I looked at prices, but DSPs capable of 200 MHz
>operation were pretty pricey then and they may still be. May reduce battery
>life too. Attractive, though, because they can do a lot in addition to
>serving as the FM demodulator. If too expensive at VHF one could be
>mplemented after a mixer stage but then you have to pay for the mixer,
>filter etc in addition to the DSP and it starts to lose it advantages for
>counting RF cycles accurately as the frequency goes lower. Hmmmm ....
>
>
Prices have come down big time, mostly because of the advent of DSL and
cable modems. Check out TI. Yes, these DSP can do all the math you want
to perform afterwards on the same chip that does the detection. Also,
you may be able to pull routines from a library if the manufacturer
provides them. But in this application a DSP may still be overkill.

Regards, Joerg

http://www.analogconsultants.com

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