Curious, I ask...going somewhat on a tangent...


"biascomms" wrote in message
. uk...
Steve N. wrote:

I think the tone detector-PLL is not too good a method. Getting the
phase
shift to do a cancell will take an adjustable all-pass phase shifter.
Not
impossible, but complex as well.

The PLL can correct phase as well as frequency of the cancelling tone -
it's
trivially easy.
Bob --
Everything gets easier with practice, except getting up in the morning!

AMEN!

Bob,
The OP said a DSP was too complex, so I was keeping with that restriction
(and assuming that he may have been an analog type of guy)
How do you propose to get the 180? An Exclusive OR type PD has *approx* 90
degrees for a type one loop and 90 should be possible with a type 2...so you
need another 90. Not impossible by any means, of course. If the VCO is a
state variable implementation or a phase shift type, you may be able to get
another 90 degree section for "free".
The PLL lock time will allow a burst of tone at the start *and end*...(see
next).
Then, when the loop is idling, with no tone to cancel you need a gating
circuit to disable its tone from the output since ther'll be no tone to
cancel and time for it to turn on/off. Sounds like a juggling exercise.
Comment?

The PLL lock time will allow a burst of tone where a notch/equalizer
won't and it sure seems a notch is very simple - a hand full of parts in a
twin "T" or possibly narrowing it up with a augmented twin-T (twin-T and
amp)...or there may be another band reject, I think based on the Wein bridge
or some other active filter topology.

Now that I think about it, I suspect there may me a way to just change where
the tone in inserted into the path to eliminate it from the dispatcher's
speaker/phones.. But that assumes access to the system and the OP may have
just wanted to stick something in the output jack for the dispatcher...
There may even be a "dispatcher tone injection level adjustment." (:-)
Perhaps it is set too high now...

This is the type of problem that I *know* has a really simple solution.
It's just knowing all the constraints and getting the clue for that answer.

73, Steve, K9DCI.