3flp wrote:

The PLL will keep the shift constant.
You'll also have to detect when the PLL is not locked and remove the
counter-tone on this condition.

Use a CMOS switch (4016 or 4066) to mute the tone.

No idea how well this would work... but
I'd cost only a few $, for the 4046, an opamp or two and some Rs&Cs.

It works really well - I used to use this method to remove the 96 kHz
(approx) interference in "scrambled" cable TV signals.

Bob

--
Everything gets easier with practice, except getting up in the morning!