On Thu, 31 Jul 1997 10:35:16 +0100 Oyvind Kaurstad
<oyvind.kaurstad@NOFAC.ABB.NO> writes:
><SNIP>
>
>>>are hard to design no matter what the frequency range.  The same
>>>problems
>>>will be encountered at 35 MHZ that I just described.  Your best bet
>is
>>>to
>>>probably modify an existing RC receiver
[...]
>
>This is a very good idea.
>But there may be a slight problem. The receiver is designed
>to have a crystal connected, not a "frequency generator".
>
>This may or may not pose a problem, depending on how the
>circuitry is designed.=20
>
>Maybe it is possible to make the PLL circuit behave in a crystal-like
>manner?
>
My first attack would be to replace the crystal with an inductor and
varicap, so the crystal oscillator becomes a VCO.  Arrange to take a
sample of the oscillator frequency back to the PLL chip and the RF design
of the project is done.

If that doesn't work out so well, remove the feedback components so the
R/C receiver's crystal oscillator becomes an amplifier.  Then feed in an
external signal.


>One would have to get the schematics of the receiver to know.
>
>But making a similar receiver without the servo-control options should
>not be impossible. Only a signal-strength indication would be
>required.

Start with a receiver of the AM type.  It AC-couples the detected pulses
to the pulse demultiplexer and DC couples the average carrier level to
the IF amplifiers to adjust their gain so the pulses are always about the
right level.  This AGC line should be suitable for signal presence
detection.  It will detect any signal: AM, FM, or PCM, since they all
occupy the same bandwidth.  It may be desirable to speed up the time
constant to allow faster scanning.