Why on earth would you want to possibly compromise the excellent performance
of the K2 when there are cheap purpose built FM transceivers in abundance on
the market and second hand they can be picked up for peanuts?
Even without this in mind the immediate problems are:
Receiver.
Even with using the narrow band FM we have been bullied into adopting in the
UK for repeaters, the +/- 2.5kHz deviation will not pass through the 2.0 to
2.5kHz filters found in the K2. The effect is to cut the carrier off on
deviation peaks producing what has been nicknamed here the "Norman Collier
effect"
by causing the squelch to close at this instant and chopping up the received
audio to make it unintelligible. As a repeater license keeper and builder am
acutely aware of this effect by mobiles trying to force a +/- 5kHz signal
though our now mandatory 7.5kHz (+/-3.75kHz) bandwidth filters.
I just does not work!
For +/-5kHz deviation an IF filter of 15kHz (+/- 7.5kHz) is the norm. To
incorporate this in the K2 would mean extracting the IF signal soon after the
1st mixer Z6 and before the K2 1st IF filters. A totally separate IF, squelch
and FM detector strip would be needed with most probably 15kHz filters to cope
with older transmitters. Additionally this would really need to be an
outboard unit with its own audio amp and speaker. IC's such as the now
obsolete?
MC3357 would take care of all the IF, squelch and detector functions.
Transmitter.
FM on transmit could be produced by feeding audio via a suitable coupling
capacitor into the K2 VCO control line, though the deviation would probably not
be constant on all bands. There is a problem with the TX audio as the peak
deviation has to be strictly limited in its amount. Even the use of a
compressor just does not give the degree of control needed, so an audio peak
limiter
stage and subsequent following low pass filter has to be inserted.
All in all do you really want to do this when again it could compromise the
excellent performance of the K2?
Bob, G3VVT
Keeper (license holder) GB3LD/GB3LF
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