Re: Phase or frequency modulation?

On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
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john larkin <jl@glen--canyon.com> wrote:
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On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator.  After
multiplication and mixing, the signal appears at 145 Mc/s.
>
Listening to this signal on an Icom 706 MkII transceiver I found it was
barely intelligible, with severe high frequency cut.  At first I
suspected my modulator but I checked the audio output of the Icom with a
good-quality signal generator and found the response was:
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200c/s : -3dB
400c/s :  0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5    : -10dB
2k0    : -13dB
2k5    : -16dB
3k0    : -18dB
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(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
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The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation.  Most references to modulation in the
2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
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Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?

 
Is your "crystal oscillator" a packaged VCXO?

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No
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They generally lowpass
the frequency control input, the varactor thing, pretty hard.

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The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
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The crystal oscillator is a modified Colpitts with the reactance valve
tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current).
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As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
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The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s.  I
have tried removing most of the top-cut capacitors between the audio
clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
>

 
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.

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The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A
signal generator.  This has an internal meter which allows the
modulation level to be accurately set and monitored.  The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
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The sig-gen tests confirmed what my ears were already telling me.

>
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.