Re: Phase or frequency modulation?

john larkin <jl@glen--canyon.com> wrote:

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:
>
200c/s : -3dB
400c/s :  0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5    : -10dB
2k0    : -13dB
2k5    : -16dB
3k0    : -18dB
>
(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.
>
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.
>
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?

No

They generally lowpass
the frequency control input, the varactor thing, pretty hard.

The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm

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).

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.

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.

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.

The sig-gen tests confirmed what my ears were already telling me.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk