Re: Phase or frequency modulation?

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

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