Re: Phase or frequency modulation?

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

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

On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

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

On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com>
wrote:
 

On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

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.

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

 
Why not measure the FM and see who the bad guy is?

 
The FM was coming from a high quality signal generator, I have no reason
to suppose it was causing such a bad frequency response.

 
I meant measure the actual FM from your crystal oscillator.

 
I have no way of making those measurement other than with a VHF
receiver.
 

I wonder if the crystal's Q limits modulation bandwidth somehow.

 
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver.  The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my
usual sources.
 
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
 
 

Crystals modulate much faster than f0/Q, interestingly. The mechanical
movement and the piezoelectric coupling are a single degree of freedom,
governed by an ordinary differential equation.

ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.

Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics