Re: Phase or frequency modulation?

Don <g@crcomp.net> wrote:

Liz Tuddenham wrote:

Ralph Mowery wrote:

liz@poppyrecords.invalid.invalid says...

>
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?

>
Frequency and phase modulation should come out of a transmitter almost
identical.  As they start off different in the first stage of the
transmitter the audio frequency going into the modulastor stage must be
modified so the end results is the same for FM and PM.

>
Yes - but it would appear that PM is the de-facto standard for
commercially-built 'amateur' 2-metre transceivers despite all the
literature calling it FM.

 
Yes, your sentiment seems "on the money."
 
This thread apparently advanced from an IC-706 to a DIY receiver. If
your schematic indeed pertains to your DIY receiver - excellent work
showcased in an excellent thread! (It taught me a few things, thank
you.)

It is based on an idea that has been in the back of my mind for many
years.  The change from an exchange-powered to a mains-dependent
telephone service in the UK was the final factor that made me decide
that I neded an alternative communications system.  I thought that,
while I was making one, it might be a good idea if it was EMP-proof, so
I decided it would have no semiconductors.  It has turned into an
interesting project.

For what it's worth, the IC-706 probably uses the 75 micro-second pre-
emphasis standard prevalent in the Americas and Japan. Meanwhile,
Europe favors a 50 micro-second standard.

Those are the standards for wide-band broadcast FM, but they are outside
the range of permitted modulating frequencies for narrow-band amateur
work.  The time constant for the Icom appears to be around 2000
microseconds.
 

Although Phase Modulation is popular in Asia:
 
    <https://pmc.ncbi.nlm.nih.gov/articles/PMC11501851/>
 
it's doubtful the IC-706 was developed for an Asian market.

Apparently it was.  There is a Japanese version with a slightly
different specification from the European one.    You may have hit upon
the real explanation, I haven't checked to see if mine is a Japanese one
which has been sold in the UK by accident.


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