Re: Phase or frequency modulation?

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1rf3e3v.uj0bkl16o5mqqN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> 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.

 
It's the former, and "it's a feature, not a bug".
 
As I understand it: the normal convention on the ham bands is to apply
a 6 dB/octave high-pass equalization to the transmitted voice signal
prior to frequency modulation.  The time constant puts the "knee" of
the curve above the voice band.
 
During reception, the signal from the discriminator is fed through
a corresponding low-pass filter ("knee" below the voice band) before
being fed to the audio amplifier and outputs.
 
It's a process similar to what's done in commercial FM broadcasting,
but with different time constants in the filters.
 
As I understand it, this was done for two reasons:  to reduce the
incursion of high-frequency noise into the audio signal, and to
allow compatibility with PM transmitters (which don't require or
use the high-pass filter).
 
So, what you are observing is probably this:  the signal you're
transmitting (from your own modulator or from your test oscillator)
is frequency-modulated, but the necessary pre-emphasis of the
upper frequencies is not being applied.  When the signal is
demodulated by the Icom, it's passed through the de-emphasis low-
pass filter, and what you're observing in your frequency chart is
the response of that filter.

That makes complete sense and explains what I measured.  (As an aside,
it probably makes the manufacturer's specification look better too.)

The question now is where to put the pre-emphasis in the feed to the
modulator?   If I put it in front of the clipper it will be rendered
ineffective by clipping - but if I put it after the clipper it is liable
to over-deviate the signal.

<http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm#INAMP>

There is already some pre-emphasis generated by the 100pf capacitor
coupling between the anode of the first valve and Grid 1 of the second
valve.  I can further increase the pre-clipping effect by substituting a
470nf capacitor for the 2u2f capacitor which currently by-passes the
cathode resistor of the clipper.

If the pre-emphasis is needed after the clipper, I can take the feed
from the anode of the clipper directly to the top of the 100k pot
through a small-value capacitor and omit the top-cut network.  My worry
is that this could cause 'sideband-splash' into adjacent channels and
lead to complaints.

[...]

So, you could try putting your Icom into "9600-baud" mode, and look at
the discriminator-output signal at the "to TNC" output jack.  I'm
going to guess that you find that you see a pretty flat frequency
response from the receiver when you feed it a signal from your
experimental modulator.

Thanks, I'll look into that.


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