Re: Phase or frequency modulation?

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

In article <1rf4563.inglu7k0vp30N%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
 

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.

 
If you push the signal into clipping, the very act of clipping is
going to create a whole bunch of nasty high-frequency harmonics and IM
that sound terrible.  In that situation, applying further emphasis to
this grunge is probably the opposite of what you want to do.  Rather,
let the THD/IM be rolled off by the receiver's deemphasis filter, and
consider this an act of corporal mercy to the listener's ears :-)
 
If you put the emphasis after the clipper, then you end up having to
set your deviation limit based on a worst-case prediction of high
frequency content in the signal, and this may force you to use
a lower deviation level than you'd otherwise prefer.
 
So, my vote is "emphasize, then clip".

I've now altered the circuit to do it that way:

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

The circuitry between the two valves determines the pre-clipper
frequency response:  The 100pf capacitor in conjunction with the 680k
grid leak resistor gives a 6dB/octave slope below 2.3 Kc/s.  Above that
frequency the 220pf capacitor has almost the same signal voltage at both
ends, so it doesn't have any effect. 

As the audio frequency rises towards 3 Kc/s, the 100nf cathode bypass
starts to take effect, increasing the gain of the clipper and
simultaneously reducing the audio signal on the cathode.  This has the
effect of bringing the 220pf capacitor into action to contribute another
pole to the cutoff of frequencies above 3 Kc/s.  In this respect, it
acts a bit like a Sallen & Key filter with a sharper corner than the two
RC circuits would have had if they didn't interact.

The clipper is 'soft' to some extent, so it doesn't generate such strong
and wide-ranging spurious component as a hard clipper would.  The 220pf
capacitor across the output level control deals with some of the worst
harmonics.  The second stage of filtering is in the grid circuit of the
reactance valve.

--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk