Some phasemodulation thoughts

[Rene Schmitz]

Hi all!

Martin wrote:

>I just had the same idea on the weekend. 

Must be some sort of syncing :->

>Analog, this would mean:
>For every 180 DEG of phase modulation one HWR is needed, at the end one
>needs two summer stages to add up all HWR signals with appropriate sign.
>
>The precision HWR gives more dc accuracy and lower drift, but is slow.
>The emitter follower HWR is faster, but needs another junction
>(complementary emitter follower) for 1st order temp compensation.

One can clamp the HWR with a backward diode, preventing the opamp output
going to the negative supply.

>Over the weekend I did a gnuplot simulation, and it worked imedeately.

Great. 

>In reality we will have some artefacts, however:

>1. the sine shaper (ota or diode network) is of course nonideal

There are those sine shaper networks at your site, I simulated arround with
the last circuit from that page, I think this will fit nicely in here. Btw,
I will make that a module anyway. Should give results similar to the Serge
WS, but continuous.

Note the difference to my proposal: I would use such a +/-360 deg
sineshaper instead of an OTA that only goes to +/-90 degrees. That means
less wrapping stages. I can have the shaper do part of the job, if its
modulated 270 degrees in either direction. 
What I'd have is adding modulo 3/2pi with the wrapper, and then using a sine 
"lookup" table with values from -3/2pi to +3/2pi. The input saw hereby only
modulates +/- pi. For ammounts of shift smaller than pi/2 the modulo adder
is out of the action! 

>2 . the single segments of the PWL may not match exactly, but this could
>be trimmed

That is no issue with that rectifiing circuit I have mentioned in the
earlier post. Provided that precision resistors are used (only two
different values!) it has a single trimming point which sets the interval
at which the foldover occurs. Trim it once to 1.5 * amplitude of the saw
and you're done. But then the problem is the delay.

No better idea for 3. Maybe the ADC/DAC route would be better.

>4. this is PM, so at low modulation speed the percieved frequency change
>will be small. This circuit can not do these fantastic analog frequency
>sweeps, of wide frequency modulation, from sub audio to 10kHz, or so,
>if you know what I mean. So, analog frequncy modulation is still necessary,
>but maybe not the through zero variant, with all the implicit problems.

Right, wide range frequency sweeps are difficult. If my maths is right this
requires an exponentially rising voltage at the modulation input. It would
hit the rails pretty quick.

Bye
 René


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