[sdiy] all analog FM synthesis?

[Mattias Rickardsson]

At 13:52 2005-03-02, Johannes Öberg wrote:
>Mattias:
> > How do you mean? How would you tell the VCO to run backwards with an AD/DA?
>
>I wouldn't; I would use the A/D -> microcontroll -> D/A to do
>sinetable lookups. So it would basicly become a digital PM synthesizer
>with analog PM input :-(

Ah, I thought the "all analog" subject was still a little bit valid... ;-)

>Btw, I'm to silly to understand your saw phase shifter. It doesn't
>work with anything else than a saw, does it ?

I don't think so, no. (And it's not mine, it's Magnus'.)

>How is a modulus function (if it's not called modulus in english, I
>mean the function of getting the rest from an integer division) done
>in analog?

(I'm quite sure it _is_ called "modulus" in English, (as opposed to in 
Swedish where it's called "modulo"), but "rest" is probably called 
"remainder". Or "raindeer". Anyway:)

One way is to use an ADC/DAC combination, and subtract this signal from the 
original signal. If you keep the number of used bits in the ADC low enough 
to give every DAC output level good precision, then you will get a modulus 
function.

If you don't need lots of "modulus wraparounds", you won't need many bits 
of resolution, and then you can build the ADC yourself out of comparators, 
and the DAC out of an R-2R-construction. As a pure coincidence, your 
construction will probably turn up almost exactly the same as if you would 
have analysed the modulo function itself and built it without the ADC/DAC 
in mind. Such is the beautiful world of science. :-)


/mr