[sdiy] oscillator jitter / phase noise

[Czech Martin]

> > For the phase noise I could imagine current source noise,
> > integrator noise and perhaps most important comparator
> > noise. Also noise on the power supply comming from other
> > modules could be considered.
> > 
> > Anyway, has this ever been investigated?

Where? Most work I see concentrates on narrow
band rf oscillator phase noise,
because this is a problem for nearly all on chip plls.
So far I have seen nothing interesting for audio frequency
oscillators of the kind we use in synth gear.

> It's the bandwidth resolution which is your limit. You want 
> to zoom in like
> hell on a single overtone to view the sideband spectra.

Well, I thought that the interesting noise must be low frequency,
in order to create those phasing patterns. i.e. 1Hz and below.
Noise in higher frequency (1kHz-20kHz) would make the signal sound
noisy (hissing), but avering out the instantenious frequency.

I also thought that it might suffice to look at the fundamental
and the side bands that go down to DC. Of course a very long DFT
must be made to get good resolution in the low end.

 
> Another way to do it is to give your frequency counter a 
> heavy dose of steriods
> and make time-interval samples back-to-back and analyse the 
> data according to
> the methods used in the time and frequency world. When you toss Allen
> deviation and Modified Allen deviation analysis on a signal, 
> you can start to
> break up the different phase noise noise power spectrum 
> curves and individual
> amplitudes. Also, induces signals like overtones of line 
> voltage also show up.

Unfortunately my frequency counter can not dump data.
But couldn't a time measurement be made via audio sampling?
I found this:
http://www.analog-synth.de/avr/index.htm

Using the square output of the oscillator (given that the comparator
is good enough, else use the original saw or tri)
it should be possible to make such statistics
for every cycle. This time domain analysis has also the advantage
that no computer memory constraints apply, only hard disk space.

With 48kHz sampling rate the time resolution apears to be rather poor:
20.8 us. But is this really the case? Could the real resolution
not be a little bit better due to interpolation techniques?

Since most people do not have access to old gear, audio sampling
would be also interesting. It is easier to obtain a DAT tape
of a pure VCO recording then to get hold on the machine.


> 
> Maybe I should put a synth under my arm as I go to work 
> tomorrow and do a
> little evening-session?


This sounds like a very good idea!

m.c.