[sdiy] oscillator jitter / phase noise

[Magnus Danielson]

From: "Czech Martin" <Martin.Czech at Micronas.com>
Subject: [sdiy] oscillator jitter / phase noise
Date: Wed, 2 Jun 2004 17:08:03 +0200
Message-ID: <D9D56E8FA1A73542BE9A5EC7E35D37FFF390A2 at EXCHANGE2.Micronas.com>

Hi Martin,

Hmm... you seem to be tossing wood into another interesting topic for me! ;O)

> Just some idea:
> 
> In some old postings it was said that "old" vco designs
> "could" sound better because there is some random process
> leading to phase noise/jitter.

Yeaaazzzz...

> This would then lead to very interesting phasing pattern,
> if two or more oscillators beat against each other.

... or 16 of them.

> Obviously a quartz stabilised oscilator, DCO or software
> oscillator will not have this feature.

Actually, it does have that feature, just _much_ less. All "clock" signals
have phase modulation. It's a fact of life. Most crystal oscillators have a
"fair" amount of phase modulation for it's purposes (clocking digital designs)
but it can be a real task to get low phase modulation and also long term
stability (which is another phase modulation component!).

> To me it seems the only explanation, since the actual
> waveform is not very important (after my experiments).

Interesting that you (too) have come to that conclusion.

> 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?

To some degree.

> What do owners of old gear say?

Well, my gear may not be that old, but I definitly say it's a real effect.

> If not, what would be needed to investigate this?
> Would a very long audio sample be sufficient to make 
> statistics and meaningfull spectral measurements?
> Or is the usuall 20kHz + bandwith too limited? 

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.

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.

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

> (currently very busy with room acoustic measurements, i.e.
> room impulse response measurements)

Yeah, another topic I enjoyed you having tossing alot of wood into! ;O)

Cheers,
Magnus