[sdiy] Moog wander

[Magnus Danielson]

Hi all,

Sitting here reading the thread and feeling up to nothing special I decided to
walk down the lab and see what my Mini is doing. One of the things I noticed
was that I need to cook up a better trigger circuit, need to ponder a little
bit over that. What I did see is that when I look over longer time stretches
(100 s or so) I definitly see a random walk in frequency. This would imply
f^-4 of the PSD, but it may be flicker frequency (and thus f^-3). Need to look
more carefull about it. I did some quick Allan deviations measures and found
that tau = 100 ms seems to be the most stable one (about 1.8 mHz at 167 Hz
carrier). My trigger noise certainly bites me for shorter taus.

Since my counter input is fairly wideband (good for 500 MHz frequency) I can
expect some noise there, so I must come up with some sharper edges to counter-
act that. I need to solve that problem anyway so I'm on it.

I clearly see a 50 Hz peak in the modulation, but beyond that it is clearly
noise. I am not looking right at the VCO, I look at the normal output (which
is certainly not ideal from rise-time perspective) but I also input a test
signal to compare. For shorter counter periods it is definitly trigger noise
limited.

Oh, and yes... I think you are all too hooked up on the jitter aspect of
modulation where as I think one needs to ponder over the wander aspect (as
well). I think I have the necessary tools more or less at hands.

Cheers,
Magnus