AW: Jitter in VCOs

[Haible Juergen]

	>I've noticed that several synth
	>enthusiasts believe in "best" technology for implementing their
gear ... I
	>wonder if that's not a mistake ?!

Right! 


	>I've been pooring over the VCS3 schematics for a few weeks and
I suspect
	>that the VCOs there are capable of considerable jitter since
the
	>single-ended switches and reference levels will drift
dynamically in time
	>(this is a universal properties of all VCOs).  In other words
the spectrum
	>sidebands would be fatter for such an oscillator, how much
flatter would
	>depend on jitter amplitudes and frequencies.

Maybe. 
Or look at the way they get the reference voltage for their expo
converters.
A transistor reverse biased as zener diode. Must be noisy as hell.

BTW, more conventional VCO designs which use opamps to sum the CVs
can sound very different, depending on the opamp they use (1/f noise).
In my SEM VCO clone, the cheap (unselected) 741's I used produce
quite some jitter in the 1Hz range. LF356s which I alternatively used
were
much cleaner. Maybe the optimum is somewhere in the middle. Maybe
Oberheim selected his opamps (?) to get what were low-noise ones
by the standards back then, but are not from todays point of view?
(Only guessing. Don't know anything about Oberheims selection methods.)

	>Lot's of people think that a square wave is a aquare wave,
regardless of
	>whether it's been generated with op-amps or discrete transistor
circuits

Not so many on this list, anymore 8-).
There are no square waves in the real world at all. And *trapezoids*
have quite some degrees of freedom.

JH. (recovering perfectionist)