Summing node/S+H generator question

[Magnus Danielson]

>>>>> "h" == halfmast  <halfmast at clubmet.metrobbs.com> writes:

 h> As my first experiment with sample and hold, I sampled some white noise
 h> and applied the output to an attenuated CV input of my VCO.  A problem
 h> arose as I lessened the attenuation.  A peak detector connected to the
 h> summing amplifier of the VCO would register 2.5v (which is really only
 h> half of the s+h's theoretical swing--it gives +-5v unattenuated), the
 h> VCO's frequency would go up past an audible level, and then the peak
 h> detector reads 12.5v. The thing is, the oscillator can handle such a
 h> swing when my LFO is controlling it; so why can't I get the full
 h> swing from the S+H?  I should probably also add that it would never
 h> reach the high frequencies without becoming unstable, and that the S+H
 h> never has problems when its not connected to the VCO.  Any ideas?
 h> Thanks in advance for any input.

I come to think of the rise-time of the signal. What if the S/H will
have a real quick responce? What happends if you suddenly see a step
which has a very quick rise time and a step large enought, would you
bet that your filter would be stable under these conditions?

Try using some lowpass filter to dampen the risetimes, preferably a
variable filter so you can move close to the edge.

It is also worth noting that the behaviour that a curcuit has under
normal conditions and under high noise levels is apprechiably
diffrent. For instance will a high noiselevel at the input of the PLL
change the loopgain of that PLL to some degree dependent on the phase
detector responce. Nonlinearities like comparators etc start to behave
strangely. I say this to point out that noise is not just any signal,
it is a particularly mean signal to deal with.

Cheers,
Magnus