Soft sync and medicine to cure it.

[Harry Bissell]

Hi Bjorn (and everybody)

The soft-sync should not be a problem if you take some precautions.

1) Power supply decoupling (like Bjorn said)

2) Don't use the same IC (opamp, comparator) in two VCO's. This prevents them
from being decoupled individually.

3) Keep the reset path SHORT and low impedance. Fat traces, and no loops. A
little
inductance can not only add to reset time, it can couple to other traces.

4) Physical separation between oscillators is a good thing. Ground rings or
planes between them will help.  Separate PCB are the ultimate !!!

Bjorn:  What method did you use to measure the spikes ?  Voltage, current ?
Thanks.

H^) harry

Bjorn Julin wrote:

> Hi.
> I made some measurements lately on softsync
> issues, by measuring the spikes bleeding trough
> to the common emitter of the expo convereter.
>
> Depending on the tecknique used in the switching
> mecanism of the core (as you allready know i suppose)
> the spikes are quite different in its amplitude.
>
> Just by simple bypassing with a apropriate cap
> between the most pos and most neg rail you can
> reduce the spike significantly.
>
> To my suprice, by placing a FET in series with the
> sourcing current from the osc did not reduce the spikes?
>
> Juergen or  (others) ,did you got any improvment doing so
> in your's designs? And has any one done any soft sync measures?
>
> reg
> BJ
>
> >From: "jhaible" <jhaible at debitel.net>
> >To: "Tim Ressel" <Tim_R1 at verifone.com>, <synth-diy at node12b53.a2000.nl>
> >Subject: Re: Junction Noise
> >Date: Thu, 7 Sep 2000 00:55:08 +0200
> >
> > > Well, you math weenies have one on me. I can tell you that I have
> >studied
> >PRN
> > > sources with FFT analysers, and the output falls into well defined
> >buckets. In
> > > between the buckets there are definite holes: frequencies with no
> >energy.
> >My
> > > idea was to frequency modulate the clock with another unrelated PRN.
> >This
> >would
> > > smear out the energy and fill in the holes. This I believe would make
> >for
> >better
> > > "noise".
> >
> >If you just play your PRN sequence _once_, you should have a continuous
> >spectrum.
> >(But only for a limited time, unfortunately.)
> >The more often you repeat the same cycle, the less "smearing" you will get,
> >and
> >at infinite repetition you'll have discrete frequencies.
> >So altering the sequence, or the playback speed of the same sequence,
> >between
> >every cycle, must surely help to preserve the "smearing". With another PRN
> >serving
> >as a modulation source, the variations will not be unlimited either. You
> >will
> >get some repetition as well - only much later. It would surely be an
> >improovement
> >compared with one single PRN.
> >The big question is, will this improovement be better than simply investing
> >the additional PRN in an increased length of the first one, rather than
> >splitting in
> >two PRNs and finding an optimal modulation algorithm ?
> >What if for a certain number of flipflops the linear arrangement of all
> >flipflops
> >*is* the best algorithm ?
> >(I cannot answer this.)
> >
> >JH.
> >
> >
>
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