<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=us-ascii" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.23588"></HEAD>
<BODY>
<DIV dir=ltr align=left><SPAN class=815431418-11112018><FONT color=#0000ff
size=2 face=Arial>You might find that the filter will oscillate easier if you
use good film caps. I had problems getting a four-pole filter to oscillate
at low frequencies with monolithic ceramic caps, but it oscillated strongly down
to the minimum frequencies with WIMA polyester caps.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=815431418-11112018><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=815431418-11112018><FONT color=#0000ff
size=2 face=Arial>I just built an SVF with silvered mica caps (which I usually
use in oscillators) and it is being finicky with self-oscillation below about
50Hz. Maybe I'll swap those out for WIMA polyester and
see.</FONT></SPAN></DIV><BR>
<BLOCKQUOTE
style="BORDER-LEFT: #0000ff 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px"
dir=ltr>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <B>On Behalf Of </B>Pete
Hartman<BR><B>Sent:</B> Sunday, November 11, 2018 9:20 AM<BR><B>To:</B> Tom
Wiltshire<BR><B>Cc:</B> synth-diy@synth-diy.org DIY<BR><B>Subject:</B> Re:
[sdiy] AS3320 VCF chip - some queries<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV dir=ltr>
<DIV class=gmail_quote>
<DIV dir=ltr>On Sun, Nov 11, 2018 at 10:32 AM Tom Wiltshire <<A
href="mailto:tom@electricdruid.net">tom@electricdruid.net</A>> wrote:</DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
class=gmail_quote>2) The resonance response is tilted to the high end.<BR>It
seems to require a lot more resonance CV (ok, current) to make the chip
oscillate at the bass end than at the treble end. Similarly, the amplitude
of oscillation increases as the frequency rises. Why is this? Is there
anything I can do about it? Should I even care?<BR></BLOCKQUOTE>
<DIV><BR></DIV>
<DIV>If I compare this with the behavior I've seen in other lowpass filters
that self oscillate, even without CV over the resonance, it seems
consistent. Typically I need the frequency at 50% or higher to get it to
start oscillating -- I can then commonly turn it down and get lower
frequencies, but my experience is that if the filter is going to oscillate,
it's always easier to start the oscillation at a higher frequency. And
typically if I turn it "too low" then it stops oscillating
again.</DIV></DIV></DIV></BLOCKQUOTE></BODY></HTML>