[sdiy] VCO reset time for BBD-based PM

[Magnus Danielson]

From: "JH." <jhaible at debitel.net>
Subject: [sdiy] VCO reset time for BBD-based PM
Date: Wed, 2 Jun 2004 22:52:45 +0200
Message-ID: <001101c448e3$9783f480$2870b9d9 at jhsilent>

Hi Jürgen!

> > Hmm, a discharge of 300ns is 1/3000th of the cycle of a 1kHz. That would
> > mean that at 1kHz you're off by 0.3Hz and at 10khz you're off by 3Hz....
> 
> I'd say that's ok - just so - for an audio VCO.
> 
> But once you want to clock a BBD for making a Physical Modelling oscillator,
> it looks pretty bad. With a BBD length of 512 stages, you're already in the
> 1ns
> range of reset time for the accuracy quoted above. Or use some compensation
> method and longer reset times.
> 
> Re-introducing Harry's dreaded BBDs into the thread (;->)
> But I'm serious about this application. So who makes a HF VCO that goes
> up into the MHz range and keeps the tracking error in the ns range ??
> HF compensation allowed, PPLs not allowed.

It is a little-known fact it seems that the 1N4148 has a very fast recovery
time, well below 1ns infact. This can be used to create very quick edges if you
are in the mood for it. However, for some just below 1 ns isn't enought, so I
use a pulse of about 17 ps every now and then at work. There are gear down to
7 ps commercially available (if I hade the money to spend, guess what...).

Now this might sound like a strange comment, but you can use a diode's fast
recovery behaviour (which not all diodes have!) to sharpen up the edge. You
could also use another hard-doped diode like the tunnel-diode (hard to come by)
for similar purposes.

Should I make some experiments maybe?

There is however ways to get inverter ring oscillators CV-controlable, this may
be an option if the controlability is important but not necessarilly the
waveshape.

Cheers,
Magnus