VCOs

[Bissell, Harry]

The problem is even worse than diode leakage. C-MOS schmitt-trigger
threshholds vary with manufacturer and aren't guranteed. Sawtooth amplitude
is proportional to the hysteresis. And the C-MOS gate has a very limited
ability to recharge the cap. especially with the series resistor and diode.
It's a cute circuit though.
	Thierry Rochebois has a VCO design on his website (don't know the
address but someone in this group will...) that generates a Triangle wave,
Charge Time = Discharge Time. I haven't tried it but it looks like the way
to avoid the problem. Any Comments on his design?   Harry

> -----Original Message-----
> From:	Martin Czech [SMTP:martin.czech at intermetall.de]
> Sent:	Friday, February 05, 1999 7:51 AM
> To:	synth-diy at mailhost.bpa.nl; MHELIN at tne01.ntc.nokia.com
> Subject:	Re: VCOs
> 
> > Hello,
> > 
> > Are there any VCOs that produce saw waveform by recharging the
> > cap instead of discharging it as usual? I mean something like
> > this: http://www.uta.fi/~helin/sch/s-vco.gif. Usually in
> > relaxation type ocillator the cap discharge time has to
> > be constant, discharge can't be detected by comparator,
> > but in this case the charging can be detected (here by the schmitt-
> > trigger, but comparators could be used as well). Are there any problems
> > with this kind of method (like temperature or supply dependency,
> > possible current spikes etc.).
> > 
> > -Mikko
> 
> Charging, discharging, where's the difference ?
> A saw requires a controlled slow slope and a very fast one, that 
> usually has less control (because of the short time).
> I like the above mentioned circuit, because it minimises dead time,
> this is one of the high frequency errors of a saw osc.
> I think this is possibly the fastest arrangement for discharge
> *SENSING* you can find. 
> 
> One should not forget that even CMOS is beeing used, the schmitt input
> impedance is dominated by the ESD-Protection junctions inside
> the chip. Leakage can be a topic there. Most manufacturers guarantee
> <1uA over temp. This is a whole lot. One should test the circuit for
> leakage
> therefore (just disconnect the expo converter and see what happens).
> If a good cap is used, the observed droop or rise makes it easy to
> calculate
> the leakage current. 
> 
> A shortcomming could be the diode parasitic resistance as well as the
> fact that the diode resistance as such varies wideley as the current
> through the element changes (e.g. ladder filter). Ie. I would expext a
> very fast charging in the beginning, but then when approaching the upper
> level the charging gets slower. However, the schmitt will switch off
> about 2.5V or so, far away from the usual 5V. It would be interesting,
> if the diode resistance has a LARGE influence on charge time.
> The voltage across the diode influences the switching behaviour,
> that may be undesired, eg. for sync.
> 
> Maybe a faster diode with better recovery time and also low leakage
> could be used.
> 
> I had similar plans (but now I dropped sawtooth ideas completely),
> but I wanted an active switch for charging. Leakage problems get worse
> for BJT , because of beta.
> 
> OTOH you might also like to have sync. In this case you'll need a second
> diode arrangement, I want to sync charge to a predetermined level,
> in this case an active switch, BJT JFET or MOS would be good.
> If this sync switch takes also normal charging, even better.
> 
> m.c.
> 
> 
> Since the diode is charging, the expo source has to discharge, you can
> use npn, that's good.