VCOs

[Martin Czech]

> 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.