[sdiy] More VCO fun

[ASSI]

[off-list]

Hi David,

On Thursday 03 December 2009, David G. Dixon wrote:
> One thing I am finding with this updated circuit is that I need an
>  even bigger reset RC to ground the integrator cap.  I'm up to
>  3.3us now (33pF * 100k) and I'm still only getting to within about
>  100mV of ground (resetting a 1nF cap from +5V).  I am thinking of
>  going to 47pF, but I know this will wreak havoc with high
>  frequency tracking.  It also defeats the purpose of having a
>  high-speed comparator (although LM311 isn't exactly "high-speed"
>  as comparators go...).  Of course, I'm using 2N5485 (still)
>  instead of 2N4391, although my simulations all show that this
>  should actually work better.

I'm assuming your circuit is very similar to Ian's, as I'm really 
commenting on that (I don't think I ever saw a link to your circuit).  
Both the switch itself and the control are not ideal for what you want 
to achieve.  The JFET used is a depletion transistor, which means it 
is switched on when V_GS becomes zero (or indeed very slightly 
positive, but not so much that the G-S-diode opens).  After reset, the 
comparator is held at -12V (since both pin 3 and 6 are tied to this).  
This is only good if you are using a transistor with a high cutoff 
voltage that actually needs such a high V_GS to be switched off.  The 
one you are actually using has much lower cutoff and should not be 
driven so deep into depletion.

During reset the gate capacitance will be discharged through R23-R24 
to ground only.  Again, if you have a transistor with a high cutoff 
voltage this is probably going to work out, otherwise the transistor 
may never actually switch fully on until quite some time has passed 
(the gate RC is about 10-15ns here and is getting larger as zero V_GS 
is approached).  That is if there wasn't the "timing" cap, which 
increases the effective RC by about 3 times.  The simulation should 
show this... anyway, for a JFET with low cutoff you need to come much 
closer to zero V_GS to really switch them on and I suspect you never 
really do that before the comparator calls it a day and pulls down 
again.

It would be much better to use a comparator that has a push-pull 
output stage or follow up with one.  Alternatively, if you want to 
stay with that circuit feed some ~mA current into the output of the 
LM319 via a resistor from +12V and put a Schottky diode from output to 
ground.  This way the comparator will switch faster, the JFET will 
open much better and you should be able to reset in quite a bit under 
1µs.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

Wavetables for the Terratec KOMPLEXER:
http://Synth.Stromeko.net/Downloads.html#KomplexerWaves