[sdiy] Ultrasonic VCO question for Ian Fritz (or whoever)

[Grant Richter]

I would recommend the core design of the ultrasonic VCO for general purpose
use.

The Wiard system was supposed to have a video module (but parts are long
obsoleted) so the VCOs needed to be "roughed in" for use up to 100Khz and
above.

Ian's use of a LM318 for the hysteric comparator is brilliant and sets up
the VCO for use in the frequency decade above audio.

The original design used a digital waveshaper to allow "drawing" the
waveform on a set of 10 slide pots, providing a frequency division by 10.
For this the oscillator normally operates in the decade above audio.

But operated only in the normal audio frequency range, boy howdy, does that
core design work good.

And the last time I bought NJM318s from Mouser, they were only 33 cents
apiece, quant. 100.

> From: Ian Fritz <ijfritz at earthlink.net>
> Date: Mon, 28 Oct 2002 10:10:20 -0700
> To: Scott Bernardi <sbernardi at attbi.com>, Synth-DIY list
> <synth-diy at dropmix.xs4all.nl>
> Subject: Re: [sdiy] Ultrasonic VCO question for Ian Fritz (or whoever)
> 
> Scott --
> 
> Thanks for your interest. It's always great to see this old material be of
> use. Believe it or not, I still have this module in my system and use it. A
> couple of years ago I updated the converter with better opamps (OP27 for
> the input summer, Scott!) and I put in improvements in the temperature
> compensation.
> 
> The HF tracking scheme works by forcing the comparitor to switch early at
> high frequency due to the extra voltage drop across the compensating
> resistor.  The argument I made about the resistance value needed is quite
> crude. I simply assumed that the maximum slew-rate of the 318 would
> determine the reset time. The slew rate is spec'ed over a range, so one
> can't expect to predict the resistor value exactly. Additionally, the
> compensating resistor is tasked with taking care of the Rbe error in the
> converter, which it cannot really do correctly because of the different
> functional dependence involved.
> 
> I probably didn't actually measured the total value of the tracking
> resistor, so 4k is just a rough estimate. Maybe it is 5k. The integrating
> cap is actually 120p vs the 100p value in the EN schematic.  So the RC time
> could be 5k x 120p = 600 ns.  I consider this very good agreement with the
> value of 570 ns estimated from the maximum slew rate.
> 
> I hope this explanation clarifies things for you.
> 
> Best regards,
> 
> Ian
> 
> 
> At 06:38 AM 10/28/2002, Scott Bernardi wrote:
>> I direct this question to Ian Fritz, since you are the author of an
>> Electronotes EN #112 article on a Ultrasonic VCO (built with a 3080,
>> JFET buffer, and LM318 schmitt trigger). Anybody can answer.
>> In the article, you say you compensate for the 570nS dead time with a HF
>> track trimpot in series with the integrating cap. The Mikulic style
>> sawtooth oscillators use the same scheme. You suggest a value of around
>> 4K total.  What is the formula for calculation of this resistance?
>> Doesn't seem to be t = RC,   R = t/C.
> 
>