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

[Ian Fritz]

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.