JH's VCO

[Martin Czech]

:::I have been just playing with the thyristor-based VCO Juergen posted
:::some time ago (well, just the core Q1-Q8, J1,J2 and U3, I don't need
:::the interclock prevention circuitry yet). This circuit promises a high
:::scale range due to the high speed of the thyristor switching. I intended
:::to use this VCO in a wavetable oscilator, so I needed a good ultrasonic
:::range operation (to be scaled down by the sample counter). 
:::
:::I observed accurate tuning within only about 4-5 octaves, and my VCO 
:::goes significantly flat over about 30kHz. I reckon this might be due to 
:::the general purpose transistors I used.
:::
:::My question is whether anybody tried this out and achieved wider tracking
:::range, especially in the high frequency range ? Do you use some fast swi-
:::tching transistors ?

My nose says that thyristors may turn on quite fast,
but may need considerable time to turn off.
Total saturation....
I don't have the design handy, iare there means to
discharge the (saturated) emitter-base junctions
to turn off? 

I rember how slow darlingtons turn off, if the big npn has no path
from base to emitter. A 100 Ohm resistor made it 10x faster !


Just my nose.

m.c.