wide range Schmitt trigger oscillator

[elektrouwe]

I guess all of us Lunettics have built circuits with the standard RC Schmitt trigger oscillator consisting of a resistor, capacitor, and a Schmitt trigger inverter IC (ST).
It's a beautifuly simple circuit, but has some drawbacks :

1) tuning
if you replace the resistor by a potentiometer, you can tune the time constant T=R*C. Because T is 1/Freq, the frequency curve wrt potentiometer value is a nasty hyperbolic function

2) range
for high frequencies == low potentiometer resistance, you must add a series resistor of some kOhm to the potentiometer, to prevent shorting the ST output to the capacitor. This limits the tuning range to typically 1:100

3) power consumption
high frequency means high charge/discharge currents to/from the capacitor. That means beside the more or less static power consumption of the ST we have a dynamic power consumption proportional to frequency. This is bad for battery powered Lunetta circuits and even worse for power starved circuits, because you can kill a good sounding starving sweet spot by changing frequency.

I found a solution to these problems by adding 2 more resistors and rewiring the potentiometer. It is published here:
https://www.edn.com/wide-range-tunable-rc-schmitt-trigger-oscillator/


I have successfully tested the design idea with an ICL555 on breadboard.
Of course any inverting Schmitt trigger like 74HC14, CD40106 as well as opamps and comparators should work.
The catch is :
The maximum voltage at the potentiometer must never reach the ST hysteresis voltage , else frequency will jump to some MHz then. Hysteresis voltage is a well defined 1/3 of the output voltage in case of a CMOS-555.
Other STs, especially 74 and 4000 CMOS logic have huge tolerances for hysteresis (>=20%). If you use one of these, a R3,4 should be replaced by a resistor in series with a trimmer to adjust the desired max. frequency.
Finding the resistor value that limits the highest frequency is a bit tricky because the tuning is very sensitive for high frequencies.
BTW: the same problem occurs with the old standard ST oscillator when reducing R : 0 Ohm means infinite frequency ( some MHz in reality) so R must also be carefully selected there.

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