Do you really need a 50 percent duty cycle wave?

I do not, the extra power usage doesn't bother me that much anyways. I'm mostly worried about the noise on the falling edge.

 

I breadboarded the circuit using the 6 MOSFETs in a CD4007. It's oscillating at around 20 30 Hz with a 1M resistor and 0.1uF cap. I doubled the cap and the frequency dropped to about 10 7 Hz.

I did some calculations for the Schmitt inverter in post #12. Something wasn't right about the frequency not scaling by a factor of 1/2 when capacitance was doubled.

I measured Vt+ and Vt- and did the calculations to determine the expected frequency. For 1M ohm and 0.1uF, that would be:



The axial ceramic cap I used still had some tape residue on the leads. Once I cleaned the leads, I started getting the expected results.

 

I've done some more testing and i think the noise on the falling edge is due to the diodes reverse recovery time. I'm using 1n4007 diodes which have a typical recovery time of 30us and that does match what I'm seeing on the scope.
Unfortunately i don't have any other types of diodes, is there anything i can do to resolve the issue without having to buy new ones?

 

if you have a spare transistor, you can use it as a diode, just for testing.
The circuit still function without it, so you can remove it and check

 

Using a BC547 transistor instead of a diode reduces the noise to 5us (which i assume is the recovery time of such a transistor).
Removing the diode entirely causes the capacitor to discharge through the output, which is why the diode is there in the first place.

 

Removing the diode entirely causes the capacitor to discharge through the output, which is why the diode is there in the first place.

You were suspecting that its causing it and you didnt have another diode, so its one quick solution to check and see if that was the case...

 

Getting rid of the 1k power consumption:

Replace R6 with a pot to adjust discharging duty cycle
Note: Will not oscillate without a load (R5)

 

Getting rid of the 1k power consumption:
View attachment 182206
Replace R6 with a pot to adjust discharging duty cycle
Note: Will not oscillate without a load (R5)

@iimagine please explain how the circuit works and derivation of voltages at each nodes

 

hhsing - This thread died over 5 years go. If you have a related question, it is better to start your own thread and reference this one for clarity.

al