I'm trying to build a low frequency oscillator (think about 0.01Hz – 5Hz) using a TL072 as an astable multivibrator. Here's the circuit in LTspice:
And here's the simulated output:
Where does the 2.8s initial delay come from? It appeared once I raised R to 1M to decrease the frequency to suit my range. Also, where would one put a potentiometer for frequency adjustment? I also suspected using an electrolytic capacitor in this configuration would damage it as it's repetitively getting a negative voltage, so I limited C1 to 100n.
Best Answer
Model 1 mV of offset voltage and this won't happen. The simulated op-amp is starting up in a precariously balanced-on-a-point condition that is totally unrealistic.
To adjust the frequency, change R to a pot (better with a resistor in series). You can use a film capacitor. Ceramic caps are another possibility, but suffer from voltage coefficient (this circuit does not produce linear sawtooth anyway).
Note that period, not frequency, is linear in R as well.
For shaping to sine waves it might be better to use an op-amp + comparator oscillator, which will give you nice linear triangle waves, and can be adapted into a VCO which is linear frequency for input voltage by switching the integrator to +/- gain with comparator state.