I have to make the frequency variable of an square waveform generator built with the help of an astable. I modified a circuit a found on the internet and everything seems good with only one exception: I cannot bring the astable to a low frequency without destroying the signal.
Whenever I raise the resistance of R13 (thinking about it as a potentiometer) to make the frequency lower, the signal waveform is getting worse and I can't figure out why.
I really don't have enough experience (I'm student) to make it work between 20-60 kHz without ruining the signal waveform. I thought if I add a variable resistor before the capacitors I can limit the current so the charging time of it, but I don't know why the signal is destroyed at a certain frequency. Do I have any chance to make it work like this or should I just think about another method? Or what should i modifiy to make it work?