/diy/ - Do It Yourself

I'm trying to make a reliable square wave generator but google just shows operational amplifiers and other circuits which are not easily adjustable. It doesn't needs to go down into the negative range. All I want is to adjust the duty cycle and the frequency, from 0Hz (ideally) to at least 1Mhz. I tried with a 555 but their maximum frequency is about 200-300kHz if I'm not wrong. I would try the CMOS 555, but I haven't found any circuit that allows me to use them properly. The best thing I could find is this circuit, but the duty cycle range depends on the value of the frequency potentiometer, what makes the circuit very buggy and unreliable.

Can someone point me what IC or circuit should I use?