qwerty68 said:
I managed to get both the 555 circuits to work. I could adjust both frequency and duty cycle. The catch was that the duty cycle doesn't remain constant, it changes when the frequency changes so it's not very useful.
I wouldn't waste my time with those circuits.
I have never worked with opamps so I have no clue to their properties in use or what is required. What would you have chosen? Could you suggest some models you think might work?
It depends on your performance requirements. If, for example, you're driving +/-12V square wave at 100Khz, and you want the 'switching' portion to be, oh say 1% of the waveform, then you can calculate the required slew rate as follows:
1/100Khz = 10uS period
10uS*.01 = 100nS switch time.
So, you need to slew 24V/100nS or 240V/uS. If you used a slow 741 at .5V/uS, you wouldn't get much of a signal. Use this analysis, then use one of the opamp manufacturer's web search facilities to find an amp that has the minimum performance capabilities.
And does this design keep the duty-cycle constant when the frequency is changed?
The duty cycle in this design is totally dependent on the center-tapped variable resistor. The frequency is totally dependend on the other variable resistor (shown as "gain" on the drawing)
Another idea that might work well is to use the saw tooth at the 555 astable's trigger and put it through a comparator just as I thought of with the function generator. I read that suggestion somewhere last night.
That's another good way to do it.
I saw on my pc that it produces a nice tooth and the duty cycle remains constant as far as I can theoretically overview it.
I read something about the general purpose of the 741 being appropriate for comparing the sawtooth. Do you have any experience with the 741? In a simulation it works well.
The 741 is a bad choice, as shown in the above discussion.