Re: PWM GENERATION OF 25nS - 70nS outputs
That is more RF circuitry than digital...
To get square wave at that speed you are talking about frequency components of tens of MHz, at least... You must use transmission lines, matched source and load to get it working.
And that isn't PWM because you have fixed off time. So it's variable frequency with constant off time. You can check out constant on time DC/DC for ideas, just invert output.
If I was doing that, I'll try create something like NE555 with LTC1394, fixed OFF time, and adjustable current source for charging on time capacitor...
You can find some ideas in Linear AN94 for delay circuit.
So use pic just to adjust charging current.
EDIT:
Just replace R1 with variable constant current
http://www.electronics-tutorials.ws/waveforms/tim47.gif
And build NE555 on steroids...
Then ON time depending linearly on C and current. Off depending on R2 and C.
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