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I am wanting to design a circuit that uses an astable multivibrator as a clock source. However the difference being that over time the clock is gradually sped up until it reaches some multiple frequency.

I have found circuits that have a speed that that decreases to zero (an addition of a cap) but not one that speeds up.

Any ideas or help to point me in the right direction would be great! Thanks jme

endolith
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jme
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  • Some more details would help. The min and max frequency, how fast you want it to ramp, once it speeds up does it just stay at the max, do you just need a square wave output, how accurate/stable/linear does it need to be... – Clint Lawrence Mar 22 '10 at 10:15
  • Linear or logarithmic ramp? – endolith Mar 22 '10 at 14:59
  • I think if you showed us the circuit that goes the other way, we could perhaps modify it to go the way you want. – gbarry Nov 15 '12 at 01:35

2 Answers2

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One approach would be to use a voltage controlled oscillator (VCO) and a ramp generating circuit.

The CD4046 has a VCO block in it or there are some function generator ics, like the MAX038. You'll need to find something that covers that range of frequency you need. To produce a ramp, it could be as simple as a capacitor pulled up (or down) to the input of the VCO.

If you need a linear ramp, then driving the capacitor with a current source would be more suitable.

Clint Lawrence
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Do you mean something that ramps the frequency up, from zero, until it reaches a maximum value? I'd use a small MCU for that, like a PIC or AVR.

Leon Heller
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  • Thanks for the quick reply. Yeah that's exactly what I wanted and I could easily program it but I don't want to do it with a MCU! I cannot find any circuits that do something like this automatically. – jme Mar 22 '10 at 08:53