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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

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  • \$\begingroup\$ 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... \$\endgroup\$ Mar 22, 2010 at 10:15
  • \$\begingroup\$ Linear or logarithmic ramp? \$\endgroup\$
    – endolith
    Mar 22, 2010 at 14:59
  • \$\begingroup\$ I think if you showed us the circuit that goes the other way, we could perhaps modify it to go the way you want. \$\endgroup\$
    – gbarry
    Nov 15, 2012 at 1:35

2 Answers 2

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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.

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  • \$\begingroup\$ Thanks Clint. The VCO is something that I will most likely use! \$\endgroup\$
    – jme
    Mar 23, 2010 at 6:52
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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.

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  • \$\begingroup\$ 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. \$\endgroup\$
    – jme
    Mar 22, 2010 at 8:53

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