On May 2, 2005 05:30 pm, KY1K wrote:
> You're the second one to suggest a PIC digital solution.

...well, looks like I'm in good company to suggest a PIC, hehehe!

> I think it's not practical because you would have to clock the PIC at
> 10 Mhz (or higher) in order to get small increments at the 55 Khz
> output. Doesn't the clock frequency limit the smallest length of time
> that the PIC can be driven high?

...wrong direction, this is a digital attempt at an analog solution.
I'm suggesting to modify the actual running frequency using a varactor.

Run the PIC at External RC speed of about 220 to 240kHz and tap the PIC 
OSC2 output for your frequency out of 55...60kHz.

At the OSC1, to achieve RC speed of about 220kHz...240kHz, you need to 
calculate the appropriate values of R and C required for your PIC to 
run at 240kHz operation. The unmarked cap is a large C to block DC 
voltage between your varactor DC circuit and your PIC.
Some varactors will run at about 300pF when at about 1...3v, so you may 
need to add 1 or more in parallel, or modify your large cap, so that 
the series capacitance only tunes the appropriate range.

These are just numbers out of the blue for an imaginary 240kHz operation 
(assuming T=RC, or 1/240kHz=27k x 150pF), so you need to find the 
correct numbers for correct operation.

> >                               Vcc
> >                                |
> >                                Z = 27Kohm
> >                large R large C |
> >Cntrl Voltage>--+--WWW--+---|(--+--PIC (OSC1)=220..240kHz
> >                |       |       |
> >        large   _       _var    _
> >        cap.    ^       ^       ^ = 150pF
> >                |       |       |
> >               gnd     gnd     gnd
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