Johnny Chang ( snipped-for-privacy@gmail.com) ha scritto:
:: i'm trying to find a very inexpensive way to generate a 120Khz sine :: wave to act as a carrier wave for a cheap system.
Would you like to use that in a powerline communication system?
there is also a need for a 20Mhz clock for the processor. Would there be a way to convert this crystal oscillator's square wave into a
120Khz sine wave or am i better off building my own circuit?
There are a number. Unfortunately you can't divide 20MHz down to exactly 120kHz - the ratio is 166.66 : 1 which is not an integer ratio. 18MHz divided by 150 does give you 120kHz and you can buy 18MHz crystals off the shelf from Farnell.
Once you've got a 120kHz square wave you can low pass filter out the higher harmonics (360kHz, 600kHz etc) or you can feed the edge through a tapped delay line clocked at some high frequency and us the taps to build a finite impulse response filter, which is an easier way of reducing the lower frequency harmonic content, though you'll still need some kind of analog low pass filter to get rid of the higher harmonics.
Hope this helps.
-- Bill Sloman, Nijmegen
Since you have a processor, you could use the DDS technique of a phase accumulator to a lookup table driving a DAC. Without knowing what your instruction cycle time is and what kind of timer peripherals there are it's not possible to say how clean the output would be. You'd probably need at least enough of a filter to remove the phase accumulator switching time.
Advantage is that you can change the phase and frequency on the fly from the processor.
Tapped delay line?!? I do hope you mean one that's clocked!
You could use a 4017 and a pack of resistors to get your first approximation of a sine wave with several of the lower harmonics knocked out -- this would ease the filtering considerably.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
Sorry about that - I meant to type "shift register" rather than ""delay line". After all, tapped delay lines aren't clocked. You can do something vaguely similar with a tapped delay line and a pack of resistors - I've done it a couple of times - but it wouldn't make much sense to do it here, as tapped delay lines cost quite a bit more than CMOS shift registers
The 4017 is - in fact - a shift-register based "Johnson" counter
For this application you don't really need the decoding gates, though it makes the circuit a little easier to understand.
-- Bill Sloman, Nijmegen
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