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Help our fsk modulator requiring high frequency accuracy

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Newbie level 3
Jan 20, 2011
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Want to create a ultrasound fsk modulator ( sine wave ) with fo at 20KHz and f1 at 20.5KHz

First, we used XR2006, but its frequency accuacy is only 1%, which is not good enough for our project,

Then, we changed to use a avr mega microntroller, but its pwm output is too slow.

Now we start to think about using mcu + DAC ( high speed ), letting mcu output more than 30 values during a sine circle and DAC translate it to voltage. Is that a good design> We are not sure.

We wonder whether there is a more precise XR2206 in market or any other popular solution for problems like this.

Thanks for any suggestions.

Do you need the source to be a sine wave really? Or would
it be enough to work with digital and simply filter the higher
harmonics at the end?

If the latter, you might find a programmable down counter
from a higher, higher accuracy reference could give you
the two tones simply by toggling the right bit. I've made
10MHz 16-bit counters from 74ACxx counters and logic
comparators; I bet a uC could do as well for speed. Use
one input pin to decide, every carry, whether to count
down from 500 or 488 on a 20MHz beat, followed by a
squaring FF and low-pass filter.

Or you might be clever and figure out how to get two
bits rigged right for a poor man's DAC and a "modified
sine wave" output which would be more cleanly filtered.

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