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I'm looking to create several sine waves on a single circuit. All must be under 20Khz frequency and each must be unique. Mostly it will be 5-10 frequencies needed.

As I found - almost all crystal oscillator are in Mhz frequencies and only one kind is 32Khz (which is still too high).

I should be able to get this wave on the other side using FFT.

Ideas? :)

Thanks

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    \$\begingroup\$ How about the expected spectral purity of each sine wave? That parameter could lead to different solutions. \$\endgroup\$ – Mario Vernari Jul 21 '11 at 11:37
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    \$\begingroup\$ Do you want the frequencies to be fixed or adjustable? \$\endgroup\$ – Jim Jul 21 '11 at 15:23
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    \$\begingroup\$ What's important to you? Frequency stability? Frequency accuracy? (5.000000 kHz) Signal to noise ratio? Distortion? Are you doing multitone testing? \$\endgroup\$ – endolith Jul 21 '11 at 16:48
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You could easilly divide the 32KHz crystal frequency with a Binary Counter (such as the 4040) to give 16KHz, 8KHz, 4KHz, 2KHz, 1KHz, 500Hz, etc...

Then some clever filtering can create a sine(ish) wave from each of those square waves.

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    \$\begingroup\$ Then you can better start from a 12MHz crystal. Very common value, and many more dividers. These are the first multiples of 1kHz you can get from 12MHz: 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20, 24, 25, 30, 32, 40, 48, 50, 60, 75, 80, 96, 100,... \$\endgroup\$ – stevenvh Jul 21 '11 at 10:55
  • \$\begingroup\$ Also, a 32kHz crystal is actually 32.768kHz, so you don't get the nicely round numbers from your answer. \$\endgroup\$ – stevenvh Jul 21 '11 at 11:57
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    \$\begingroup\$ You get nice round binary numbers ;) 16,384Hz, 8,192Hz, 4,096Hz, 2,048Hz, 1,024Hz, 512Hz... \$\endgroup\$ – Majenko Jul 21 '11 at 12:03
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    \$\begingroup\$ Yes, but for frequencies binary doesn't make sense like it does in addressing, for instance. Note that most of the time the 32.768kHz is just used to get a 1Hz signal. (yes, I noticed the smiley!) \$\endgroup\$ – stevenvh Jul 21 '11 at 12:55
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    \$\begingroup\$ @roman - What Matt didn't tell you is that it's not that easy to filter out a sine from a square wave, especially not when it has to be done for multiple frequencies. I'm going to leave this to him to answer :-). (The DDS would have created a nice sine) \$\endgroup\$ – stevenvh Jul 21 '11 at 15:08
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Similar questions were asked here and here.

In this answer I talk about DDS, direct digital synthesis, which has replaced classical analog oscillators like Wien bridge. The DDS technique is crystal-based so has the same stability and accuracy.
Here you'll find a design for a simple DDS. DDSs which use special function ICs typically achieve a wide frequency range with very high frequency resolution.

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