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I am looking for a way to generate 10 to 20MHz TTL-compatible clock signal. I used something like microchip unit to produce clock signal before in school but I didn't use any other way to generate clock pulses. Someone suggests to use 555 timer but I one of the handbook I have told that 555 timer won't support clock frequency as high as 20MHz. Is that true? I need a pretty stable clock signal which will be run for more than 60 hours continuously once it is on. Any other suggestions?

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    \$\begingroup\$ It's too fast for a 555, if you don't need to vary the frequency probably a crystal oscillator module like this one is easiest digikey.com/product-detail/en/ECS-2200BX-200/XC1188-ND/827295 \$\endgroup\$ – PeterJ Dec 29 '13 at 2:03
  • \$\begingroup\$ Hi Peter, thanks for your reply. It sound like what I am looking for. From the spec, it said that the output is TTL compatible, so does it output square wave? Is oscillator stable and long life? Thanks. \$\endgroup\$ – user1285419 Dec 29 '13 at 2:14
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    \$\begingroup\$ It's a square wave minus a small rise/fall time you'll get with anything, there's a diagram in the datasheet link. Yes it'll be quite stable (exact numbers are in there as well) and I'd expect it to last for years. \$\endgroup\$ – PeterJ Dec 29 '13 at 2:18
  • \$\begingroup\$ @PeterJ, Sorry Peter, but I posted an answer along the sames lines as your comment. I was already typing it in when I saw your comment... \$\endgroup\$ – Ricardo Dec 29 '13 at 2:20
  • \$\begingroup\$ @Ricardo, no problems, am just on my way out the door and didn't have time to write an answer. \$\endgroup\$ – PeterJ Dec 29 '13 at 2:21
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What you want is a crystal oscillator module like this one from Farnel or the one PeterJ recommended. They come in 4 pin packages like in the images below. They will generate your clock for you.

Crystal oscillator image Crystal oscillator image Crystal oscillator image

Both modules mentioned have tolerances of 50 ppm and operate at 5V.

It's not to be confused with regular 2 pin crystal like this below. Those are part of the circuitry of external clocks for MCUs, for example.

Crystal oscillator

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    \$\begingroup\$ To avoid confusion, the two-pin device should just be called a crystal - with additional parts, the crystal becomes part of an oscillator. \$\endgroup\$ – Peter Bennett Dec 29 '13 at 2:29
  • \$\begingroup\$ @PeterBennett Right. I've edited my answer accordingly. Thanks for clearing that up! \$\endgroup\$ – Ricardo Dec 29 '13 at 2:32
  • \$\begingroup\$ Note that Farnell tends to be pretty expensive. You often can find cheaper parts at Digikey. \$\endgroup\$ – flup Dec 29 '13 at 9:54

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