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I am a newbie to circuit design and I need a 50MHz clock. I went to a local electronics store and got a 2-pin radial Quartz crystal. I do not have a data sheet for this. The only markings for the part are on top that says "FS50.00" [the double quotes are mine]. The packaging seems to be referred to as metal can (long rectangle with round sides).

Does anyone have any ideas which part this might be and how to make this thing oscillate ? I have tried some circuits with a resistor at 3.3v but it does not look like I am getting anywhere.

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  • \$\begingroup\$ Hi and welcome to EE. What do you need this clock for? Is it for a microcontroller like an arduino maybe? If you add some details helping you might get easier. \$\endgroup\$ Commented May 14, 2014 at 17:13
  • \$\begingroup\$ @Vladimir, this is a peripheral clock. \$\endgroup\$ Commented May 18, 2014 at 1:31

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Chances are pretty good that a "50MHz" crystal is actually a 3rd overtone crystal, and if you make a simple Pierce oscillator (eg. with a 74HCU04) it will oscillate at 16.6667 MHz. If you only want it to oscillate at some frequency, not 50.000 MHz, then you can use the common circuit (load capacitors, some M ohm bias resistor, 1/6 of an unbuffered gate).

To get the 50MHz, something like this should work, if it's a 3rd overtone crystal:

enter image description here

You will want to reduce the inductance to more like 8 turns, some experimentation may be required. Circuit is from here.

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  • \$\begingroup\$ Thanks for posting a circuit for this. I do not have a 74HCU04. So you think the crystal I have is a 5V part ? Also is the output across La ? Are all capacitors ceramic ? \$\endgroup\$ Commented May 18, 2014 at 1:30
  • \$\begingroup\$ Is it possible to pull this off without inductors ? Yes I have been reading up on this and I also think this might be a third overtone, but still do not understand how the above circuit and others work exactly and how specific capacitances are picked for a given crystal. \$\endgroup\$ Commented May 18, 2014 at 1:38
  • \$\begingroup\$ If you want it to oscillate at 50MHz you need to force it to, which means that the fundamental mode has to be suppressed by some kind of frequency dependent behavior outside the crystal. The easiest and most practical way is with a small inductor, though there are other ways (expensive high speed op-amps using active filters). Or just buy a crystal oscillator for a few dollars. \$\endgroup\$ Commented May 18, 2014 at 13:20
  • \$\begingroup\$ Correct me if I am wrong (a) f0 in this circuit is the oscillation frequency of 31MHz with the fundamental of the crystal at 10.66 MHz. (b) Cb and La have LC oscillation at 10.66MHz and essentially ground the fundamental oscillations. If a and b are correct, then my calculations suggest La should be 2.194 uH for 16.67MHz. How are you getting 8 turns on La (which I am assuming would make it 0.62uH) ? \$\endgroup\$ Commented May 18, 2014 at 17:49
  • \$\begingroup\$ @banal You would want the circuit to be resonant at the 3rd overtone frequency of 50MHz, not at the fundamental. The number of turns may not be exactly correct- inductance is proportional to number of turns squared, but there's some parasitic inductance there too. It could be 9 turns. \$\endgroup\$ Commented May 18, 2014 at 18:07
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Getting good performance out of a 50 MHz clock, particularly on a breadboard, can be tricky. I suggest you buy a clock oscillator, ready-made. Try eBay, and search on "50 MHz clock", or try Digikey or Mouser. If you're going the eBay route, you should search the data sheet for the model to make sure it uses the proper supply voltage.

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  • \$\begingroup\$ Yes the shop was asking if I need a 4 pin one, but I did not know the difference between crystal and crystal oscillator ! But then again the 4pin oscillator did not have data sheet either. Do you know of any programmable oscillators that can generate any clock in a range so I do not have to design circuits every time I need a clock ? \$\endgroup\$ Commented May 18, 2014 at 1:34

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