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I am building a 50MHz clock by using ABRACON ACOL-50-EK crystal clock oscillator. I have connected 3.3V supply (with a 0.1uF capacitor to ground) to Vdd (pin 14). GND (pin 7) is grounded and output is from OUT (pin 8). I am getting a sine wave and I do not think that would be a good clock signal.

Unfortunately ABRACON has not given any circuits that I could refer to and I got the capacitance hint from another circuit around a different oscillator manufacturer. [Before I connected the 0.1uF capacitance, I was getting ~25KHz oscillation which made me think, may be I got the wrong part - that went on for a while]

I suspect many of you have done this before, so what am I missing ? How is the 15pF output capacitance figure into the design ? How is the 0.1uF calculated ?

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  • \$\begingroup\$ What sort of scope / settings / leads etc are you using to test and do you know about calibrating them? I'd normally expect a square-wave output from one of those it sounds like it may be more of a measurement error. Probably not required but it may also be worth trying it with a small load such as a 1K resistor between the output and ground to see if that makes a difference. \$\endgroup\$
    – PeterJ
    Aug 23, 2014 at 4:06
  • \$\begingroup\$ Peter is correct. Check your scope's bandwidth and bandwidth limit. If your bandwidth limit is set to 20MHz, or your scope's bandwidth is below the first several harmonics of 50MHz, you will not see something that looks like a squarewave. \$\endgroup\$
    – John D
    Aug 23, 2014 at 4:15
  • \$\begingroup\$ There's not much to these things-- hook up clean power of the correct voltage and they just work. You did notice that the pin diagram is shown from the bottom? If your scope probe is set to X1 the BW may only be 20MHz and you'll see a sine wave. \$\endgroup\$
    – Spehro Pefhany
    Aug 23, 2014 at 4:16
  • \$\begingroup\$ This unit is square wave output as data sheet. Some less common unit does have sine output. \$\endgroup\$
    – EEd
    Aug 23, 2014 at 6:09
  • \$\begingroup\$ @PeterJ You are spot on. As I wrote below to alex, I am on a 100MHz scope. How do I check the waveform now ? I have a 10X on my probe, not sure what that does though. And if I had a better scope, what does it mean to have CMOS compatible clock ? What voltage levels should I see ? I guess I can match the voltage levels to the sine wave I am seeing now ? \$\endgroup\$
    – banal
    Aug 23, 2014 at 15:48

1 Answer 1

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If you are using a 100 MHz scope, then a 50 MHz square wave will look like a 50 MHz sine wave as all of the harmonics will be killed (150 MHz, 250 MHz, 350 MHz, etc).

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  • \$\begingroup\$ Yes I am on a 100 MHz scope. Damn, I did not think about harmonics in a square wave at all. I guess I am out of luck with this scope ? Is there any other way to check make sure the waveform is good ? I do not want to fry the micro. \$\endgroup\$
    – banal
    Aug 23, 2014 at 15:44
  • \$\begingroup\$ So long as the voltage is not too high, it should be fine. \$\endgroup\$
    – alex.forencich
    Aug 24, 2014 at 3:26

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