I am sending the clock signal of a DE0-Nano FPGA to one of its GPIO pins. As I increase the frequency, the signal becomes more and more distorted. Is this an issue with the GPIO pins having small buffers? My scope has a bandwidth of 200MHz and my probes are also rated for 200MHz. What might be the reason for this?

Ultimately I want to modulate data on top of this clock signal.

1) Is there a way I can output a clean clock signal at a high frequency like 100MHz?

2) Can I modulate a clean square wave of data on top of this clock signal?


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  • 3
    \$\begingroup\$ Quick test: run all the signals at 20 ns/div and look at the rising edge. I bet they all look the same. \$\endgroup\$ Jul 10, 2017 at 21:05
  • \$\begingroup\$ What type of modulation? \$\endgroup\$ Jul 10, 2017 at 21:18
  • \$\begingroup\$ Your question is not how to output the clock, but how to correctly capture it. Please look up the Nyquist sampling theorem - you will learn that to see a clean 100 MHz signal with likely 1-ns edges, you would need at least a 500-MHz or 1 GHz scope, with at least 2 Gsps sampling rate. \$\endgroup\$ Jul 10, 2017 at 21:34
  • \$\begingroup\$ My scope samples at 1GS/s and is rated to have a bandwidth of 200MHz \$\endgroup\$ Jul 10, 2017 at 21:36
  • \$\begingroup\$ For the modulation I want to output either a low or high voltage for every rising clock edge. \$\endgroup\$ Jul 10, 2017 at 21:37

1 Answer 1


Fist, a 200 MHz oscilloscope and a 200 MHz probe will only give you a system bandwidth of 141 MHz (ish).

We talk about calculating "system bandwidth" on our blog here: https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2016/09/01/what-is-oscilloscope-system-bandwidth-and-how-do-i-find-the-bandwidth-of-the-scope-probe

Basically, the bandwidths compound.

Second, a bandwidth of 141 MHz means you can a 141 MHz sine wave with 3dB or less of attenuation. So, for a 100 MHz clock, it's not going to look like a square wave.

This app note actually looks at a 100 MHz clock example on page 7 and talks about how to evaluate your oscilloscope's bandwidth for your app.


The ringing you're seeing is probably some inductive or capacitive effects of your probing system.


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