Electronic – Why is 30MHz Clock signal distorted on oscilloscope


I'm trying to probe a 30MHz clock signal, coming from an FPGA and routed to a CCD sensor.

There are no other components, other than connectors, between source/destination.

When I probe this signal with my oscilloscope, I get a distorted waveform which over shoots and undershoots. The probes I am using are the standard tektronics probes that come with the MSO2024B that we have. They have been calibrated to the scopes 5khz test point and show no overshoot.

Distored 30MHz Clock Signal

The clock signal should be 3.3V peak to peak.

The scope probe ground clip is connected to the 0V reference point near the regulator for the 3v3 rail.

I am investiagting why we are seeing noise on our power supplies after the FPGA has powered up and starts providing the 30MHz clock to the external CCD.

Best Answer

There are two issues here: Bandwidth and Measurement technique.

Bandwidth: Your measured signal is bandwidth limited. A 30MHz square wave has lots of harmonic content and your scope has a relatively low bandwidth (200MHz) so the higher frequencies are attenuated and you only see the sum of the first few odd-order harmonics (30MHz, 90MHz, 150MHz, ...).

Here is a square wave approximation made up of the 1st, 3rd, and 5th harmonics (taken from this question which looks quite a bit like the trace you posted: square wave approximation

Tektronix has a Technical Brief on this subject that might be of interest:

Understanding Oscilloscope Bandwidth, Rise Time and Signal Fidelity

Measurement Technique:

You are using a rather long ground clip which contributes to overshoot. This answer to another E.SE question illustrates the improvements in measured overshoot by using a spring clip on the scope probe tip instead of the longer ground tip.

Before: Initial measurement

After: Measurement with spring tip

Here is the measurement setup: Measurement setup

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