FAQs from Rohde & Schwarz

Scope rider shows strange Waveforms


With my Scope rider I am analyzing a rectangle signal with a repetition rate of 500 KHz:

To see the envelope I use slower time bases. Down to 20ms/div everything is correct. But when I use slower values i.e 50 ms/div to 200 ms/div, the waveform which is shown on the display looks very strange:

What is the reason for that?


The effects are caused by undersampling.

At 100 ms/div the sample rate of the RTH is just 125 kSa/s, at 50 ms it is 250 KSa/s. This is too low to detect a signal of 500 KHz correct.

The sample rate increases to 2.5 MSa/s at at timebase of 20 ms. For a square or a sinus signal this would be enough to detect it correct.

Even at such a high sample rate, undersampling may occur if pulse signals are evaluated. If the length of the pulse is too small, even 2.5 MSa/s may be not enough.

The distance between two sample points is 400 ns. Pulses which are in that range or even shorter, are not detected correct.

The attached screenshot shows an example. The pulse length is just 100 ns.