Why does the waveform change below 18MHz in electron spin resonance experiments?

[Dogsnake]

Homework Statement

This isn't exactly a homework question, but my lab partner and I came across a phenomena whilst doing an electron spin resonance experiment, and I was wondering if anyone could help explain it.

The sample we found electron spin resonance for is 1,1-diphenyl-2-picrylhydrazil.

A glass phial containing the sample was placed between two Helmholtz coils, so that the B field was uniform, and within the output coil of a radio frequency oscillator. The drop in voltage due to resonance was measured by an oscilloscope in X-Y mode.

Whilst the radio frequency oscillator was set at 18-35MHz, the waveform shown by the oscillator at resonance was the first shown in the attached file (this was the expected shape). Below a frequency of about 18MHz for the radio frequency oscillator, the waveform looked like the second shown below (excuse my drawing skills, but that's pretty much what it looked like)

The question: Why has this happened?

A qualitative explanation would be fine, as we don't plan on doing experiments on the anomaly itself, we just need to know what's happening. I've spent a while perusing relevant textbooks and can't find a reference to it.

Thank you.

 

[turin]

What voltage are you measuring? You have a background B-field for a reference orientation, and you are perturbing the system with RF, much like NMR (MRI), no? Do you also have a pickup coil?

What does it mean to set an oscillator at 18-35MHz?

I will assume that X is input and Y is output. You can immediately determine from your Lissajou patterns that the output frequency of the top one is doubled, and the output frequency of the bottom one is tripled. I can't understand exactly what you're doing or measuring, though, so I don't know why you get that extra node.