I'm lately fascinated with the whole "beat" concept, and have been doing some experiments. I'm trying to capture the beat waveform on an oscilloscope in run mode, but am having no luck. I'm putting a 30MHz and 37MHz tone into a combiner, then into one channel on the scope. I can only capture the beat waveform when i do a single shot capture. It's exactly what i want to see, the nodes are 7MHz apart, but i want to see in in run mode so i can vary one of the frequencies and see the waveform react. None of the triggering methods i've tried will allow me to do this. Is it even possible? If not, why? At least that would give me a better fundamental understanding of the oscilloscope's capability.
The beat frequency is a half of the difference (see Wikipedia here), so in your case $3.5MHz$. This should be your triggering frequency to see the beat. Yet, in your case, neither $30$ nor $37$ is a multiple of $3.5$, so it is no surprise that your don't see a static picture. Try making one of the frequencies a multiple of the half of the difference, feed this frequency to the second channel and trigger off it. For example, $30$ and $36MHz$ will beat at $3MHz$. Then $30MHz$ will be a multiple of $3MHz \,(3 \cdot 10 = 30)$ as well as $36MHZ \,(3\cdot 12=36)$ . Feed the beat to the first channel and also feed $30MHz$ (or $36MHz$) to the second channel and trigger off it.