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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.

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  • $\begingroup$ I'm not sure I understand the question. You're feeding 7MHz into the scope and it shows 7MHz. Changing the frequencies will show the 7 MHz signal changing. Run mode should still trigger happily on the 7 MHz signal. You could feed the 30 and 37 MHz signals into other channels of the scope, and see them all vary side by side. Or are you trying to say that the 30/37 signals are not steady? They can't be when 7MHz is the trigger signal. Please clarify the question. $\endgroup$ – hdhondt Aug 20 '19 at 5:00
  • $\begingroup$ You could build a detector. That is, a circuit whose output follows the amplitude (a.k.a., "envelope") of the input waveform. Then you could trigger off that. $\endgroup$ – Solomon Slow Aug 20 '19 at 13:45
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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.

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  • $\begingroup$ According to Wiki and other sites, the beat frequency is just the difference of the two tones. Either way, I'm going to try as you suggested, putting 30 and 36MHz tones into the scope and triggering off a 3MHz, 30MHz and 36MHz signal to see what happens. $\endgroup$ – wesley delk Aug 20 '19 at 10:03
  • $\begingroup$ Both views are correct, of course. From Wiki: "subjectively, the frequency of the envelope seems to have twice the frequency of the modulating cosine" - And yes, this shouldn't matter anyway. Good luck! $\endgroup$ – safesphere Aug 20 '19 at 14:41
  • $\begingroup$ Actually with 30 and 36, the modulated frequency will be 33 that also is a multiple of 3, So you can try just triggering of the beat itself as well rather than of the second channel. The frequencies of course should be pretty precise. Can you also test in Hz or kHz instead of MHz? $\endgroup$ – safesphere Aug 20 '19 at 14:58
  • $\begingroup$ My signal generators only go down to 100KHz, so I tested at 100 and 110KHz and was able to capture the beat signal in run mode. I was triggering off another channel with a function generator set to 100Hz (not sure why, it was already set to that). I guess without syncing all my components together, 30MHz was just to fast and not as precise between sig gens. $\endgroup$ – wesley delk Aug 20 '19 at 18:33

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