Envelop detection with low sample rate? I have a signal up to $3\textrm{ MHz}$. The ADC that sample it has a rate of $1.5\mu s$. So a full $T$ of the signal is $0.3\mu s$, and I can only sample each $1.5\mu s$. It sounds not enough, but I don't need to reconstruct it, but to create an envelop detector from it, over time, based on lets say $1000T$ (periods).

So, over $1000$ periods of $3\textrm{ MHz}$, I will have: $300 \mu s/1.5 \mu s=200 \textrm{ samples}$. From these $200$ samples, I need to create some envelop detector, or continues curve to then later check if it has some large amplitude changes.


How can I chose where to sample the input signal so that I can get the "right points" of it- means mostly its maxes, where my sample rate is much slower then the signal ?
Should I use a moving average to get this curve ?
Is there another good approach expect from taking more periods?($&gt;1000$)
Is the ADC sample-and-hold fast enough to even capture any envelope peaks or even half cycles?  If not, all bets are off.

If your ADC does have a fast enough capture time, then randomizing your sample times might be a good bet at capturing a few near extrema.  If you know, a priori, the approximate shape of your waveform, then Monte-Carlo sampling might allow one to estimate the waveform amplitude from the statistical distribution of enough sufficiently time-randomized samples.  Assuming the envelope changes slowly enough.