enveloping curve - LTPSICE
Then run that signal through a rectifier, or even better a precision rectifier:


Source: https://www.electronics-tutorial.net/analog-integrated-circuits/precision-rectifier/precision-full-wave-rectifier/

Then put a low pass filter on the output (vo) with a pole that is lower than the frequency of the envelope. You may also want to put a voltage buffer/follower between Vin of the precision wave and the voltage point that you want to measure the envelope. 

OR

Use the AM modulator that is built into LT spice