The output of a simple mixer contains the sum and difference of its two input frequencies.
However, like amplitude balance, the phase difference is sensitive to the input frequency and typically will vary a few degrees.
This appears to be a distant co-channel user on the same input frequency as Repeater 7.
From these recordings, one can calculate the electrical impedance of the muscle at a given input frequency.
Both resistance and reactance depend on the input frequency of the signal.
For input frequencies much lower than corner, the ratio is small and therefore the phase angle is close to zero.
When the input frequency is high, S-Transform have a better clarity in the time domain.
The oscilloscope's response will drop off rapidly as the input frequency is raised above that point.
Note the modulation at twice the input frequency and the elevation of mean activity level.
As each is tuned to a slightly different frequency, one of them will resonate at a time, according to the input frequency.