The fundamental wave of 300mV signal is 100Hz, which can be filtered by a second-order low-pass filter and only the fundamental wave is retained. The cutoff frequency can be designed in the range of 100Hz. The specific circuit is as follows:

The output of the low-pass filter is connected with a hysteresis comparator, which can output a square wave. The specific circuit is as follows:

It is not difficult to amplify a signal of 300mV, and an inverse proportional amplifier can be used to maintain harmonics. The key is to determine the effective signal bandwidth and choose the filter with appropriate bandwidth. The specific circuit is as follows:

If it is converted into a digital signal for harmonic analysis, firstly, it should be sampled by AD, and the sampling frequency is 4~ 10 times (at least 2 times) of the effective bandwidth. Considering that harmonic analysis needs FFT transform, it is suggested to use DSP as the main processor, and the specific algorithms are ready-made