How to calculate the instantaneous frequency of a signal?
Instantaneous frequency is a poorly defined concept over wide-band (single channel) data, but can be a useful estimation given narrow-band signals of sufficient duration.  One trick for using an instantaneous frequency estimation algorithm which is usefully accurate only in one frequency band is to resample your data to move the frequency band of interest into the bandpass of the algorithm.  You may need to bandpass filter your data before resampling so that the source band ends up entirely within the algorithm's bandwidth.

In your case, you can try high-quality resampling your data to double the sample rate to extract instantaneous frequency estimates from 50 to 100 Hz.  Triple for 75 to 150, etc., assuming the sample rate and anti-aliasing mechanism captured spectrum in those ranges.