Hi,
70 MHz +/- 20MHz... so max is 90MHz.
If this is a purely sine signal, then according no nyquist you need more than 180 MHz sampling rate.
200 MHz is OK, but 400 MHz is better.
If the waveform is not sine, then you need to determine what max overtone frequency you need to detect. Then the sampling rate needs to be more then twice the overtone frequency.
400 MHz sampling rate is high, check if you can use "undersampling technique".
For digital signals I recommend to use comparators and a pure digital sampling.
Klaus