I checked the BK precision 1856D manual. I found that although this frequency counter can measure upto 3.5GHz, it has 2 separate input channels to measure two range of frequencies.
One input to measure between 0.1Hz to 100 MHz and the other to measure between 80MHz to 3.5GHz. Our beat frequency desirable range is <100MHz for stable ALS. Also, the beat PD response falls off beyond ~150MHz . Should we be happy with this frequency counter and use it in the 0.1Hz-100MHz range or look for one with a better measuring range?
P.S. Right now we are using the spectrum analyzer in the control room set to frequency range from 10MHz - 140 MHz for beat note search.