Reciprocal Frequency Counter Mean
I don't know how the SR620 does it, but this is how I suggest doing it. 

Suppose you want at least 10 readings per second. Divide the 35000+/- frequency by about 3500. Maybe you pick 4096 or 2048, say the latter. That gives you an edge every 58.514285714 ms, dividing by 2^n where n = 11. 

Use that with a reasonable clock, say Fosc = 25MHz. You will have  1462857 clocks per cycle, giving you solid 6-digit resolution at 17Hz sample rate. If you need more resolution, increase n or Fosc. If you increase n you reduce the sample rate, of course. If you increase Fosc the logic has to work faster and the sharpness of the edges on your 35kHz becomes more important. 

To get the measured frequency F from the number of clocks k, 

F =\$\frac{F_{OSC}\cdot 2^n}{k}\$



I see you've added the clock frequency to your question- at 2GHz, the edges of the input signal have to be very sharp, the comparator very fast, the reference voltage and signal amplitude very stable etc. to get full benefit from the high clock frequency.