Perhaps you never got a straight answer because it's one of those things that is so simple it's hard to explain. Frequency counters used to be kind of a big deal, but nowadays they can be built very cheaply and are often part of DMMs.

What it does is simple, it counts the number of times the input signal cycles. This tells you a few things right away. If you want to count a frequency accurate to 1 Hz, you need to count for an entire second. If all you need is 10 Hz accuracy, then 1/10th of a second will do. Simply count the cycles for 1/10th of a second, multiply by 10, and the result is the frequency in Hz.

Basically, then, the counter needs some sort of imput circuitry that allows it to not load the circuit under test, some kind of comparator or zero crossing detector, and some counting circuitry, as well as a way to display the result. It also needs an accurate time base to tell it how long to count.

This used to take loads of flip-flops and LED drivers, but today low cost microprocessors like the PIC are fast enough to count up into the low VHF range, so a counter adequate for HF work can be built for about $15 using this sort of part (and some looking around to get a good price on an LCD).

Calibrating the receive frequency is a little trickier. You need a signal source of known frequency. One way to do this is to use some sort of oscillator and use a counter to verify it's frequency, then listen to it on the radio. "Listen for what" is a little harder question, since it depends on the mode you are using and the particular radio. In practice, it may be hard to get closer than a few hundred Hz without special equipment.

Hope this helps.


Link:  WB8RCR's PIC Counter