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The word "limit" should not be interpreted as a fixed, discrete boundary that separates "useful" from "useless". The world is analog, so it's best to consider what is really meant in a given circumstance.

For a frequency counter to function properly, all you need is to preserve the zero crossings and some minimum amplitude, so that's a fairly forgiving application. That said, I am surprised to hear that your counter's 22GHz input uses a BNC. High-frequency counters often have a couple of connectors, one covering up to a couple of GHz and another above, but I've never seen or heard of a BNC being used in a commercial instrument for 22GHz -- could you let us know the instrument model? Or is it a homebrew counter?

That aside, in other situations you might need to worry about impedance mismatch. And that's where a typical BNC connector has unfavorable characteristics as you increase frequency. The instability of the ground connection is also a problem (particularly in mobile applications, where the modulation of impedance can cause the BNC to act as a surprisingly good vibration sensor). BNCs are also manufactured to fairly loose tolerances, so if the application demands accuracy and precision over a wide frequency range, BNCs aren't going to be a good choice.

So, it's not as if signals abruptly stop emerging from the BNC once you go above, say, 4GHz. It's more complicated than that. And whether a BNC is useful or not in a given circumstance depends very much on what performance you need in that particular case. The frequency "limit" will vary accordingly.

-- Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070