Although less exotic than the SA and VNA tools I use during development, I find I make use of basic frequency counters pretty often (mainly to calibrate equipment).

A frequency counter does only one thing: measures the frequency of an RF signal.  What’s important for a good counter is frequency range, timebase accuracy, speed of measurement, and of course cost.  I try to only purchase counters with an OCXO frequency standard although a good TCXO is often sufficient.  An external reference input is very useful when you need very high precision so you can slave the counter to a higher accuracy lab frequency standard (OCXO, GPSDO, or Rubidium).

Having had quite a few counters, I’ve concluded that if you need more than ~200MHz, the older HP/Agilent counters offer the best value, especially if you’re willing to spend $200-250 on eBay for one with their excellent OCXO timebase option (4).  If you aren’t buying a counter with an OCXO, I would make sure to get one with an external 10MHz reference input so you can slave it to an external high-accuracy standard.  If a counter isn’t accurate, what good is it?

I have a couple of HP5385A counters that work to 1GHz; one has the TCXO timebase and is quite accurate; the other has the superior OCXO timebase; both are excellent.  I also have an HP 5386A with OCXO that is good to 3GHz.  For higher frequency measurements, I have an HP 5347A that integrates a power meter and frequency counter up to 20GHz, its maximum frequency resolution is 1Hz (plenty for higher frequency measurements); it only has a TCXO timebase so I almost always use it with an OCXO or rubidium lab standard; it is larger/heavier than the other counters, but it’s hard to get an affordable counter with that much bandwidth.

For counters that are no longer officially in calibration (per certificate), I use a Trimble Thunderbolt GPS-disciplined oscillator (GPSDO) to calibrate them annually.

Recently, many companies have started adding bonus frequency counter functionality to their arbitrary waveform generators; for example, my SDG2042X has a counter input that is good to 200MHz.  The counter’s internal reference is better than 2ppm, but it supports an external 10MHz reference so I slave it to an external DOCXO standard for higher accuracy.  It requires around -10dBm (200mV) at 200MHz, less at lower frequencies: -20dBm (63mV) works at 35MHz.  Overall, it does quite well up to 200MHz and has some nice features lacking in older counters such as min/avg/max and sdev, duty cycle reporting, deviation from a reference in ppm, etc.