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Looking for used frequency counter recommendations

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alm:

--- Quote from: Wallace Gasiewicz on September 30, 2023, 11:58:43 am ---You can indeed find older freq counters that work just fine and will accept an external time base.   
But one of your problems is the small voltage signal you wish to measure. Many counters do not measure minute signals.Besides that counters only measure the biggest signal they area exposed to and there may be off the air signals in the radio that may confound your efforts.

--- End quote ---
20 mV is not extremely small either for a general-purpose counter. All the counters I mentioned in my post have a sensitivity of < 20 mV or better below 100 MHz. (PM6672 < 15 mV up to 75 MHz I think). Shielding may certainly be a concern at this level, especially since the counter inputs are often high impedance for these low frequencies.

BillyO:

--- Quote from: alm on September 30, 2023, 12:26:42 pm ---
--- Quote from: Wallace Gasiewicz on September 30, 2023, 11:58:43 am ---You can indeed find older freq counters that work just fine and will accept an external time base.   
But one of your problems is the small voltage signal you wish to measure. Many counters do not measure minute signals.Besides that counters only measure the biggest signal they area exposed to and there may be off the air signals in the radio that may confound your efforts.

--- End quote ---
20 mV is not extremely small either for a general-purpose counter.

--- End quote ---
Agreed, that Data Precision 5845 and my HP 5302A have both been verified to work down to below 10mV.  Although the HP is only specified to 25mV.  That said I don't think I tested them at the OPs required frequency.

Wallace Gasiewicz:
Thank you for clearing up the sensitivity issue. I have not used these expensive freq counters, just run of the mill older HP stuff.

 I have en entirely different suggestion. For the money you are willing to spend perhaps purchase a good spectrum analyzer. This will give you a good frequency readout and has multiple other uses in radio repair.Another suggestion is a Service Monitor like the HP 8920-7835 Family of Service Monitors.  I use an HP 8935 a lot repairing Ham radios. I do not use a freq counter much at all any more.This way you get a spectrum, signal generator that can be audio modulated and a really good freq counter.  It also has a swept freq gen for the SA. 
It is sensitive enough to pull off the air signals well and it is sensitive enough to use a 10X scope probe to probe for signals on a board. 
Audio generator for use with mic input, very useful especially in evaluating SSB signals. Mine has two audio generators. You can put two tones into the mic for some adjusts or use one for modulating the RF for testing Receive and one for the Mic input.You can AM or FM modulate your signal.
They also usually have an input for an external reference. 
The scope function is not really very useful however.

I find the Service Monitor almost all you need for servicing most radios.

https://groups.io/g/HP8924-Family-Spectrum-Analyzers/messages

nctnico:
The downside of using a typical spectrum analyser I see is that it has 50 Ohm inputs only. So you will be loading a signal if you connect it directly and using a standard 10x probe won't work. You'd need a passive Lo-Z probe or an active AC probe (DIY models can be bought cheaply) to get the signal into the spectrum analyser.

Besides that I'm not sure you get more than 4 to 6 digits of resolution. I think this should be enough for the OP's purposes as there will likely be drift over time as well.

OTOH the dynamic range and other use cases are plenty for a spectrum analyser. I'm not into radio at all but even I use my spectrum analyser a lot more compared to my frequency counter. Mainly because the oscilloscopes I use can tell me the frequency of a signal with enough precission in most cases.

Wallace Gasiewicz:
I use a standard 10-X probe all the time with my HP 8935. Seems to work fine.  I realize the magnitude of the signal is not easily quantifiable  however. I have readings down to one Hz, so easily 9 digits, since the primary bandwidth is one GHz. 8 digits below 100 MHz.
I service radios all the time. The 20 mV level signal is not sometime I see very often. It seems very low for a reference oscillator. Perhaps a mention of the radio involved with perhaps a schematic of the circuit in question?
My oscilloscopes give me a freq but my Agilent 54820 scope is not terribly accurate in freq readout. And it does not have an Ext ref input. 
Any freq counter is only as accurate as its reference generator and the ones in scopes seem to have a lot to be desired if you wish to have a really accurate freq.The same goes for spectrum analyzers. freq is dependent on what reference they use. Many older SA had very good OCXO.

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