frequency counter "check" function?

[xyzzy]

Greetings!

I recently acquired a couple of used frequency counters (Tek CDC250, Fluke 1925A) without manuals.  Both have a "check" function that's easily accessible enough that it seems like it must be something one would use often.  Selecting "check" on either counter results in a display of 10 MHz.  Obviously, this is the frequency of the internal oscillator.

My question is as follows: what useful information is this feature intended to convey?  If each counter only has one precision oscillator, how can it independently measure its own oscillator frequency?  Is the counter hard-coded to simply display 10.000000 MHz, or is it actually measuring something?

Thanks!

[Fraser]

You are correct if you were interested in accuracy... a counter that checks itself without an external reference of known accuracy cannot tell you anything about its accuracy.

BUT... the 'check' function is designed to test all the counter divider and processing chain for a 10MHz signal. It is a confidence check that the counter is actually functional. That is all.

A word of caution here as well. Many high quality frequency counters incorporate a 'Pico' fuse inside the input socket or on the input board. This Pico fuse can fail due to an RF overload, yet the counter will still appear OK in its 'check' mode. I had this on an Advantest frequency counter. The seller thought the front end was blown when in fact it was just a simple Pico fuse that needed replacing 



Regards

Fraser

[xyzzy]

BUT... the 'check' function is designed to test all the counter divider and processing chain for a 10MHz signal. It is a confidence check that the counter is actually functional. That is all.

That makes sense.  Thanks!

[SeanB]

I remember those fuses, available from 50mA to 5A AFAIK.