Here's a different way I checked my frequency counter. I was lucky to have a counter that will work low enough into the AF Ranges.

Originally I would need to use a signal generator, along with a receiver to zero beat onto a known frequency standard. In most cases this would be Radio 4 on 198Khz Longwave. It was never the easiest of things to do, with coupling cables etc.

This way was so simple to check if there was any drift after 2 years from my original calibration I did.  Inside is the local VXO that the counter uses as the reference frequency for comparing with the sample frequency your checking.

This approach is good enough for the resolution my frequency counter supports, but may not be suitable for other types with higher resolutions.