A long time ago, I overhauled an 8-bit digital frequency meter composed of integrated circuits and discrete components. I think the idea is the same.

1, with a high-precision crystal oscillator inside, generating a reference time (for example, 1 microsecond);

2. Gating signals (such as 1 ms, 10 ms, 100 ms, 1 s) are generated by the above high-precision crystal oscillator;

3, amplifying, limiting and shaping the external measurement signal to obtain each falling edge;

4. Measurement method: according to the measured frequency, automatically or manually select the measurement frequency or measurement period (and then convert it into frequency);

5. Self-check: this machine is used to detect the frequency of high-precision crystal oscillator in the machine;

(In fact, it is how many times the pulse of the reference time has passed during the time of recording a gate signal).

One more thing: the self-calibration function only checks the logic, counting and display functions of the machine.

Can not be based on the accuracy of high-precision crystal oscillator inside the calibration machine.

If you want to check the accuracy of the internal crystal oscillator,

Then you need a crystal oscillator with two orders of magnitude higher accuracy than the local crystal oscillator.