Go to digikey.com search for oscillator.
Select oscillators, or VCO
Pick 12.8mhz
There are 5,000 oscillators and 150 VCOs (have fun)
I have a frequency counter with a 10mhz xtal that has a “trim” capacitor. I calibrate with a short-wave radio. The 10mhz osc and WWV radio can be heard at the same time. The difference in frequencies can be heard as a tone. When the difference gets below 30hz it is hard to here. Look at the file strength meter or listen for background noise to here below 20hz. I usually get the field strength meter to swing up/down in 10 seconds. 10,000,000.1 the .1 makes the 10 second beat.
Are you making a 2.5ghz 8 digit counter?
2.400,000,0ghz the fastest you can count. Counting 100 times/ second.
240.000,00mhz 10 counts/second
24.000,000mhz 1 count/second
To display 50.7hz you do not need an accurate counter you need to count for 10 seconds.
In 100 seconds you can count 50.71hz.