The Hewlett-Packard (H-P) 5342A Microwave Frequency Counter was introduced in the 1978 Hewlett-Packard catalogue, and even by the H-P standards was an extraordinary instrument. It is able to count frequencies anywhere between 10 Hz and 18 GHz, with completely automatic scaling. How did it work? Well, that’s complicated. For complete details, the full 436 page “5342A Microwave Frequency Counter Operating and Service Manual” [PDF] is available on-line. Basically, the counter used conversion to an intermediate frequency with which its ECL native counter could keep up, mixing a local oscillator with the input frequency and scanning automatically to find the signal, with two offset frequencies and averaging allowing finding frequency modulated signals without aliasing.
Here, a vintage counter is upgraded by installing the crystal oven option for better stability and re-calibrating the counter against an H-P cæsium frequency standard (atomic clock).