yes, I was serious about 100 MHz. I guess I really only need a 25 MHz bandwidth although 50 MHz would be better.
A little about my application:
I’m building wavelength stabilized lasers, and one way to measure their stability is to shine both lasers on the same photodiode. This works the same as adding any two sources with different frequencies, you get a beat note with the sum and difference frequencies.
I’m working with HeNe lasers at 633 nm, and I believe that I’m stable to the range of 1-5 MHz for short time scales. So, I’ll take one laser at 632.81600000 nm, and one at 632.81600667 nm, combine them on a photodiode, and get a 5 MHz signal out. I want to track the relative stability of the two lasers, so I want to convert his beat frequency to a voltage. A 25 MHz bandwidth gives me enough range for a larger constant wavelength offset between the two - wider bandwidth just gives me more range.
@analogon - thanks for that, it is an interesting idea. I guess I was hoping for a more integrated solution but I’ll keep this in mind.
I’ve seen a reference to using a philips SA568 PLL, and taking the error voltage as the measurement. This part seems not to be available, and I’m currently looking for alternatives. Seems that digital PLLs are more common - but perhaps I could use a comparator (schmidt trigger?).