Hey y'all,
I'm thinking it'd be a fun (steep) learning project to build a tracking gen for my HP 8566B. I figure a sane goal is to cover the first two bands, 0-2.5GHz and 2GHz-5.8GHz.
Now I'm a total RF n00b, so please bear with me, and mind that the point here is to learn, more so than to get the best quality tracking gen at the lowest cost - whether in time or money. So, I'm thinking a "discrete" design, with a PCB and modern RF components is the way to go (for me), rather than plumbing together existing RF modules.
This approach will get you up and running on the 0-2.5 GHz band. Also see Dan Doberstein's notes
here.
The high band requires a completely different conversion scheme. There will be few if any opportunities to reuse the same hardware that you use for the 0-2.5 GHz band, so you really have to treat them as two different designs.
I started looking into combined wide-band PLL/VCO chips like e.g. ADF4355 to provide the offset frequencies, but I'm not sure that's the best way forward. Specifically, the specs look lackluster in phase noise and harmonics when division is employed.
The ADF4355 is fine, along with any number of similar parts. Hint: look for evaluation boards on eBay.
Valon Technology sells some economical synths that would work well as LOs.
Remember that the signal will ultimately be picked up by a tuned receiver (meaning the 8566 itself). Isolation is really the only thing to worry about, but a good ALC circuit is also helpful. You can make do with trace subtraction on the analyzer to 'calibrate' the measurement, although load VSWR won't be accounted for.
You don't care much about harmonics, unless you're testing antennas and you want to be nice to other users of the spectrum. Phase noise is also not important. In fact, many people actually use broadband noise sources instead of TGs. The measurement takes longer than it would if you had a real TG, and dynamic range is worse, but it gets the job done. Or you can use a sweep generator with the analyzer set to MAX HOLD.
Before I had a VNA, I used my homebrew TG for 0-2.5 GHz work, swapping in an HP 86290B plugin for 2-22 GHz measurements in MAX HOLD mode. HP 8620C mainframes and plugins are so cheap nowadays that it's a no-brainer to use them as the basis for a tracking generator, even more so than when I built mine.