So one day soon I'm going to have these two SMA-mounted photodiodes that work up to about 10 GHz. Conveniently, they have built-in bias tees but don't have built-in load resistors, so they need a decent-quality termination.
They're part of a differential Doppler interferometer producing tone bursts of very roughly 200 cycles, immersed in wideband white noise, to the point of being completely invisible on a scope.
I need to build/buy/cobble together a splitter/combiner that works from DC to 8 GHz. I can buy resistive ones, but they have a 6- or 7-dB insertion loss, which means that by the time I'm done attaching an amp with a 3-dB noise figure, I'll wind up with about a 3000K noise temperature. Those photons are expensive, so this is clearly a non-starter.
I'd also like the two paths to be phase-matched to within 5 degrees or so, and the gains to be equal to within, say, 0.3 dB. That way I have some hope of getting 15 or 20 dB of laser noise suppression. On the plus side, because of the signal characteristics, I don't care much about the time domain response, and a few amplitude whoopdedoos aren't worrisome as long as they aren't too deep or too broad.
So, I'm thinking of just siamesing the two detectors together via matched pieces of RG-402 semirigid coax (0.141 inch) and either a T connector or some sort of artistically sculpted microstrip thing, e.g. a vanilla Wilkinson coupler tuned to some higher frequency.
I'm sort of leaning towards the tee connector to start with, because the actual input resistance of the average connectorized amplifier is around
25 ohms, iirc, so the match should be reasonable, and the degradation in the noise figure due to going a bit off the noise match shouldn't be too bad.True? False? Smart? Stupid? Other suggestions?
Cheers
Phil Hobbs