I recently got a 8753C / 85046B VNA set, and being confronted with a 75 ohms N connector for the first time, of course I immediately ruined it plugging in a 50 Ohms SMA adapter (for those who don't know - like myself a few weeks ago - the 75 Ohms N connectors have smaller centre pin diameters than the 50 Ohms counterparts, and consequently smaller sockets as well, so plugging a 50 Ohms male into a 75 Ohms female may ruin the female - whatever that may mean for "real life"...
). Since an S parameter test set is quite an expensive piece of equipment, I'ld prefer to fix it, hence I took it apart. Only the combined return loss bridge / power splitter module was damaged, the other return loss bridge seems to have a different connector version attached that is tolerant to 50 Ohms N plugs.
So here's some photos of the disassembled coupler / splitter box which probably explains why the stuff has been so incredibly expensive when new...
The first photo shows the whole unit, consisting of the power splitter subassembly on the lower right and the return loss bridge / impedance matching network in the centre. The power splitter is a functional separate unit and is required to feed a portion of the VNA RF generator signal back to the reference input of the VNA that's necessary to lock the PLL inside the VNA. Actually, the R input of the VNA contains the same circuitry as the A and B signal inputs except that it's got an additional feedback line to the phase lock assembly (which could be attached to the A and B inputs for testing purposes as well). Hence the R input is used for closing the RF generator frequency control loop as well as providing a reference for the RF signal strength. The RF generator input is at the lower right SMA connector, routed to a resistive splitter (designed as thin-film strip-line assembly on a ceramics/glass substrate). The connector at the right bottom side is the output to the step attenuator and forward/reverse switch assembly that selects between S11/S12 and S21/S22 measurements, i.e. if the powered side of the DUT is the input or the output. The upper right-side connector is the refenece output, fed from the power splitter via a delay line (the coil of sub-miniature coax cable) and a resistive attenuator.
The return loss bridge / impedance matching assembly is actually beyond my understanding (I probably didn't try hard enough...), but at least I can report what goes where
:
The bottom centre SMA carries the RF input signal from the forward/reverse switch assembly, i.e. the input signal that's routed to the DUT. Just left of that (only visible as a yellowish insulator) a wire is connected to the left RF choke in the coupler/splitter assembly. This carries the optional "Port 2" bias DC signal for the DUT. The N connector at the left-hand side is the "Port 2" test connector with the broken receptacle. The top SMA connector outputs the signal coming from the N connector to the B input of the VNA.
The two right-hand side ferrite toroids are wound with tiny, rigid coax wire that's silk-insulated at the outside. The left-hand side toroids are wound with silk-insulated silver wire. There appear to be two tiny ferrite beads at the feed-through from the PCB section to the strip-line section in the upper area, just sandwiched between the FR4 (or hatever it is) and the glass substrate section below. Considering this assembly works from 300kHz to 2GHz and also matches the 50 ohms impedance of the VNA to the 75 Ohms DUT Ports, it's quite amazing and I'm sure the HP engineers needed quite some experiments, time and effort to get it running satisfactorily. Moreover, the manually assmebled unit continaing considerable amounts of precious metals and specifically produced components must have cost a small fortune when new. But the best news (for me) is that the centre connector of the N port is a removable item that can be replaced individually (if a spare was available) or repaired - and that's probably the route that I will take...
The two other photos show the individual sections of the splitter / coupler assembly in higher detail.
Cheers,
Thomas