HP 85046B Directional Bridge / Splitter Photos

[TurboTom]

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

[orin]

For contrast, here is a 50 ohm version.

The blue flecks are loctite from the screws.  This bridge is sick - with a 15dB excess loss from the input to the test port.  It came with that nasty soldering job on the inductor.

[orin]

Differences:

B has 50 to 75 ohm pad on the input AND on the coupled port output.
Bias inductors are different values.
Microstrip line to test port and internal reference is wider on the 'A' - so it's no doubt 50 ohm on the 'A' version and 75 ohm on the 'B' version.
The A has some kind of tuning capacitance between the microstrip and case (small metal squares that appear to be soldered to the substrate).
The machining of the case near the coupled port is different on the 'B'.

There ain't no converting the 'B' version to 50 ohms IMO, even if you were able to scrape off the three 75 ohm resistors* and glue (conductive epoxy) in 0201 50 ohm resistors in their place.  I have heard of gluing in replacement 50 resistors in blown bridges being successful though.

*two are hidden on the clear substrate under the PCB

[hendorog]

Sweet pics thanks. Anyone seen what an 85047A bridge looks like inside?

[AF6LJ]

Nice pictures, I feel your pain...
I almost did that once but I took a second look at the N Female and the little voice in the back of my mind said... "There is something wrong with this picture" So I looked again and realized I had nothing to adapt to it.

[orin]

Sweet pics thanks. Anyone seen what an 85047A bridge looks like inside?

Yes... here's an 'R' coupler.  You can't really see anything.  The A/B couplers also have bias components.