Converting an HP 75 ohm S-Parameter test set to 50 ohm? (HP 87511A/B)

[DaJMasta]

I don't know how feasible it is, but from the block diagram it should be possible.... I just have no way of seeing inside the test set in question in advance, and wouldn't want to buy one only to find it was very challenging to modify.

The test set in question is the HP 87511B variant, half the price or less than the A variant because it's 75 ohm port impedance, and from the service manual, the difference seems to be a set of four minimum loss pads that are inside of the directional bridge assemblies.  Basically I'm wondering if these minimum loss pads are going to be soldered in place or if they're just going to be inline with the rigid coax - because it would be a very easy modification to remove them and replace them with an appropriate adapter.

I've found the service manual here, and in the block diagram on page 44 is where I'm getting most of what I know about it: https://doc.xdevs.com/doc/HP_Agilent_Keysight/HP%2087511A%2C%2087511B%20Operation%20%26%20Service.pdf

In this discussion of modifying a higher bandwidth test set ( http://k6jca.blogspot.com/2014/03/modifying-75-ohm-hp-85046b-s-parameter.html ), they didn't open the directional bridge, they just swapped them, and there is a difference between the two, at least in this variant.  The service manual for that 85046A/B test set has electrical schematics that will probably match the board in the test set, since both the 85046 and the 87511 seem to be compatible with the same VNA (4395A, in my case).


So does it seem feasible that inside those directional bridges is an easily removable pair of minimum loss pads?  Has anyone attempted this before or does anyone have pictures of the insides?  I can just wait around to see if a 50 ohm version comes around at a better price, but getting my hands dirty and saving some money has its appeal.

[capt bullshot]

Looking at the block diagram, page 7-2, I'd guess the reflection bridges itself are different between the 50Ohm and 75Ohm version.
The MLPs are attached at the analyzer side to match the analyzer 50Ohm to the 75Ohm bridge. So I'd think you'll have to swap the whole bridge assembly (which includes the minimun loss pads).

[DaJMasta]

Yeah, them being grouped into the directional bridge is of concern, but I was sort of hoping they would have made them a bit modular, since everything else in there is all 50 ohm.  That said, HP did a lot of custom stuff for their equipment, and I'm sure these test sets were low enough volume that the manufacturing benefits of making a directional bridge with modular minimum loss pads for reduction in different inventory or whatever are probably very, very low.

I'd certainly give it a shot and crack one open if I had the 75 ohm test set, but unless I can get some confirmation I'm certainly not going to spend for a 75 ohm variant.  Since this test set only goes up to 500MHz, physical modification could probably be done reasonably without catastrophic damage..... but I'm still hesitant.

[radioactive]

After looking at the block diagram in the linked manual,  I suspect that the directional coupler is separate from the "minimum loss pads".  The directional coupler appears to be terminated in 50 ohms.  I don't know the device, so can't help out with what is actually in there, but I suspect the pads are just resistors due to the use of "minimum loss pad" wording.   See here:  https://www.microwaves101.com/encyclopedias/l-pads

This configuration is often called the "minimum loss matching pad". It is used to match one real impedance to another real impedance, a typical application is to match 50 ohm stuff to 75 ohm stuff (we'll use that as an example below). Both source and load see matched impedances when this is done correctly.

If They are resistors, then the insertion loss for the pads  75->50 ohms would be ~5.7 dB,  so you would have to take that into consideration.

[DaJMasta]

Yeah, I'm not sure if the test set identifies itself to the analyzer, so there could be some light digital work or just picking another configuration option in the software somewhere - I would imagine that's relevant to the displayed data on the analyzer, so there's gotta be some way to compensate for it.  That said, maybe once you do your SOLT calibration for both ports, I guess it would just be reduced dynamic range in the 75 ohm configuration.


If it's just some resistors on a bit of low loss PCB in that module, the actual mod could be pretty straight forward.

[radioactive]

Should be pretty easy to convert a 2-resistor L-pad (5.7dB insertion loss) to a 3-resistor Pi-type attenuator.  Assuming they are SMD, then you could replace the series resistor with a 36 ohm and the parallel with a 154 ohm.  Then you could solder a third 154 ohm on top of the other two  (triangle looking configuration).  This would give you the 50-ohm, 5.7dB insertion loss while still maintaining good return loss.

Then, there is still 75 ohm transmission lines and connectors to consider.

[Bud]

i'd be careful with assumptions. HP being HP did not cut corners. If the pads are parts of thecoupler assembly they may have been implemented as resistive material on ceramic or glass substrate.

[SpencerTC]

If you don't need the entire spec'd range you could just use some more [in-line] min loss pads either before or after your test cables and cal it out...this will decrease performance a bit, but I've done it on 8712's, 8714's and 8753's and they work just find and give good data.

[DaJMasta]

That's certainly true, but at 20ish dB of insertion loss from the source to the receiver with the 50 ohm set, that would be pushing 40dB of loss as a baseline with the 75 ohm test set and additional external pads.


In any case, my opportunity to see has passed, I got my hands on the 50 ohm version of the test set, so there's no longer any need to find out.  Would be curious if anyone else attempts/has attempted it, though.