Need some pointers on building UHF/microwave 50 ohm termination/power splitter

I must ask, what is the purpose of this?

Hi all,

I need a couple of accessories to enable me to make some phase measurements with my HP network analyzer. I'd thought I could pick these up on ebay easily enough, but note the lack of availably with surprise and dismay. I need to therefore contrive two precision parts:

Firstly, 50 ohm load that's essentially non-reactive up to 1.3Ghz. Power handling only need be a few tens of miliwatts. N-type connection.

Secondly, a 50 ohm power splitter (one feed-in; three outputs) N-type connections, again flat up to 1.3Ghz. No switching needed, thankfully.

If I can't source these parts elsewhere, how feasible is it to make them up and can anyone point me to any designs on the web that might fit the bill?

I'm aware that the introduction of any stray reactances into the devices will render all subsequent measurements invalid so I need to get these parts right. At least 1.3Ghz capability should be achievable for a hobbyist with care.

Thanks, P.

--

"What is now proved was once only imagin\'d" - William Blake

can supply some of your needs as well.

Hi Paul,

Take a look at Mini-circuits

They seem to stock what you are after.

Have a range of splitters and loads.

Paul Burridge wrote:

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David Huisman
General Manager
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You can make a pretty good 50 Ohm termination with a PCB-mounting SMA connector. Cut off the centre pin of the PCB-end of the connector leaving

0.5mm or less protruding (careful of your eyes, the pin can go shooting off pretty fast, it's hard metal), and then solder two 0.1% 100 Ohm 0603 resistors between the centre pin and the outer (ground) part of the connector. The resistors should be diametrically opposite.

I made one with 1% resistors and got the following: s11 < -30dB up to 6GHx and s11 < -47dB up to 500MHz

It helps to tweak how flat you lie the resistors on the teflon at the back of the connector, but without a working VNA you just have to accept what you get.

If you want a termination with a N connector, then you could use a really good adapter with the SMA termination I mentioned above, or work out something similar with a N connector however I have never tried that since I mostly use SMA anyway.

I think a very accurate / flat power divider would be fairly hard to make well unless you can get boards made with microwave substrates. If you can work out your measurement setup such that the flatness etc of the divider is not so important, then that would help.

There's a guy in the UK selling a one input two output type power divider with N connectors and a 50 Ohm N termination on e-bay at the moment if that helps you.

Chris

Okay, I couldn't figure out what operational use it would have.

Thanks, Wes. I'll keep an eye on it.

The (thumping great) service manual that came with this VNA gives various levels of tests that can be performed oneself prior sending the thing away for calibration. The power splitter together with a few other basic items enables the analyzer to 'check itself' for fundamental operating fitness. I'm eager to get measurin' stuff, but need to establish some basic, satisfactory level of accuracy first. Being just a hobbyist, I don't require any absolute standard, thankfully!!

--

"What is now proved was once only imagin\'d" - William Blake

It's probably a lumped element Wilkinson power divider design and they work at a specific frequency range the optimum Z match is where the 1/4 wavelength is set.

Actually, I've noticed that on checking up the mini circuits pointers that the splitters they manufacture are only good for a given frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -

1000Mhz or similar. I'd have thought this type of stuff would have a bottom end of *DC* not some relatively high radio frequency. We're only talking about power splitters after all, not broadband filters. So why the low frequency cut-off? Are there some capacitantances utilized in these designs and if so, what are they doing there?

--

"What is now proved was once only imagin\'d" - William Blake

If all you need is a resistive power divider, the frequency limits should be generous. If you want efficient power dividers (with little loss in the splitter itself) you use wideband transformers to get the power split.

Without knowing which devices you're referring to, I would still guess that they are based on broadband transformers. "Broadband" does not mean DC to daylight. And yes, there might be some compensation caps involved too.

If you want better bandwidth, including DC, and loss is not a concern, then fully resistive dividers are what you want. An example of a two-port is here:

These are ferrite transformer hybrid splitters with fractional dB insertion loss beyond the 3dB split and typically 30dB isolation between output ports. If you want a DC splitter then they have those too- but you're not swift enough to see them. They are simply this. What do you think the loss and isolation is now?: View in a fixed-width font such as Courier.

. . . . . v . | . .-/ \\ -. . | | . [50] [50] . | | . +-[50]-+ . | | . ^ ^ . . . .

(Seems like this should be r.r.a.homebrew instead of the antennas group!)

Beware of the difference between a "power divider" and a "power splitter." (The terminology different folk use can be confusing, because there's not good consistency in the useage...) I gather, without reading every word of this thread, that you want to make network measurements, and the splitter is to provide levelling: either a reference channel or actual amplitude control of the exciting signal.

So for a two-way, you want simply an input connector and two output connectors, with a 50 ohm resistor from the input to each output. The trick is that you want as near perfect symmetry between the two channels as you can get. Ideally, each 50 ohm R will be in a coaxial environment where the output end is 50 ohms and the input end is 100 ohms, but you don't have to control that very accurately to get it to work well to 1.3GHz. With that arrangement, the source "sees" 50 ohms if each output is loaded with 50 ohms, and if you use one output channel to control the level, the level stays constant at the input connector (since that's what's being monitored) and that is an EFFECTIVE zero impedance point, much as the inverting input to an op amp is a virtual ground. The the second output port then shows an effective 50 ohms source impedance. You can show that monitoring the reference channel instead of actually using it to control the level is functionally the same for circuits which are not significantly level dependent.

For a three-way, you want 50 ohms to each output port as with the two-way, but now since the three outputs in parallel (each 50 ohms load plus 50 ohms series resistor) result in 33.3 ohms at the junction where they come together, you need 16.7 ohms in series from there to the input port, to provide a 50 ohm load for the source (assuming 50 ohm loads on each output). Note that 16.7=50/3. So you can make that splitter with 6 50 ohm resistors.

Being just resistive devices, the only thing that limits the frequency response is parasitic inductances and capacitances--not being able to make the environment "perfect". But that's only a problem on the high end, not the low end. DC is not a problem for these devices. However, they are much more lossy than a power divider, which ideally has no loss in the divider itself, at least with proper loads.

We had a need for some DC-6GHz splitters. The commercial ones we had were rated to 3GHz, and indeed they weren't all that wonderful much beyond that. A search for commercial ones that were rated to 6GHz came up empty. So I designed a 6GHz one, using 50 ohm 0805 SMT resistors, with some help from a mechanical engineer. The three output ports are in one plane, radial spokes from the central node where all four resistances come together. The input port is perpendicular to that at the center. The resistors are all soldered together at one point, zero lead length except for their terminations and the tiny ball of solder. To get good performance to 6GHz, there's a screw that adjusts the capacitance from ground to the central node. It's all housed in a hexagonal aluminum block, with SMAs radiating out. I was able to build a prototype that worked acceptably, using only PC mount SMAs soldered together. I'd have a lot of confidence, based on that, that I could make one that would work quite well to 1.3GHz even without much in the way of test equipment to check it.

And you should have no trouble at all making some very decent loads using the techniques others have mentioned. FWIW, I've had better luck using two 100 ohm SMTs radially opposed than using four 200 ohm SMTs with 90 degree spacing. I didn't investigate just why, but assumed it had to do with the parasitics inherent in the parts.

Cheers, Tom

(I'm a bit surprised you need a three-way splitter. Are you measuring two DUT paths simultaneously??)