About RF Matching of Weinschel 1870A Resistive Power Splitter - Page 1
Forum section: electronics / rf-microwave

author: NickF
author_rank: Newbie
timestamp: March 31, 2025, 05:06:11 am
content: I am trying to understand the internal structure and the matching of the RF power splitter 1870A. I used an ohmmeter to measure the resistance between the two outputs (and it is about 100 ohm) and the resistance between input and any of the two outputs (and it is about 50 ohm). Based on the measurement, I assume there are just two 50 ohm resistors between the input to outputs. I don't think it is possible to have any delta or star configuration that would match the measurements. If I terminate the two outputs by 50 ohm loads and measure the input resistance, I measure 50 ohm, which is what we expect. However, if I connect two 50 ohm loads, one at the splitter input and one at either output, the measured resistance at the remaining output is 83.3 ohm. We get the same value using formulas. The puzzling part to me is why does the splitter specification indicate a VSWR of 1.05 at the outputs (between DC - 2 GHz). The VSWR mismatch between a source with impedance of 83.3 ohm to a load of 50 ohm is about 1.67, significantly higher than 1.05.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 31, 2025, 06:13:26 am
content: Are you sure the splitter is good? Those numbers don't sound right. Your splitter might be fried.A resistive splitter has no inputs or outputs - all ports are identical to each other and labelling is just for convenience.A two-way resistive splitter should consist of three 16R667 resistors in a Y configuration. So measuring between any two ports should give you 33R333. This will result in each port seeing 50R if (and only if!) the other ports are terminated in 50R. Of course, in operation you're talking about impedance rather than resistance but resistance is valid for this kind of test.I ran across a piece of HP equipment that included a 4-way resistive splitter that was made of five 31R6 resistors. That seemed so weird that I did the calculation and came up with an impedance of 52R. It would have made more sense to use 30R1 resistors, but it was a GPS splitter so maybe operating at 1.575 GHz with what appeared to be standard 1% surface-mount resistors required some tweaking.

author: NickF
author_rank: Newbie
timestamp: March 31, 2025, 06:27:31 am
content: Hi Ed, I am confident the power splitter is good. We have multiple similar power splitters and they all measure the same. I used them for a long time and I never thought about analyzing their impedance and I was surprised by my finding. Probably I lack some bit of knowledge about how they work.I know what you are referring to, about devices with identical internal configuration for all three ports. We also have that type, but they are known as "power combiners" or "power dividers". Their internal resistance measured between any two ports is about 33 ohm (formed by a star configuration of three 16.7 ohm resistors) and when we connect two 50 ohm loads at any two of the tree connectors, we see (or measure) the expected 50 ohm. They are not used as power splitters when high accuracy is expected - we use them to combine the outputs of two signal generators for third order intercept measurements.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 31, 2025, 06:49:30 am
content: Okay, sounds like you're way ahead of me.They're doing something weird inside that box. This 1 watt device can tolerate 1 Kilowatt peak on the input port!And the input has a single number for SWR while the outputs have an SWR that varies with frequency. So there's a limiter on the input that's throwing off the ohmmeter measurement.Your measurement of 83R3 equals two 16R67 legs plus 50 ohms on the other output. It's like the input port isn't DC coupled. Do you get the same results for all your tests if you reverse the polarity of your ohmmeter? It's almost like there's a series diode on the input port.

author: NickF
author_rank: Newbie
timestamp: March 31, 2025, 08:18:16 am
content: Quote from: edpalmer42 on March 31, 2025, 06:49:30 amI am not way ahead, I merely discovered sooner I don't know what is going onIt is an interesting point that you raised, about the 1 kW peak power, which is tolerable only at the input. It reminds me on the attenuators in PI configuration, where the input resistor is much higher power than the output resistor (although the resistor values are the same). I initially thought that they are using two beefy resistors inside, which can handle the 1kW, but this theory fails because of the "input connector only".I don't think you are right about the two 16R67 + 50 ohm at the other output and the diode. The resistance between the two outputs is 100 ohm and I cannot see how you can combine two 16R67 and one 50 ohm to obtain 100 ohm which I measured.It does not matter if I reverse the probes when I measure. To my knowledge, all modern multimeters are measuring resistances with a voltage that is low enough (0.3V ... 0.4V) so it cannot open any semiconductor junctions - if a diode were in circuit, I could not measure any resistance. The very wide frequency range and tight specs would indicate to me there is something purely resistive inside. Maybe somebody will figure this out.

author: gf
author_rank: Super Contributor
timestamp: March 31, 2025, 09:27:42 am
content: Quote from: NickF on March 31, 2025, 05:06:11 amThere is nothing wrong with it - it is just not what you expect it to be. It is obviously a (two resistor) power splitter for leveling loop (ratio measurement) applications. It is not a (three resistor) power divider.See here:https://docs.keysight.com/kkbopen/what-differentiates-a-power-splitter-and-power-divider-and-when-should-one-be-utilized-over-the-other-577941638.html

author: NickF
author_rank: Newbie
timestamp: March 31, 2025, 09:59:57 am
content: Thanks for providing the link to the document - it confirms there are just two resistors inside. The original question I have still remains - how can the datasheet say that VSWR is less than 1.05 below 2 GHz, when the calculation indicates it is 1.67?

author: gf
author_rank: Super Contributor
timestamp: March 31, 2025, 11:11:15 am
content: Quote from: NickF on March 31, 2025, 09:59:57 amThe key is the leveling control loop. Reflections are not prevented, but the the leveling loop keeps the voltage at the splitter input constant. Therefore it looksas ifa voltage source with zero source impedance was connected to the splitter input, and as if each output leg had 50 Ohm output impedance (independent of the loads connected to the output legs).

author: NickF
author_rank: Newbie
timestamp: March 31, 2025, 11:45:47 am
content: I think you are referring to the automatic control level loop of the signal generator, which strives to maintain the output level constant, irrespective of impedance (amplitude?) changes of the load. The thing is that from the point of view of the signal generator, the splitter + 2 loads equivalent impedance is exactly 50 ohm, so I don't know if the reflections between the splitter outputs and their loads will affect the level of the signal generator. I need to digest this, to see if I understand and agree with the process. Thanks for the food for thought!

author: gf
author_rank: Super Contributor
timestamp: March 31, 2025, 12:14:44 pm
content: Quote from: NickF on March 31, 2025, 11:45:47 am... where the power detector of the leveling loop is connected to one output leg of the splitter (as shown in the referenced document).That's the typical application for a two resistor power splitter.Note that not the generator's internal no-load voltage is kept constant, but the voltage at the power sensor which is 1/2 of the splitter input voltage (50Ohms / 50Ohms voltage divider). Therefore the splitter input voltage is kept constant as well.EDIT: Alternatively to adjusting the internal generator level to keep the voltage at the detector constant, you can do ratio measurements.QuoteThe power sensor is indeed hopefully very close to 50Ohms as well.But the input impedance of an arbitrary DUT is not necessarily 50Ohms.

author: NickF
author_rank: Newbie
timestamp: April 01, 2025, 01:41:44 am
content: Quote from: gf on March 31, 2025, 12:14:44 pmIt is not necessary to have the leveling loop connected to the power detector. We can use a power meter and an operator can set a new frequency and adjust the level to nominal value.Quote from: gf on March 31, 2025, 12:14:44 pmI am not sure about this. If the equivalent resistance of the power splitter branch where the power sensor is connected and the signal generator impedance and the 50 ohm branch is indeed 83.3 ohm, then the voltage at the power sensor may be different than 1/2. What is certain is that both outputs of the power splitter will get same level.Quote from: gf on March 31, 2025, 12:14:44 pmI fully agree with you, this is the case with real instruments. I am approaching the matter from an idealistic point of view and so far I still cannot say what is the VSWR at the output of the power splitter. If I were to measure the VSWR of the power splitter output, I would use a VNA, terminate the input and an output with 50 ohm matches and measure the VSWR at the remaining output (and I don't know if this is the correct procedure, but it is my best guess). And I expect the VSWR will be around 1.67. In fact, I will do this hopefully today and report my results.

author: NickF
author_rank: Newbie
timestamp: April 01, 2025, 03:02:58 am
content: I measured an output port of a 1870A power splitter and the VSWR is around the expected value of 1.67. I used two 50 ohm terminators at input and second output. Probably this is not the correct test procedure to measure the output VSWR of power splitters, but it is the only one that makes sense to me.

author: NickF
author_rank: Newbie
timestamp: April 01, 2025, 03:52:20 am
content: I found a document that describes the calibration procedure of power splitters and directional couplers.There is an equivalent reflection coefficient which is being calculated based on measurements that involve the use of Open, Short and Match terminations.It is a significantly more complex operation compared to what I was expecting.https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez51/1EZ51_1E.pdf

author: gf
author_rank: Super Contributor
timestamp: April 01, 2025, 08:10:09 am
content: Quote from: NickF on April 01, 2025, 01:41:44 amThen the operator becomes part of the (manual) leveling loopQuoteIf splitter and sensor are ideal, the |Vsensor| / |Vin| ratio is always 1/2, independent of the generator's source impedance and independent of the load impedance connected to the other output of the splitter, where Vin is the voltageat the input of the power splitter(not the generator voltage!) and Vsensor is the voltage at the sensor.Edit: If you look a the schematic, how could |Vsensor| be anything other than |Vin| / 2 ?An ideal lossless 50 Ohm transmission line between splitter and sensor would not change the ratio either.[ Things change, of course, if components are non-ideal. ]

author: gf
author_rank: Super Contributor
timestamp: April 01, 2025, 09:30:26 am
content: Quote from: NickF on April 01, 2025, 03:02:58 amA splitter is a 3-port network. Let's assume port 1 is the input and ports 2 and 3 are the outputs, and that your 50 ohm terminators are ideal. With terminators on port 1 and 3 you measure S22 at port 2, and with terminators on port 1 and 2 you measure S33 at port 3 (or the VSWR corresponding to S22 or S33).But that's not how theequivalent VSWRof a splitter is defined. The equivalent VSWR of port 2 is not only a function of S22, but also a function of S21, S31 and S32 (see the document you referenced, orhere).

author: NickF
author_rank: Newbie
timestamp: April 01, 2025, 10:15:55 am
content: Thanks for all the info. The document from Keysight is easier to read and related only to power splitters.While I still don't have a clear image about the equivalent VSWR, it seems to be related to the mutual interaction of the ports of the power splitter.

author: gf
author_rank: Super Contributor
timestamp: April 01, 2025, 10:31:48 am
content: This one may also be of interest:https://www.dinfo.unifi.it/upload/sub/laboratori/emc/Dispense%20Misure%20Elettroniche/power%20splitter%20vs%20divider.pdf

author: Miek
author_rank: Regular Contributor
timestamp: April 01, 2025, 10:35:41 am
content: There's some more information about equivalent output SWR here:https://docs.keysight.com/kkbopen/how-do-you-measure-the-equivalent-output-reflection-coefficient-or-equivalent-output-swr-of-a-splitter-589738422.htmland in the mentioned paper here:https://pdfcoffee.com/understanding-microwave-power-splitters-johnson-1975-4-pdf-free.html