Build 50 ohm power splitter (question for RF guru) - Page 1
Forum section: electronics / projects

author: eurofox
author_rank: Unknown
timestamp: July 24, 2014, 10:58:14 am
content: I'm not an RF dude and ask opinion of people with knowledge in this area.I'm working on a ultra fast pulser, I build my own power splitter (because it is extremely expansive) with 16,5 ohm resistors in T connection, they are connected to SMA terminals and there is more than 1 cm space around the 1% resistors. It is build in a quite massive aluminium box.I just wonder what kind of bandwidth I can reach this way? I know that some RF foam is available and wonder if it is useful in my case.I order as well to improve the design 33 ohm 1% SMD resistors (1W), in parallel I will get the requested 16,5 ohm.Thanks in advance for your input.eurofox

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2014, 05:34:44 pm
content: If the layout is correct, chip resistors are good into the 5-10GHz+ range. And whatever fractional nanoseconds that corresponds to. Mind that low values will tend to look inductive (physically, they look more like transmission line than terminating resistor), and high values, capacitive (same reason, opposite direction).Obviously, something as high as a 1M resistor ain't gonna be 1M impedance at 10GHz, not when the footprint pads themselves have lower capacitive reactance between them through the substrate alone.Tim

author: acbern
author_rank: Frequent Contributor
timestamp: July 24, 2014, 06:18:29 pm
content: the problem is not so much what chip resistor you use but how the layout is, how tightly controlled your microstrip impedance is and so on.to judge you would have to say much more about how you did it. the other question is what you need in terms of reflection, amplitude/phase match (i.e for your application, what does 'work' mean?). 10ghz can be reached, sure, even with FR4 if done right and with some constraints, but in the end you will have to measure this with a vna.

author: eurofox
author_rank: Unknown
timestamp: July 24, 2014, 07:37:58 pm
content: My pulse generator is in the area of 30ps rising edge, amplitude from 200 mV to 800 mV.The resistors got about 1 cm from SMA terminals and between each other in Y shape, from top and bottom of the aluminium container 1 cm as well.The resistor are based on 2 different 1% 1/4W in parallel to get the 16,5 ohm.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2014, 07:49:33 pm
content: How much power do you need? Adding resistors in parallel will increase parasitic capacitance.

author: eurofox
author_rank: Unknown
timestamp: July 24, 2014, 08:00:28 pm
content: Quote from: ejeffrey on July 24, 2014, 07:49:33 pm1/4W is ok, I don't think that resistors of 16,5 ohms are available, you have to build them with 2 resistors in parallel.Maybe on special order but then I can better buy an Agilent splitter, is only 1400 US$

author: KJDS
author_rank: Super Contributor
timestamp: July 24, 2014, 09:19:01 pm
content: Will this do? I've got a few lying about, yours for £20 plus postage.http://www.e-meca.com/rf-power-divider/divider_specs.php?divID=3&specsID=22Alternatively, I'd go for using 16.8R resistors, or a triangle of 51R resistors. Use chip resistors otherwise the parasitics will kill you. Have the parts touch each other at the junctions.

author: G0HZU
author_rank: Super Contributor
timestamp: July 24, 2014, 09:54:03 pm
content: There's several ways to try and make a successful design. However, you do need access to a decent VNA if you are looking for high performance.You can either adopt an iterative approach armed only with the VNA and experiment with star and delta configurations of various SMD resistor packages to see which is best.Or you could try and get decent models of the resistors you are using (up to many GHz) and try and simulate the design. This can be very useful in terms of learning 'faster' than the VNA approach but ultimately you will still end up with the VNA and the 'best' combination of resistors and teasing/snipping it with a scalpel and maybe even adding some tiny slivers of copper tape to get best balancing of the various parasitics in the resistors and the layout. Then when you get a good result, make a neat (final) version.I would start off with the delta configuration using a pair of small 100R SMD resistors stacked in order to get 50R for each arm. The resulting parasitics from this may well mean it performs better than one with a single small 50R SMD resistor per arm but it all depends on what spec you are trying to meet for phase balance/loss/match for the three ports.

author: Marco
author_rank: Super Contributor
timestamp: July 24, 2014, 10:05:50 pm
content: Quote from: KJDS on July 24, 2014, 09:19:01 pmThose are quadrature combiner/dividers, phase shift together with the limited frequency range is unlikely to give you two nice pulses as outputs.Eurofox, why didn't you make a delta splitter? It uses 3 Z0 resistors.http://www.microwaves101.com/encyclopedia/Resistive_splitters.cfm(The Owen splitter they describe is also interesting, more loss but also more isolation.)PS. there'ssome 4GHz splitters on ebayfor 30 bucks. Of course they have female connectors, which might be trashed. (Why aren't male connectors standard on equipment? Seems more reasonable to me to put the easily broken gender on the cable.)

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 24, 2014, 10:08:19 pm
content: If you translate 30ps rise time into bandwidth using simple rule of thumb formula:BW (in GHz) = 0.35 / TR (nanoseconds) or in your case BW(GHz) = 0.35/0.03 = 11.6 GHzSo you need power splitter for 12 GHz. Example for 7.8 GHz shown on picture below. Geometry will be different for 12 GHz. Also note that white material is NOT FR4 - you need low loss high frequency substrate such as Rogers "Duroid" or Alumina. Search for "duroid" on Ebay - you'll probably find some.In order to get correct geometry for your power divider you can search for free tools online or use commercial tools such as Agilent ADS (Advanced Design System) or Microwave Office etc. - they are very expensive (>$25K however). If you use right data for substrate and keep the dimensions withing +- 5% of simulated then you will probably get something close to desired result even if you have no access to VNA.I would try to find power divider for ~12GHz first.If you work with this kind of frequencies you kind of start to appreciate why some of that stuff cost so much.

author: KJDS
author_rank: Super Contributor
timestamp: July 24, 2014, 10:26:56 pm
content: Quote from: Marco on July 24, 2014, 10:05:50 pmA quadrature splitter has the outputs 90 degrees apart in phase, which these don't. Whilst they aren't perfect for the job, I suspect that unless the eorofox has access to a 6GHz VNA. or preferably a 12GHz one, then they will work better than anything he can make.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2014, 10:29:45 pm
content: Quote from: eurofox on July 24, 2014, 08:00:28 pm16.5 Ohm is an E96 value and available from all the major manufacturers. Or you can use a delta configuration with 49.9 ohm. If you can't get those easily, the nearest available from the E24 series (51 ohm or 16 ohm) are pretty close, especially the 51 ohm. You may find that the reflection from the slight impedance mismatch is less objectionable than the extra reactance of additional components.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2014, 10:38:52 pm
content: Quote from: KJDS on July 24, 2014, 10:26:56 pmWhat you linked to looks like a wilkinson power divider. As you say, the outputs are in phase, not in quadrature like a hybrid. Still, a 2 GHz wilkinson divider is not a great choice for a pulse splitter, it will definitely screw up your rise time. A simple resistive power divider is actually not that hard to get right the first time, and you should easily be able to beat that device even without a VNA -- at least in terms of preserving the sharp rise-time of your pulse.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 24, 2014, 10:40:48 pm
content: You guys have no faith in simulation tools. VNA is very useful, but it should not be just trial and error approach.

author: eurofox
author_rank: Unknown
timestamp: July 24, 2014, 10:50:18 pm
content: Quote from: Alexei.Polkhanov on July 24, 2014, 10:08:19 pmThis calculation I did as well, my RF instruments are limited, counter up to 20Ghz, generator up to 3,2Ghz, SA Rigol + TG 1,5Ghz, USB SA up to 3Ghz.Sampling scope up to 11Ghz.

author: eurofox
author_rank: Unknown
timestamp: July 24, 2014, 10:53:27 pm
content: Quote from: ejeffrey on July 24, 2014, 10:29:45 pmYou are right, I checked in my stock and I was supposed to have almost all current values and didn't find it but it is available and just order them.

author: G0HZU
author_rank: Super Contributor
timestamp: July 24, 2014, 10:57:58 pm
content: Quote from: Alexei.Polkhanov on July 24, 2014, 10:40:48 pmI do RF simulations pretty much every day at work using Genesys, AWR and Sonnet but in this case I think the simulation accuracy will only be as good as the models for the chip resistors.Modelithics do some decent models up to many GHz for various PCB substrates/thicknesses but I still think the 'final' trimming of this (homebrew) circuit can only be done with a decent VNA and a scalpel etc.Note: The 7.8GHz splitter PCB you posted an image of isn't a broadband design.

author: KJDS
author_rank: Super Contributor
timestamp: July 24, 2014, 11:01:44 pm
content: Quote from: Alexei.Polkhanov on July 24, 2014, 10:40:48 pmI have a lot of faith that I'd get one working quicker sat at a 20GHz VNA with a time domain option than anyone would with CST Studoi, MWO, ADS or any other simulator. If I were doing the design to put into volume production then I'd start by spending a lot of time characterizing the resistors, then even more time with CST studio to put together a full 3d EM model, then building and testing, but for a one off, build something and tweak it with a scalpel. It's too simple to spend much time playing with maths up front.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 25, 2014, 01:22:50 am
content: Yes, this is true. Unfortunately only few manufacturers like TDK (http://product.tdk.com/en/technicalsupport/tvcl/) have models for caps and resistors that can be used at these frequencies. I am not even sure how accurate these models are.There is no doubt that with VNA things are much easier and faster, but if you don't have access to VNA that covers these frequencies what do you do?

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 25, 2014, 05:55:42 am
content: If you don't have access to a VNA then you will never know if your design is correct. No amount of simulation will replace a proper real worl test.

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2014, 10:17:36 am
content: Couldn't he read in the data from the sampling scope and take the FFT from the pulse source and the split source and divide them to get the transfer function?

author: acbern
author_rank: Frequent Contributor
timestamp: July 25, 2014, 11:59:59 am
content: Quote from: Marco on July 25, 2014, 10:17:36 amnot really, unless the sampling ferquency would be much much higher to get a meaningfull response.if this is for a serious measurement and not just somehow a splitter doing something (at 30ps tr it does not spund that way), the only way to get a reliable result (without a wideband VNA and lots of work and costs to build an RF PCB, buy appropriate RF resistors and so on) is to buy something. this is not an issue of simulation, the parasitics, the tolerance, and the unknowns of the parts models, will be the main problem. as this is for pulse measurement, a flat phase ressponse is needed too over the whole frequency range. in order not to distort the pulse, the 11.6ghz (0.35 factor) will not do it either.most cost efficient are used picoseocnd pulse lab splitters on ebay. they are also built for exactly that application.

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2014, 12:28:18 pm
content: He could measure if it were flat to 11 GHz at least, which would still be a worthy effort for a DIY splitter (and not that much short of the connector limited bandwidth).Weinschel splitters seem to be the better deal compared to the HP and Picosecond pulse labs splitters ... somesold for 70 bucksnot that long ago and there's one on there for 150 bucks buy me now at the moment.

author: eurofox
author_rank: Unknown
timestamp: July 25, 2014, 01:22:45 pm
content: Quote from: Marco on July 25, 2014, 12:28:18 pmI just score this onehttp://www.benl.ebay.be/itm/Weinschel-93459-Model-1515-Power-Divider-DC-to-18-GHz-SMA-fbb22-8-/400732429250?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item5d4d839bc2&_uhb=1Based on the discussion my idea was not so bad to use resistors in "air" isolation, no PCB.Like I said in the post I'm not an RF dude, I manage very well and have instruments till 3-4 Ghz.Just bought a 1Ghz probe for my MSOX3104I really appreciate all answers, it was interesting. Above 4-5 Ghz we enter in a complete new world that look more to a microscopic plumbing than electronics.Thankseurofox

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2014, 02:31:14 pm
content: Do take a good look at the fingers inside the female connectors on the splitter ... you never know how well they've been treated (as I said before, it would be better if all connectors on equipment was male).

author: KJDS
author_rank: Super Contributor
timestamp: July 25, 2014, 03:56:58 pm
content: I've just built one, it isn't pretty but it's working reasonably. just used two 50ohm resistors, so good input return loss, which is ok going into identical matches. There's not much room to get a soldering iron in to put a third resistor in.Amplitude balance to 6GHz is excellent, phase balance was better than 4 degrees to 6GHz.

author: KJDS
author_rank: Super Contributor
timestamp: July 25, 2014, 03:58:42 pm
content: Phase and amplitude comparison between the two channels.

author: eurofox
author_rank: Unknown
timestamp: July 25, 2014, 04:26:00 pm
content: Quote from: Marco on July 25, 2014, 02:31:14 pmI will follow you advice and put a permanent "sex changer"

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 25, 2014, 04:54:59 pm
content: Weinschel - nice splitter. Looks like they are "Aeroflex" now. It does not seem to have any side panels or screws that you can take out to pick inside and see how it is built.I would be dying of curiosity

author: KJDS
author_rank: Super Contributor
timestamp: July 25, 2014, 05:06:45 pm
content: Quote from: Alexei.Polkhanov on July 25, 2014, 04:54:59 pmI'll guess that it's a thin film circuit with the resistors being part of the process.

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2014, 01:31:38 pm
content: Glad the OP has found something suitableHowever, I do feel a bit frustrated by the lack of spec/requirements data posted up. When the alternative was a precision splitter from HP costing $1400 I assumed you were looking for something very precise with a high spec. All my comments were aimed towards producing a 'precision' RF splitter up to many GHz.Last year I did make what I would class as a precision splitter when I was evaluating my old (made in 1965?) HP RF voltmeter as it measures gain and phase to 1GHz. It was just a temporary lashup splitter but my target spec was:VSWR better than 1.05:1 on all portsPhase tracking <1 degreeAmplitude tracking << 0.1dBThe aim was to get this performance across the whole range of the instrument and I designed it using my own (measured) models of the chip resistors I used. I also did a full EM simulation of the PCB.I used 100R resistors in parallel to make up the 50R resistors for a delta splitter because the parasitics were far better when compared to a single 50R chip resistor.I can try and dig it out and remeasure it and post up the results if anyone is interested but this was very much a 'precision' splitter that was designed on a simulator and the measured results matched the simulator pretty much spot on.The secret is to get accurate models of the chip resistors and I measured the 100R resistors right out to 3GHz to get a decent model to put into the simulator. I also select on tested each of the 100R resistors on a decent DMM to get as close to 100R as possible for each. I chose the best 6 resistors from a strip of maybe 30.But I think it will be a lot tougher trying to make (let alone model) something decent that runs to 10GHz. I've never tried it.

author: eurofox
author_rank: Unknown
timestamp: July 26, 2014, 01:59:12 pm
content: Quote from: G0HZU on July 26, 2014, 01:31:38 pmI really appreciate your post and from other members here, it is very instructive.My first try was with a Wilkinson splitter but got too much loss of signal.Next I try with a T splitter with normal 1% resistors (16,5 ohm), I have a quite big collection but no 16,5 and assume it was not available and did no check and just use 2 resistors in // and understood it was not perfect. Since one member told that it is a standard value I check it and immediately order it.I was also “hunting on ebay” for an existing splitter, all I could find that was affordable was outside EU …. Big delay, high shipping cost, customs clearance, duty to pay ? and finally was lucky to find one in UK that is virtually unused according the ebay poster.eurofox

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2014, 02:17:10 pm
content: I think you will get good results with the Weinschel splitterI've found my old evaluation jig for my HP8405A VVM and here's the splitter section from it. I needed a precise 6dB split when checking the performance of my HP8405A after I had repaired and had aligned it as per the service manual.The idea was to make something that was very, very precise in amplitude and phase tracking out to 1GHz and also I wanted the VSWR to be very low.It wouldn't be good enough for what you need but it does show what can be achieved with a few SMD resistors costing less than 1 penny each.Note: You can't really see it but the PCB artwork under the SMD resistors has been optimised using Genesys+Sonnet EM to tune out any remaining parasitics from the stacked 100R resistors. I've got the VNA warming up and I'll post up a few measurements.I've written 3GHz on it so I'll test it right out to the limit of my old VNA (3GHz) and post up the results. I remember that it was remarkably good up to the wanted 1GHz bandwidth so prepare to be impressed

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2014, 02:58:16 pm
content: OK here's the first few plots and it is actually much better than I remember!I remember that I did spend a bit of time adding/removing solder to optimise the performance but the results are definitely 'precision' class. I'm prepared to admit there may be an element of luck in the tolerances of the milling as this PCB was milled on my T-Tech router. Therefore, I seriously doubt I could replicate these results on a second unit but I'd hope to get very close.The first image is the return loss on the sum port. It is better than 35dB at 3GHz although there will be a degree of measurement uncertainty here on my old VNA. However, it measures about 50dB return loss for a precision SMA 18GHz load at 3GHz and this is not the same load I did the calibration with.The second plot is the amplitude tracking. Again the results are staggeringly good even to 3GHz. I aimed for <<0.1dB at 1GHz and met this easily. The two traces are virtually on top of each other on a 0.1dB/div scale.

author: eurofox
author_rank: Unknown
timestamp: July 26, 2014, 03:08:50 pm
content: I just wonder if you put the resistors in the middle leaving space between resistors if you cannot improve the parasitic effect because now since they are so close to each other there should be a capacitance effect and again I'm not an RF expert.

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2014, 03:13:58 pm
content: Yes, ultimately, this splitter will lose performance at higher frequencies due to the layout. I think the performance will probably degrade quite quickly above about 5GHz because I have used 'big' SMD 100R resistors and stacked them on top of each other to get 50R.But it only had to work to 1GHz to test the old HP8405ASee below for the phase tracking.The result is pretty good although it depends on the torque setting for each SMA connector. I've tried to keep it the same on both tests.Port 1 was measured for phase and normalised and the trace below shows the phase error between port 1 and port 2.You can see I met my target of <1degree error at 1GHz.See also the sum port VSWR plot that shows about 1.03:1 at 3GHz.

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2014, 03:38:35 pm
content: I checked on the Minicircuits website and they do a 4.2GHz splitter with SMA connectors (ZFRSC-42+) and it performs about the same as my homemade splitter.They cost about $70 from Minicircuits.http://217.34.103.131/pages/s-params/ZFRSC-42+_GRAPHS.pdfI had to do a bit of tweaking with tiny bits of solder to optimise mine but you can see that it's possible to get this level of performance from a low cost splitter

author: rfbroadband
author_rank: Unknown
timestamp: July 27, 2014, 04:40:21 am
content: you can push these designs up to many GHz (> 10GHz). Here are a few things to consider1) SMD resistors: From an RF point of view a resistor is not a resistor. It is a resistor with a series inductance, parallel capacitance and two C to gnd from each side. If you would calculate the transfer function of a resistor you would find that a resistance value ROpt exists where the resistor is really (almost) just a resistor. If R < ROpt the R has inductive behavior and for R>ROpt is has capacitive behavior. Example DC resistance R is 200Ohms but at 5GHz |Z| = 100 Ohms etc. ROpt depends on the geometry of the SMD resistor. 0402 resistors often have ROpt at around 100 Ohm. This is why two 100 Ohms 0402 resistors in parallel are a much better 50 termination that just one 50 Ohm resistor. I have built a 20GHz precision termination using that method that had > 20 dB return loss up to 20 GHz.2) Obviously choose an RF PCB material that supports the desired frequency range...3) choose the correct connector type that supports your desired freq. range (SMA vs SMB etc)4) be very careful on how you design your SMA to PCB connector footprint for your PCB. It is very easy to introduce resonances that basically destroy your transfer characteristic. It gets even more complex if you decide to use a multi layer board (more than just two layers)5) if you really want to push the frequency range (> 10GHz) you will have to optimize and compensate for steps in width of the transmission line relative to the with of your SMD components6) don't know how much power you want to dissipate..0402 is nice for broadband RF applications there are limits on how much power they can dissipate.hope that helps

author: eurofox
author_rank: Unknown
timestamp: July 27, 2014, 07:33:34 am
content: Quote from: rfbroadband on July 27, 2014, 04:40:21 amIt is very instructive and confirm ideas I got.What about not using PCB at all and connect the SMA terminal to copper wire the SMD resistor in the air in a Y shape?In theory we should avoid at least the parasitic influence of the PCB.

author: G0HZU
author_rank: Super Contributor
timestamp: July 27, 2014, 10:38:07 am
content: I think you can do away with the PCB for low cost and low performance homebrew splitters up to about 1GHz. However, in terms of a precision homebrew splitter for several GHz I think you have to use a PCB. Especially if you want it to be rugged and reliable.In my case I exploited the PCB in order to fine tune out the remaining parasitics from the piggy backed 100R resistors.I have a jig here that lets me measure SMD components very accurately up to about 3GHz so I measured two 100R resistors in piggy format and then used the model from this to help me choose how to 'end' the microstrip on my board such that the return loss up to 1GHz (at each port) was really low. As I said earlier, there is a subtle shape at the end of each microstrip to fine tune the performance. I used Genesys and Sonnet to help me optimise this shape at the end of each of the three microstrip sections.I used some old GIL 2032 PCB material 0.03" thick because it gave the optimal microstrip width to suit the delta pattern of 0603 resistors. I also used decent SMA PCB end launchers soldered top and bottom to give a really good transition to the PCB.So although the circuit board I posted up looks very simple, a lot of modelling was done behind the scenes to give me a good head start.If I wanted to make something (decent) for higher frequencies I have some 0.02" Rogers 4003C material here that would suit 0402 resistors and I'd use some similar looking (but better) Suhner end launchers. However, I'd have to resort to using resistor models from Modelithics because I have no way here to model a pair of 0402 resistors out to 10GHz+.So I'd be working fairly blind and I would need to use one of the 20GHz VNAs at work. I've made resistive splitters for use up to about 12GHz before but these were just on a synthesiser PCB and the performance wasn't critical in any way as I just wanted to crudely split an LO signal. So they were very much fit and forget.

author: G0HZU
author_rank: Super Contributor
timestamp: July 27, 2014, 11:19:04 am
content: I suppose if I was going down the path of the OP I'd be trying to avoid having lots of SMA transitions in the overall 'pulser' system so I'd be looking to build it all on a single PCB. i.e. the pulser and the splitter and I'd use GCPWG to give me more flexible control over the dimensions of the transmission line at various points of the design.