What's the difference between RF splitter & divider - Page 1
Forum section: electronics / rf-microwave

author: analogNewbie
author_rank: Regular Contributor
timestamp: August 25, 2018, 11:07:34 am
content: Recently, I want to calibrate frequency response of my spectrum analyzer. I found that there are 2 types of it.Agilent calls the 2*50ohm ones as splitter, the 3*16.7ohm ones as divider.Per Agilent's document(http://literature.cdn.keysight.com/litweb/pdf/5989-6699EN.pdf), I need to use a splitter to do the job. Connect the common port to signal generator and the other two to the spectrum analyzer and power meter.My question is if the ports are balanced nicely, can I use a divider to do the job?

author: Wolfgang
author_rank: Super Contributor
timestamp: August 25, 2018, 12:42:18 pm
content: Why not ?I built some of them, and they work nicely.https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-rf-splitter-and-combiner-gallery/General hint: Both the splitters and dividers in the Keysight paper are special cases of a generalized passive 3-port.You can find formulas for power transfer, reflection, isolation ... for arbitrary resistors on the web.With this you can compute splitters, dividers and also versions with unsymmetric power transfer like pickoffs or samplers.Before I build such a thing, I always user SPICE to simulate it with all possibilities of source and load conditions.

author: Tony_G
author_rank: Super Contributor
timestamp: August 26, 2018, 05:48:42 am
content: Here is a good intro from HPAK:https://www.keysight.com/main/editorial.jspx?ckey=524913&id=524913&nid=-11143.0.00&lc=eng&cc=CDAnd the original article:https://www.scribd.com/document/345373323/understanding-microwave-power-splitters-johnson-1975Note the SWR/mismatch point.TonyG

author: G0HZU
author_rank: Super Contributor
timestamp: August 26, 2018, 01:16:49 pm
content: It's best to use the splitter version and the quickest/easiest method is to use it with a wideband diode detector and a sig gen that supports external ALC. So as long as the detector response is flat and the sig gen has low harmonics (eg better than -40dBc would be nice) you should be able to simply sweep the sig gen and the splitter/detector setup will keep it level as well as providing a good source match.This assumes you use a good quality splitter and you connect it direct at the analyser input. Otherwise you can easily introduce unwanted uncertainty into the system.If you only have the divider and you want to use the detector/ALC system then the divider will degrade the port VSWR, Therefore, you would have to be careful to keep the ports fairly well terminated so I'd suggest fitting a decent 10dB attenuator in each arm of the divider and the attenuator would need to be amplitude matched and have a flat response and low VSWR. But this method is a compromise (test port VSWR will be typically 1.1:1) and is harder to manage so I'd suggest you buy or make a splitterI designed/made my own precision splitter and a power divider when I bought my HP8405A vector voltmeter a few years back. Both were designed to be precision devices to about 1GHz and to give 'very good' performance to about 3GHz. Not as wideband as the 26GHz versions from Keysight but probably very little in it in terms of RF performance up to a couple of GHz.The classic setup for this type of test would be the old HP 83752A 10MHz to 20GHz sweeper, an HP11667 splitter and a HP8374C detector diode. The HP83752A sweeper typically has very low harmonics by design and supports external (negative) ALC from the detector diode.https://www.keysight.com/en/pd-1000001888%3Aepsg%3Apro-pn-83752A/synthesized-sweeper-001-20-ghz?cc=US&lc=enghttps://www.keysight.com/en/pd-1000001915%3Aepsg%3Apro-pn-8473C/low-barrier-schottky-diode-detector-10-mhz-to-265-ghz?nid=-32706.536879091&cc=GB&lc=enghttps://www.keysight.com/en/pd-1000000345%3Aepsg%3Apro-pn-11667C/power-splitter-dc-to-50-ghz?cc=US&lc=eng

author: analogNewbie
author_rank: Regular Contributor
timestamp: August 28, 2018, 04:28:38 am
content: Thanks guys.I have bought a splitter. ZFRSC-123+.My friend tested this for me. The two arms are pefectly matched util 20GHz. (difference is less than 0.1db)The only difference is the loss is about 10db. And the S11 of input port is over -20db at 6GHz. The S11 of HP 11667C's input port is below -30db over the 20GHz range.

author: G0HZU
author_rank: Super Contributor
timestamp: August 28, 2018, 04:06:31 pm
content: I had a quick look at the datasheet for the ZFRSC-123+ and I don't think it's quite the right type of splitter to use for your applicationThis assumes you really want to minimise measurement uncertainty.When used in a levelling loop with a power meter or diode detector I think your splitter will present a source VSWR of about 1.25:1 rather than the desired 1:1 because of the effect of the ALC feedback and the internal resistor network the ZFRSC-123+ uses. So you will get some mismatch uncertainty.At a rough guess, if your analyser input VSWR is typically 1.5:1 and your ZFRSC-123+ has a source VSWR of 1.25:1, I think that the additional uncertainty caused by the mismatch of the ZFRSC-123+ will be about +/- 0.2dB. So your calibration could have an additional/unnecessary +/- 0.2dB of ripple on it. It really is best to use the classic splitter with the two 50R resistors in it like the image below.Port 1 is driven by the sig gen and port 3 goes direct to the power meter (which feeds back a DC level signal to the sig gen ALC) and port 2 goes direct to your analyser RF input. This way the source match to your analyser is defined by the quality of the 50R series resistor in the splitter.

author: analogNewbie
author_rank: Regular Contributor
timestamp: August 29, 2018, 04:31:32 am
content: I don't know how agilent or rohde schwarz calibrate the frequency response.I use a pretty simple way to do it. The signal generator, spectrum analyzer, power meter are controlled by GPIB. Set signal generator ouput 1M...3GHz0dbm. Read the power from power meter and spectrum analyzer. Calculate the error at certain frequency and modify the compensation data of this frequency.This way, I don't care how much power the signal generator actually ouput.The ALC way could make sure the spectrum analyzer always receives the same power. So the flatness of power splitter and VSWR will contribute to the uncertainty.Agilent 11667 is way toooo expensive.

author: G0HZU
author_rank: Super Contributor
timestamp: August 29, 2018, 12:05:55 pm
content: QuoteIf you do it that (manual) way then your MiniCircuits ZFRSC-123+ should be fine as the port match at the analyser input should typically remain low if we assume your sig gen has a reasonable output match when delivering a few mWHowever, the classic way to do it is with a negative feedback path via the detector diode or the power meter 'recorder' signal. This creates a continuous negative feedback loop and it means that the impedance (within the loop bandwidth) at port 1 will resemble a voltage source due to the continuous corrective nature of the feedback. So this means port 1 of the HP 11667 splitter will have the ultra low impedance of a voltage source and means that the source impedance seen by the spectrum analyser will be defined by the series 50R resistor in the splitter. So your MiniCircuits splitter wouldn't be the ideal choice in this classic caseQuoteYes, they are very expensive and I made my own version and it's probably OK to 2 or 3GHz. I've got all the other classic bits though. i.e. I have the HP 83752A 20GHz sweeper and a few of the old HP43x power meters with the recorder output here at home. I also have the classic 26GHz HP8473C detector diode here. So I've performed tests like this quite a few times. I've also done it at work many, many years ago using vintage sweep generators and a HP8405A vector voltmeter, detector diodes and the HP 11549A splitter (or a coupler).

author: G0HZU
author_rank: Super Contributor
timestamp: August 29, 2018, 09:30:02 pm
content: QuoteYou get the calibration for free if you use the classic setup that uses negative feedback via the detector diode or the power meter. This is because the power meter provides an ALC signal to calibrate the sig gen in real time***. Provided that the power meter is accurate and flat the system should be auto levelling because of the error correction the ALC loop provides. The power meter will constantly correct the sig gen level via the ALC feedback loop. The feedback also delivers a very low impedance at port 1 of the 11667 splitter so the analyser sees the 50R arm of the splitter terminated in an ultra low Z voltage source. So the source impedance of the splitter should be very close to 50R and this helps minimise uncertainty. But you obviously need to use a splitter with two 50R resistors like the 11667 to get this benefit and not a power divider with three 16.666 ohms resistors. I'm guessing that your MiniCircuits ZFRSC-123+ splitter uses a series 66.67 resistor and a shunt 100R resistor in each arm so it isn't quite the same as an 11667 splitter.I set up my HP83752A sweep generator to run in external ALC mode driven from the recorder from one of my HP432A power meters and had a go at looking at the response of my old HP8566B spectrum analyser on range 1. This range covers up to 2.5GHz and I used my homebrew splitter that I designed for my 1GHz HP8405A VVM. I designed a couple of 3 way devices like this. One was a divider and one was a splitter if you adopt the HP terms used to describe each device in your linked document.Both work very well as precision devices up to 2 or 3GHz and see below for the response of my HP8566B analyser to 2.5GHz on 1dB/div. This is therefore the ALC based setup with negative feedback so it should give a very good port match from the splitter. You can see there is a tiny dip in the HP8566B response part way through the range and this is really there on the analyser because I know about this already. There's also a 0.3dB peak down at about 25MHz but that isn't visible on the plot below because the test starts at 50MHz in 50MHz steps. Few modern analysers will be this flat across frequency unless they use a detailed correction table in software.The (measured) input return loss on my HP8566B with 10dB internal attenuation is better than 26dB across this 2.5GHz range and this also sets it apart from other analysers that might only manage 15dB return loss typical.*** When the power meter is used the loop bandwidth of the ALC is very narrow so it can take a couple of seconds to settle at the correct/calibrated level after each step change in frequency. The detector diode is much faster but probably won't be as flat as the power meter response.

author: G0HZU
author_rank: Super Contributor
timestamp: August 30, 2018, 11:49:53 pm
content: To show how it can self level automatically if you use an 11667 splitter and the classic ALC system, I set up the 83752A sweeper to feed the splitter via a cheap and nasty RG58 cable that had about 4dB loss by 4GHz. This cable has high loss and poor return loss. So without any ALC correction the power level at the splitter would droop badly by 4GHz.However, this doesn't really matter because as long as the ALC system has enough correction range it can auto correct for the loss in the cable with no need to characterise the cable loss in any way.I set the system up as the diagram below and replaced the spectrum analyser with another power meter as a means to check the flatness of the system. I then stepped the 83752A across 10MHz to 4000MHz and let the ALC power meter do the levelling. I then measured the power level at the 'test' port of the splitter with the other power meter. I have some SW here I wrote a while back that lets me test the response of all my sig gens using a power meter. So this evening I tweaked the SW to allow me to do the test above and log the response on the graph of the SW GUI as in the image below.You can see that despite the loss in the cheapo RG58 cable the system did a good job of auto levelling the signal to 0dBm across 10MHz to 4000MHz. The result isn't perfect because I didn't include the subtle efficiency corrections for the power sensor head in the ALC power meter. So there is a slight error up towards 3GHz and down towards 10MHz. Also, above about 2.5GHz my homebrew HP 11667 splitter will be losing some performance. But when you consider that the scale on the graph is +/- 1dB it's still a very good resultNote also that the cal factor correction data on the GUI display is there for the sensor head in the power meter that is measuring at the test port (as the DUT). This ensures that this power meter reads correctly as the sig gen is stepped across the frequency range. I have the factory corrections for this power meter sensor loaded into the SW and it interpolates smoothly across frequency.Sadly, the power meter in the ALC loop runs at a fixed cal factor so this is partly why there is a subtle creeping error up towards 4GHz. But the error is very small and it isn't really worth the time/effort to put another cal factor correction in to the SW for the ALC power meter. I mainly use this software to automatically check the level vs frequency response of my various sig gens and I've been tweaking this SW slowly over the years to add more features