Passband ripples in compact wideband wilkinson power divider - Page 1
Forum section: electronics / rf-microwave

author: enclis
author_rank: Contributor
timestamp: June 30, 2021, 04:01:47 pm
content: Hello!I need to build a cheap compact wideband 1x2 power divider (0.6 - 2.4 GHz) with smooth passband on both output ports, ripples should be less than 0.5 dB per 1 GHz of the band. Output port isolation does not really bother me, so there are no resistors in my design. I've already experienced in building power dividers, but that was single-stage narrowband and there was no problem, so I drew the simplest circuit in ADS for 0.5 mm FR4 18 um copper, optimized it to get better results, exported the topology to HFSS, the results of modeling are almost identical with ADS.(wilki_hfss.jpg)(s12_s13_hfss.jpeg)(s11_hfss.jpeg)I've ordered 9 different variants of my wilkinson divider optimized for different dielectric constant (4.4 to 4.1), because I don't know the exact value for the FR4 material that was used in pcb production. Spoiler alert: turned out thatallof the 9 variants give almost identical results at the output. I've measured real devices but got poor results in terms of ripples. In less than 200 MHz of passband it drops rapidly by 1.4 dB. The situation in the center is also not good. It is also a little surprising that it somehow rises up to -3dB:(comparison.jpg)Is it possible to improve the situation in terms of ripple? Is there something wrong with the topology? Maybe I'm doing the wrong measurements? Or there is nothing to complain about these really small ripples in the passband? I'm using USB-SA44B + USB-TG44A.

author: enclis
author_rank: Contributor
timestamp: June 30, 2021, 05:57:42 pm
content: Quote@evb149, if you would be more watchful you can find out that [wilki_hfss.jpg] is the same ashttps://d2xunoxnk3vwmv.cloudfront.net/encyclopedias/636541211688242689-couplerfig3a.gifexcept resistors. You've thought I just randomly put some elements in the scheme and it somehow works out?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 03, 2021, 08:43:27 pm
content: Quote from: enclis on June 30, 2021, 05:57:42 pmMaybe it's getting lost in translation (I know, I myself am from eastern Europe too originally) but man, responses like this won't make people want to help you.Anyway, I can't comment on your design however if you haven't yet I'd try to put two decent quality 10dB pads on the end of your measurement cables to improve matching. Based on the ripple frequency you could have a .5m and a .2m cable on the two sides although it might be misleading in case of low loss insertable devices.Your plots don't show output match and especially if you have process parameter variation (e.g. dielectric constant) achieving the >20dB input match can be tricky too. Considering that it's entirely possible that you're seeing the mismatch loss ripple on your plots. If you don't have attenuators at hand you can try a different length cable and see if that changes the distance between the ripple peaks.Also make sure you terminate the unused output port when you take the measurement.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: July 03, 2021, 11:15:11 pm
content: @Marsupilami is likely onto the problem. How did you measure the Device? if you used full-2-port calibration and a precision load on the other port, you should get a good result. But if you have errors in your calibration, or you only do a response calibration (or no calibration), or you use a cable then a load on the third port, then you can see the mismatch ripples very much the size you see in you measurement.Of course the best way to measure is a full 3 port calibration on a 4 port VNA. But if you don't have a way to measure it with full calibration, using high quality attenuators can really improve the S21 response.Oh, and even if you did do a full 2-port or 3-calibration but you did not use the "SOLR" method or you did not account for the delay of the thru connector during cal, it can cause this kind of issue too (common problem with HP 8753 and HP 8510 analyzers)