Fake directional couplers from eBay? - Page 1
Forum section: electronics / rf-microwave

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 04:53:47 pm
content: I ordered 2 directional couplers from one eBay seller. In lot he wrote that is Mini Circuits ZFDC-20-5. It is good directional couplers with good directivity (25+ dB) and frequency range (0.1...2000 MHz).Today I measured the parameters and was unpleasantly surprised. Both of these couplers not work normally from 100 kHz at least.I opened the case and take some picture. Found one photo in web (front,back) and see that mine is differ. On web photo ZFDC-20-5+ but if I understand is ROHS model. My PCB has elements in one side only.And on MiniCircuits schematic main line with DC Block. Here almost zero resistance between ALL terminals.Can somebody tell me is it fake ZFDC-20-5 or may be MiniCircuit make differ model with this part.number early?May be somebody have inside photo else of this model?ZFDC-20-5.pdf

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 09, 2017, 06:28:25 pm
content: If your measurements are done properly you've got a good reason to return them. It looks a bit like someone tried to repair it?Also, here's another picture, different again

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 06:57:44 pm
content: Quote from: PA0PBZ on June 09, 2017, 06:28:25 pmThinks correct. I measure directivity after normalization on open end input (TG on output, DET on Coupled end) and after it measured with 50Ohm terminator on input end.Hope so.... I wrote to seller todayQuote from: PA0PBZ on June 09, 2017, 06:28:25 pmDo not know. For me looks like it another coupler with ZFDC-20-5 caps.Thanks for picture! Yes, I see, it is differ. And how parameters of your coupler?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 09, 2017, 07:05:39 pm
content: Quote from: Safar on June 09, 2017, 06:57:44 pmIt's not mine, I found it on the net:https://www.mikrocontroller.net/topic/284709

author: slurry
author_rank: Regular Contributor
timestamp: June 09, 2017, 07:19:05 pm
content: I would also call it a fake, or a "repaired" one, but it certainly does not look original to me, it looks like the board have been cut from something else.Here is a how a late example of genuine ZFDC-20-33 looks inside, sharp even cuts and nice soldering:

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 07:23:47 pm
content: Quote from: PA0PBZ on June 09, 2017, 07:05:39 pmYes, thanks, I see. And it looks like my case. In this article author repaired coil damaged by DC. It is must to go and look coil more closely

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 07:35:25 pm
content: Quote from: slurry on June 09, 2017, 07:19:05 pmVery possible. But for me is very strange that exist several version of PCB and is all absolutely different.Quote from: slurry on June 09, 2017, 07:19:05 pmThanks.As I said I bought 2 couplers (as 1 lot) but I can't open another. But it very similar by RF params.

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 10:52:48 pm
content: Quote from: evb149 on June 09, 2017, 08:29:49 pmThanks.I made schematic diagram of my version (sorry for placed In and Out not in classical way). And I see it very similar to repaired coupler fromhttps://www.mikrocontroller.net/topic/284709.Transformer is actually two sections ferrite bead (look picture) and 0,5 transformation gain is just a wire passed through the section (green color). It also coupled with another as 2 turns twisted pair (U1, U2, U3)And as I see signal path from IN terminal (and OUT the same) is just 1 cm and 5 turns of wire to ground, so I think that is too small reactance on low frequency (<1MHz), may be 5..10 Ohm on 50..100 kHz. I think that tracking generator overloaded on this frequencies and very possible that is the reason of my measurements result.

author: Bud
author_rank: Super Contributor
timestamp: June 09, 2017, 10:53:17 pm
content: I've never seen Minicircuits boxes painted, beside very small SMA ones. Usually they alumjnum anodized as in Slurry's photo.OP: your method of measuring directivity is ,well,interesting, and aparently produced wrong result. why dont you just substruct reverse coupling from forward coupling.

author: Safar
author_rank: Regular Contributor
timestamp: June 09, 2017, 11:34:26 pm
content: Quote from: Bud on June 09, 2017, 10:53:17 pmI think it depend production time may be. I have several type, some painted, some notQuote from: Bud on June 09, 2017, 10:53:17 pmYou are right, but result is very similar. When you feed OUT and leave open IN end almost all energy reflect back and this is like feed from IN end (it may be here add some from OUT - reverse path), than you make normalization and connect terminator to IN end. All (almost) energy is absorbed in terminator and only reverse coupling path passed to CPL terminal.I made 2 measurements with different way

author: Safar
author_rank: Regular Contributor
timestamp: June 10, 2017, 12:13:26 am
content: I measured inductance of coupler terminals with RLC meter on 100kHz and L is 2.28uH on IN and 2.38uH on OUT end and very low ESR, so reactance on 100 kHz is only 1.5 Ohms!!! It is very strange because working freq of this model start from 100 kHzNot surprised that tracking generator is overloaded on this frequencies and that is the reason of my measurement results.

author: Bud
author_rank: Super Contributor
timestamp: June 10, 2017, 01:11:05 am
content: Quote from: Safar on June 09, 2017, 11:34:26 pmNot quite, you can see the math method produced a smoother curve. And this is closer to reality, because energy in the main line goes mainly one way and exibits less reflection between ports, whereas with your other method there are reflections bouncing back between In and Out ports interacting with each other and that causes ripple. I still think nobody is doing directivity measurements this way.Also, you cant measure devices well with a spectrum analyzer at high frequencies. You need a VNA for that to calibrate out the instrument ports mismatch.As to your doubts on the coupler model , you can send a email to Minicircuits, attach a photo and ask them to confirm.

author: nctnico
author_rank: Super Contributor
timestamp: June 10, 2017, 11:02:50 pm
content: There is some interesting info I picked up from another thread:https://signalhound.com/support/forums/topic/calibration-for-measuring-return-loss/It seems the impedance mismatch from the spectrum analyser can mess things up when using a directional coupler.

author: Safar
author_rank: Regular Contributor
timestamp: June 10, 2017, 11:11:22 pm
content: Quote from: Bud on June 10, 2017, 01:11:05 amAgree that measurement from different ports is more accuracy. But for me it was just to understand the qualitative characteristics, rather than to measure out with great accuracy. And by this reason I use long cables (1m) from SA, so I think is effected in resonance ripple on curve. And this method is not my invention. I see this one in several sources, first one I can remember is rf-explorer site.Quote from: Bud on June 10, 2017, 01:11:05 amI think you understand that phase response (that produce VNA) is not so important for understanding how good this coupler, frequency response is enough IMHO. Of course SA has TG normalization and I did it. BTW I have small VNA (Tiny), but it has not so good dynamical range (60..70dB). And I understand that VNA is betterQuote from: Bud on June 10, 2017, 01:11:05 amYes... I already think about itAnyway is somebody can tell me how good this coupler work in lower frequency (from 100 kHz)?

author: Safar
author_rank: Regular Contributor
timestamp: June 12, 2017, 04:18:53 am
content: Quote from: nctnico on June 10, 2017, 11:02:50 pmIMO this can help when the mismatch is small. In my case impedance difference is very big at low frequencies (see att. - VNA limited from 1 Mhz and in lower freq all is much worse)I tried to used the 3, 6, 10dB attenuators (on both ports also) but changes are very small.I think it's worth to asking a MiniCircuits...