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
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
If your measurements are done properly you've got a good reason to return them.
Thinks 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 today
It looks a bit like someone tried to repair it?
Do 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?
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:
It's not mine, I found it on the net: https://www.mikrocontroller.net/topic/284709
Yes, 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
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.
Very possible. But for me is very strange that exist several version of PCB and is all absolutely different.
Here is a how a late example of genuine ZFDC-20-33 looks inside, sharp even cuts and nice soldering:
Thanks.
As I said I bought 2 couplers (as 1 lot) but I can't open another. But it very similar by RF params.
I guess I'd try to trace the circuit and draw it out and compare it to the other circuits you see in the photos to see if you can see if it is approximately the same except for those caps. I don't know if the bodge wire is a repair or what. See if your multiple units are the same as each other if you have several.
Thanks.
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 from
https://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.
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.
I've never seen Minicircuits boxes painted, beside very small SMA ones. Usually they alumjnum anodized as in Slurry's photo.
I think it depend production time may be. I have several type, some painted, some not
OP: your method of measuring directivity is ,well,interesting, and aparently produced wrong result. why dont you just substruct reverse coupling from forward coupling.
You 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
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 kHz
Not surprised that tracking generator is overloaded on this frequencies and that is the reason of my measurement results.
You are right, but result is very similar.
Not 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.
Not 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.
Agree 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.
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.
I 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 better
As to your doubts on the coupler model , you can send a email to Minicircuits, attach a photo and ask them to confirm.
Yes... I already think about it
Anyway is somebody can tell me how good this coupler work in lower frequency (from 100 kHz)?
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.
IMO 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...