Author Topic: Need Guidance on Everyone's Favorite Subject: Grounding!  (Read 6071 times)

0 Members and 1 Guest are viewing this topic.

Offline igniluxTopic starter

  • Supporter
  • ****
  • Posts: 92
  • Country: us
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #25 on: August 02, 2021, 12:08:52 am »
So.  It could be that real parts do very well indeed, but how much better, you'll have to measure that yourself.  (Prime opportunity for a test jig and NanoVNA? ;D )

I don't think this was originally directed at me, but you inspired me to dig through the junk bin to find an old router that I was never going to use again. As luck would have it, one of the transformer modules on there was perfect for a quick and dirty proof of concept. It is an old Delta DIP-16 package, with four independent cores, each with a single primary/secondary in 1:1 ratio. Each winding measured in the neighborhood of 260 uH at 100 kHz, which appears to be typical of ethernet transformers. Leakage inductance was under 200 nH at 100 kHz for the pair that I measured. Interwinding capacitance was 8.13 pF, again at 100 kHz.

I hotglued it dead bug style onto a piece of copper clad, and connected three of the windings in the CMC --> Center tapped transformer arrangement shown in the schematic of my last post. Just to make the EMC engineers in the audience shudder, I soldered the coax center conductor and shield pigtail across the transformer instead of bothering with a proper connector. The output was loaded with two paralleled 100R, 1%, quarter watt, THT, metal film resistors. You can see the test fixture in all of its horrible glory below.

I ran two individual tests, both using an original NanoVNA and a span of 1 MHz to 20 MHz. With a 50 ohm load, return loss in the area of 10 MHz was around 16 dB. As predicted, the reflection coefficient shows the load impedance with a bit of extra inductance and resistance. The second test was performed by cutting one of the 100R resistors out of the load. Here the return loss drops to around 9 dB, but the response is much flatter across frequency. I didn't bother to measure S21, since the load is well characterized and mostly resistive. I can't imagine more than 1 or 2 dB of loss was present.

 
The following users thanked this post: T3sl4co1l

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 21606
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #26 on: August 02, 2021, 03:58:06 am »
Nice!

Can also check out what CMRR is like, or PHY side balance (CT GND, DM or CM balun(s) from winding ends to port 2)?

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline igniluxTopic starter

  • Supporter
  • ****
  • Posts: 92
  • Country: us
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #27 on: August 04, 2021, 12:14:27 pm »
Can also check out what CMRR is like, or PHY side balance (CT GND, DM or CM balun(s) from winding ends to port 2)?

I'm trying to decide how to do this. By DM/CM baluns, I assume you're referring to using a voltage balun (transformer, DC isolating) or current balun (common mode choke, not DC isolating) to convert balanced output back to unbalanced, then feeding that to port 2?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 21606
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #28 on: August 04, 2021, 02:36:04 pm »
Exactly.

The tricky part perhaps is having baluns that are good enough (potentially better than DUT?) to see it clearly.  Or if nothing else, reading both ports independently and doing it in software, if the measurement is accurate enough to take the difference of.

Note port 1 will be terminated, it will just be a resistor in parallel with the primary windings shorted together.  Well, probably slightly better to not short them completely, but use matched (<1%?) resistors to terminate them as well, so that equal resistances go from each pin to the source.  Probably, it would suffice to do, like, port 1: shunt R (50 ohm terminator), tied to primary CT; 50 ohm from CT to start winding; 50 ohm from CT to end winding.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline igniluxTopic starter

  • Supporter
  • ****
  • Posts: 92
  • Country: us
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #29 on: August 05, 2021, 01:41:18 am »
The tricky part perhaps is having baluns that are good enough (potentially better than DUT?) to see it clearly.

Right, that was my concern as well. Perhaps time to invest in some tape-wound nanocrystalline cores  8)

So, I spent a long time today consulting various texts and reading and watching media concerning CMRR. Combined with your suggestion to terminate the center tap of the primary, I have come up with the simulation below. Is this what you had in mind? I'm not sure how much the series resistors on the primary really matter, since we're making a relative measurement between the left and right circuit. So long as they are reasonably close to the characteristic impedance of the system it ought to come out in the wash, no?

The one thing that I learned in my research today is that you can ask 5 people how to test CMRR, and they will give you 5 different ways. 3 of them will be outright wrong. The other two will each agree that eachother's data seems reasonable, but they will fight to the death over a small detail in the procedure. One of the original three will then claim he corrected his mistake and now gets the same data as one of the last two. Then someone from Marki or Wenzel will come along and demonstrably prove everyone wrong, but refuse to explain the method. The war of attrition continues.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 21606
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #30 on: August 05, 2021, 02:00:05 pm »
Like this:

Oh, I suppose the "X" should be optional -- if the line is not included in the primary side CMC, it's probably better not to connect it.  Should be able to measure that too.

Maybe some other arrangement of resistors would do, with the same Thevenin resistance presented across the primary, and to the source, but whatever.

The fact that it's two 25 ohms in series between source and transformer, increases the CM source by 12.5 ohms total.  That could cause a tiny roll-off at HF, but these are low impedances either way -- likely it would amount to a dB or so near a GHz.  And the primary side CMC is way more impedance.  So who cares.  Like, ideally you'd use a balun, but again, where are you going to find one more ideal than the DUT, and that's also better than plain ass resistors at 1GHz?... ;D

Alternately, the source doesn't really care about reflections does it?  I suppose the shunt termination (R3) isn't really needed (get an extra +6dB on your output for free).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline igniluxTopic starter

  • Supporter
  • ****
  • Posts: 92
  • Country: us
Re: Need Guidance on Everyone's Favorite Subject: Grounding!
« Reply #31 on: August 08, 2021, 05:19:59 pm »
I'm finally able to show some results after a few days spent trying to create the world's best balun. I had some pretty decent results, but ultimately I decided to reevaluate my strategy. I scrapped the original transformer jig, and instead dug another module out of the junk bin. This one has four sets of three cores when viewed from the bottom, and 12 pins per side. This leads me to believe that it's exactly what I was looking for, i.e. a center tapped transformer with CMCs on either side. It was in an unusually wide SOIC package, though, so I had to get creative and cut a SOIC DIP adapter board in half to be able to easily prototype with it. I hot glued the whole mess to a 0.7mm solid copper shim, and added a couple of SMA connectors. The result is shown below. (The solder looks like sheeit, but that's just where molten solder hit some runny hot glue. All of the joints are happy and healthy.)

As for the data, it looks to me about what I'd expect. S11 floats around just below 0 dB, and S21 is ridiculously low at the 1 MHz start of the sweep, gradually rising as the frequency increases. At the target frequency of 10 MHz, S21 is down around -42 dB. Between the rejection from a ferrite over the coax jacket and the "typical" 70 dB CMRR of the AD8130, it looks like this will be a great solution.

I'm thinking of making the whole signal path be symmetrical: Input coax --> CMC --> Transformer --> AD8130 (differential receiver) --> HIgh speed opamp buffer (OPA890?) --> AD8132 (differential cable driver) --> Transformer --> CMC --> Output coax. Due to the nature of a fully differential amplifier, the isolation between individual channels is compromised when driven from a common source (since the inverting output is fed back to the noninverting input, unlike a traditional opamp). The opamp buffer helps restore this.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf