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    brendan.lynskey
    Feb 04, 2021

    2-port shunt-thru: why is GND isolator unnecessary at HF?

    in Welcome to the Forum

    In lesson 7, it's stated that a GND isolator (transformer or solid state) isn't needed for HF measurements. Although I use a J2102B, I'd like to understand why.


    I've wondered about this since reading that coax pigtails used in a VDD-VSS measurement carry a good differential signal at HF, and it's not necessary to connect separate VDD and VSS cables to 2 scope channels for subtraction.


    My guess is that at sufficiently high frequency, the distributed L of the cable presents a sufficiently high reactance to create a Z0~=50ohms, which dominates the distributed R of the cable shield. That makes the drop across the R insignificant, thus attenuating the CM signal.


    Is that right?



    7 answers1 reply
    8 Comments

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    brendan.lynskey
    Feb 05, 2021

    OK, so under 10kHz, you measured an impedance floor of:

    ~20mOhm for 1m RG316 (no GND isolator)

    ~11mOhm for 1m PDN cable (no GND isolator

    ~0.174mOhm for 1m PDN cable (with SS isolator)


    Very impressive - thanks!

    1
    S
    steve
    Feb 05, 2021
    Replying to

    You bet ;)

    Like

    S
    steve
    Feb 05, 2021

    Exactly. The error is Rshield divided by CMRR. So when I did the 100uOhm paper I had to both reduce Rshield and also improve CMRR (that's why J2102B is now a "B" and not an "A" ;) )


    Of course from this it is also obvious that shorter cables are more better. AND since the 2 shields are in parallel, making one really short helps a lot. That's why you often see components mounted to 1 port of the Bode 100 using an adapter and not a cable ;)


    Once you get to, and below, 1mOhm everything starts to matter. There are many other benefits of PDN cable too (bandwidth, temperature range, flexibility and shield attenuation). I did show the comparisons in the class, so you should have seen the shield attenuation exercises.


    1

    brendan.lynskey
    Feb 05, 2021

    Yes makes sense.


    So a GND isolator can take you so far, but you can bring your impedance floor down further by reducing the impact of the Rshield? I take it the most important side for Rshield is the port1 connection, as that's where most current flows?


    At what impedance DUT would you say that PDN cables become necessary? That's for a 2-port shunt thru measurement with a BODE100 and a J2102B?


    1

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    steve
    Feb 05, 2021

    Precisely! No, I get excited about it too ;) Rest assured this took some work to get through. It wasn't obvious to me at first. Once you understand this, you also understand why we designed our own coaxial cable... Our PI goals are very different than SI goals. We care mostly about the shield properties and SI cares mostly about the center conductor properties.

    1

    brendan.lynskey
    Feb 05, 2021

    Thanks for this.


    So the model on p18 of that paper shows 3 mutual inductances, which combine to create a frequency-dependent CMRR?


    I'm glad I took the time to study this. It was tempting to skim the paper when I looked before.


    Am I the only person who gets excited about coax cables? Physics in action, and a piece of history!

    1

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    steve
    Feb 05, 2021

    Brendan see this University Paper in case you don't have it The math is included in excruciating detail ;) The model of the cables are also shown.


    https://www.dropbox.com/s/awf2prf7oj9avmb/UNIVERSITY%20How%20to%20Measure%20Ultra%20Low%20Impedance%20%28100uOhm%20and%20lower%29%20PDNsVersion17b.pdf?dl=0

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    steve
    Feb 05, 2021

    You're on the right track. At higher frequency the coupling of the inner and outer conductors form their own common mode transformer.


    I showed a model of this cable in my Measuring 100uOhm EDICON University paper.

    1
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