[SI-LIST] Fw: Re: The Ground of VNA

----- Original Message ----- 
From: "Zhangkun" <zhang_kun@xxxxxxxxxx>
To: <istvan.novak@xxxxxxxxxxxxxxxx>
Sent: Tuesday, May 18, 2004 7:52 PM
Subject: Re: [SI-LIST] Re: The Ground of VNA


> Dear Istvan
> 
> The following is my understand about the ground loop in VNA.
> 
> When measurement is done by means of VNA, there is two impedance between the 
> ground of two ports:
>     1. In the VNA, because the instrument is single-ended connections, there 
> is finite impedance (Calling Za)between the two ground.
>     2. In the DUT, there is impedance unknow (Calling Zb).
> Za and Zb are in parallel.
> 
> When Za<Zb, Za is dominant. The measurement result will be influnced by the 
> ground loop. For example, there are two differential pair with no ground 
> plane. The crosstalk should be measured. Therefore, the ground loop should be 
> butted by transformer of 1:1. 
> When Za>Zb, Zb is dominant. The measurement result will not be influnced by 
> the ground loop. For example, in the DUT, there is complete reference plane.
> 
> I have found that in different situation, the effect of ground loop in VNA is 
> different. In current measurement about crosstalk between differential pairs 
> with no reference plane, the influnce could get to above 20MHz. Now my 
> difficulty is that I could not found one suit transformer of 1:1 to break the 
> ground loop.
> 
> Best Regards
> 
> Zhangkun
> 2004.5.18
> ----- Original Message ----- 
> From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
> To: <zhang_kun@xxxxxxxxxx>; "si-list" <si-list@xxxxxxxxxxxxx>
> Sent: Tuesday, May 18, 2004 7:18 PM
> Subject: [SI-LIST] Re: The Ground of VNA
> 
> 
> > Dear Zhangkun,
> > 
> > High-frequency instruments use single-ended connections;
> > most of the differential inputs and outputs are also formed
> > of individual single-ended connections.  When you make
> > the connection from a VNA to the DUT with two cables,
> > you make a ground loop. The ground loop creates a
> > problem only at low frequencies, where the resistance of
> > the cable braid dominates the return-path impedance.
> > At high frequencies (with most cables above a few MHz),
> > the inductance of the cable braid opens the ground loop
> > so much so that you can cross-connect the cables by
> > connecting the hot side of cable1 to the ground side of
> > cable2 and hot side of cable2 to the ground side of cable1
> > and under some circumstances you will hardly notice the
> > difference in the response.
> > 
> > Regards,
> > 
> > Istvan Novak
> > SUN Microsystems
> > 
> > 
> > ----- Original Message -----
> > From: "Zhangkun" <zhang_kun@xxxxxxxxxx>
> > To: "si-list" <si-list@xxxxxxxxxxxxx>
> > Sent: Tuesday, May 18, 2004 5:12 AM
> > Subject: [SI-LIST] The Ground of VNA
> > 
> > 
> > > Dear All
> > > Could you tell me the details about the ground of VNA? From the
> > experiment, I could find out that the ground of two ports of VNA are
> > connected. However, I do not know the detail.
> > >
> > > This is critical for some kind of analysis. For example, there is no
> > ground plane.
> > >
> > > Best Regards
> > >
> > > Zhangkun
> > > 2004.5.18
> > >
> > >
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