[SI-LIST] Re: VNA Question

 Jayaprakash,

When you do the calibration on the VNA, it will correct 
automatically for mismatches and cable losses.  From 
that point on, the reference impedance is provided by 
the VNA.  In the defining equations, the 50-ohm reference 
impedance is the impedance that you connect to the DUT, 
not the DUT's impedance.  Once you have the VNA setup 
calibrated, the impedances connected to the DUT will be 
50-ohms.  

BTW: the reference impedance does not have to be 50 
ohms, it is just a matter of convenience.

Regards,
Istvan Novak
SUN Microsystems


----- Original Message ----- 
From: "JP" <edpc108@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, June 01, 2003 1:27 PM
Subject: [SI-LIST] VNA Question


> Hello SIers,
> 
>  This is a basic VNA question, yet I don't have 
> clarity. 
> 
>    In theory, S parameters are obtained by perfectly
> matching the relavant port impedances. 
> 
> For instance,  considering a 2 port system, 
> 
>  b1 = S11*a1 + S12*a2
>  b2 = S21*a1 + S22*a2
> 
>  bi - reflected voltage
>  ai - incident voltage
> 
>  S21 is obtained by ensuring a2 = 0, which implies
> perfect match at port 2. 
> 
>    => S21 = b2/a1
> 
>  In reality, however DUT impedances need not be 50ohms
> and is often not known. How does VNA ensure
> a2(reflected component at port2) equals zero ?
> 
>  Does VNAs have the mechanism to measure mismatch 
>  and account automatically?
> 
>  Thanks in advance,
>  Jayaprakash. 
> 
> 
> 
>    
> 
> 
> 
> =====
> Balachandran Jayaprakash                             
> Celestijnenlaan,                                                   
> 3/61, 3001, Heverlee,                                           
> Leuven, Belgium.                                                  
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> 
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