FreeLists / si-list / [SI-LIST] Re: Transfer impedance

[istvan novak <Istvan.Novak@xxxxxxx>]

Sam,

Yes, principally lower Z21 means less noise coupled between the two 
points. 

However, keep in mind the definition of impedance: it is voltage out per
current in.  This means if the point of noise injection is low impedance,
you can get very low noise voltage at the point of injection, but may
get a much higher voltage at a remote point.  A typical example is if
you drive an open-terminated quarter-wave transmission line with a
square wave, where the fundamental frequency equals the quarter-wave
resonance frequency.  The input impedance of the quarter-wave
transmission line will be very low (zero for ideal losless line) at all
harmonics of the square wave, creating very low input voltage. 
The output voltage, however, may be very big, approaching infinite for
lossless line and zero source impedance.  In other words the voltage
gain V2/V1 can be very big. 

Regards,

Istvan Novak
SUN Microsystems

Sam M wrote:

>Hello all,
>   
>  I am confused abt the relation between transfer impedance (say Z12) and 
> noise coupling between two power rails. if the transfer impedance is less, 
> does it mean the noise coupling is less between the two power rails?
>   
>   
>   
>   
>       
>  
>

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