[SI-LIST] Re: Differential S-parameters

  • From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 10 Jan 2002 10:32:41 -0600

Samir,

Let me first start out by saying I have *not* fully studied
this issue or how the ATN/Agilent system works, but ....

Regardless of theory, a differential amplifier is rarely
(never?) truly balanced; one side will have slightly different
parasitics than the other.  At low frequencies, this is typically
not noticable, but as the frequencies approach the limits of
the technology, these small parasitic differences start to
creap up and play a significant role in odd-to-even mode and
even-to-odd mode conversion.

If a "pure" odd-mode signal (no even-mode) is applied to an amplifier,
the amplifier will generate some even-mode signal.  Similarly,
when a "pure" even-mode signal is applied, some level of
odd-mode signal will be generated.

In addition, circuits amplify (or attentuate) odd- and even-mode
signals differently.  For example, a good differential buffer will
amplify odd-mode while attentuating even-mode.

The end result is that odd- and even-mode signals have
completely different effects on the amplifier, and the amplifier
treats the signals differently.

When measuring a differential amplifier with single-ended
equipment, the injected signal (into the amplifier) has 
equal amounts of odd- and even-mode strength, but the 
output signal (from the amplifier) will have different
strengths of even- and odd-mode signals.

Some of the measured "odd mode gain" will be due to 
simple differential gain through the amplifier, but
some will be due to even-to-odd mode conversion.  In
addition, where you might expect/hope there will be
no even-mode signal at the amplifier output, there will
be some, again both due to even-mode gain and 
odd-to-even mode conversion.

Again, I am not fully clear on the theory behind the
existing equipment, but I do not see how the device
imbalance and mode conversion can be accurately taken
into account in a single-ended measurement.  It might
be possible and simply a limitation of my ability
to understand. ???

Pat



> Hi Pat,
> 
> just curious here... I don't really know how the differential 
> S-parameters measurement are implemented/performed within 
> ATN/Agilent system (I can find out), so I believe what you 
> said in your email "single-ended measurements and 
> mathematically converts the results to differential", could 
> you please tell us what are the limitation(s) of such method 
> in case of active devices?. 
> 
> Thanks.
> 
> Samir.
> 
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