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. > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu