Oleksiy wrote on Jan 29^th, 2009, 5:46am:

There is a paper regarding a measurement of differential NF in IEEE JSSC.
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TITLE: De-embedding the noise figure of differential amplifiers
ISSUE: IEEE Journal of Solid-State Circuits, vol. 34, pp. 882 - 885, June 1999
AUTHORS: A. A. Abidi and J. C. Leete
KEYWORDS: Amplifiers; Baluns; Cascade networks; Circuit noise; Differential amplifiers; Electric noise measurement; Losses; Measurement noise; Multiport networks; Noise measurement; Radiofrequency amplifiers
BIBLIOGRAPHIC ENTRY:
A. A. Abidi and J. C. Leete, "De-embedding the noise figure of differential amplifiers," IEEE Journal of Solid-State Circuits, vol. 34, pp. 882 - 885, June 1999.
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ABSTRACT:
Many radio-frequency circuits use differential topologies, whereas the instruments to evaluate them are inevitably single ended. Taking into account noise correlations, this work extends Friis' well-known equation for the noise figure of a cascade of two-ports to the case of three-ports, such as baluns, which surround a differential amplifier during measurements. The amplifier gain and noise figure may be accurately de-embedded with single-ended measurements on the cascade and on the baluns. The new formulas are experimentally validated.
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REFERENCES:
The links below are references to articles within the collection. Please refer to the paper itself for a full list of references.
[1] Ahmadreza Rofougaran, James Y.-C. Chang, Maryam Rofougaran, Asad A. Abidi; A 1 GHz CMOS RF front-end IC for a direct-conversion wireless reciever, IEEE Journal of Solid-State Circuits, vol. 31, pp. 880 - 889, July 1996.
[2] RF/IF Designer's Handbook, Mini-Circuits, Brooklyn, NY, pp. 2 - 2, 1992/1993.
[3] RF and Microwave Designer's Handbook., Watkins-Johnson Co., Palo Alto, CA, p. 670, 1993-1994.
[4] A. van der Ziel; Noise: Sources, Characterization, Measurement, Prentice-Hall, Englewood Cliffs, NJ, 1970.
[5] R. Pettai; Noise in Receiving Systems, Wiley, New York, 1984.
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But I don't use this method described in this paper since it is onerous.
Instead I use two 180degree-Hybrid, here these two 180degree-Hybrid are included in a calibration of NF meter such as Agilent N8974A or N8975A Noise Figure Analyzer although this method might not be very accurate.

Settings of Agilent N8975A Noise Figure Analyzer are like following.
  Pre Loss Compensation Mode is FIX
  Pre Loss Compensation Control is 1
  Pre loss Compensation= 1.41[dB]
  Post Loss Compensation Mode is FIX
  Post Loss Compensation Control is 1
  Post loss Compensation= 1.41[dB]
  Measurement ENR Table ID is "N4002A"