Technical Feature: Mixer Noise Measurements
by Allen Podell, Vice President of Technology, Besser Associates
in collaboration with Agilent Technologies
Integrated circuit mixers with broadband RF and LO gain are vulnerable to noise degradation from out-of-band sources. Both the measurement and the actual performance can be affected, depending on the mixer application.
The two noise figure measurement scenarios using mixers described within this application note highlight inherent problems with making this type of measurement. The solutions discussed for the two different measurement scenarios described below will significantly increase measurement accuracy and help to show agreement between simulated and measured noise figure performance.
Broadband LO buffer amplifiers, whether on-chip or in instrumentation, amplify noise at the signal, image, and even the IF frequency. Ultimately, this noise is added to the IF output and degrades the performance of the DUT much more than intuitively expected. Additionally, noise figure measurements, used to evaluate noise performance, are often complicated by the unsymmetrical distribution of the noise around the LO signal. Fortunately, placing a narrow bandpass filter on the output of the signal generator used for LO substitution normally solves the more serious problem of multiple noise paths. The measurement problem created by the unsymmetrical noise distribution can be solved with a simple calculation.
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