More Mixer Noise

April 6, 2012

When you publish in one of the industry’s premiere magazines, you open yourself up to criticism such as what was presented in your “Feedback” column (March 2012). The focus of Mr. Polivka’s criticism was at the end of the article (“Predict Mixer Noise Behavior,” January 2012), in which I commented about the degradation of system noise figure when image noise is not dealt with properly. He contended, quite vigorously, that the noise figure of the system is set by the front-end LNA, independent of whether or not the image noise is rejected.

I was the recipient of numerous demeaning e-mails from Mr. Polivka, in which he ignored all of the published references and measured data that support my position in the article. The following are two well-known published textbook excerpts which I supplied to Mr. Polivka:

“Practical RF Circuit Design for Modern Wireless Systems,” by Rowan Gilmore and Les Besser: “Therefore, in a broadband mixer, the noise floor at the image frequency will fold onto the RF signal noise floor when downconverted to the IF, resulting in a 3-dB loss in system sensitivity, no matter how good the preceding component noise figure. The purpose of the preceding RF filter should therefore be to remove as far as possible the effect of the image noise.”

“Practical RF System Design,” by William F. Egan: “If the circuitry preceding the mixer is high-gain broadband (same gain at all frequencies of importance), the cascade noise figure can increase as much as 3 dB.”

The facts that support this aspect of my article (which was not even the main point of the article) are not new, and are not disputed by the engineering community. There will always be those out there who try to hold on to old beliefs, and when confronted with opposing views, lash out with personal attacks instead of presenting a factual basis for their position. I am afraid that Mr. Polivka falls into this category.

Roy Monzello

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