Radio Receiver Design
This reference focuses on understanding and designing radio receivers and systems, presenting the characteristics of individual receiver components and their interaction in cascade.
TABLE OF CONTENTS Radio Receiver Design
Chapter 3: Mixers
A translation is no translation, he said, unless it will give you the music of a poem along with the words of it.
- John Millington Stone
Translation increases the faults of a work and spoils its beauties.
-Voltaire
3.1 Introduction
A mixer translates signals from one center frequency to another while keeping the modulation intact. The translation is performed for the following reasons:
Filtering
When building a receiver, we need to filter the signal of interest from other signals before we apply the signal to a demodulator. Filtering reduces noise and attenuates unwanted signals. For example, if we are building a commercial FM broadcast receiver, we want to separate the desired station from all of the other stations. It is much easier to move the signal of interest from its original center frequency to some intermediate frequency (IF) and perform the filtering at the IF. This method also has the advantage that the IF can be chosen based upon filtering needs. It is often less problematic to realize narrowband filters at low center frequencies and better to handle large bandwidth signals at high center frequencies. The realizable insertion loss, percent bandwidth, and physical size of the filter are all strong functions of the filter center frequency.
Frequency Assignments
On-the-air frequencies are almost always assigned by government agencies such as the FCC. These frequencies are often chosen based on external requirements that have nothing to do with filtering or demodulator requirements. To build the best system possible, we perform...
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