CMX973 Datasheet, PDF (2/4 Pages) CML Microcircuits – Very small 32-lead Package

CMX973 Quadrature Modulator/Demodulator
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Quadrature Modulation
Quadrature (I/Q) modulation provides a number of advantages over conventional two-point modulation schemes. Two-point
modulation requires calibration/adjustment to achieve optimum performance. I/Q modulation is the answer, providing
maximum flexibility in the digital domain and allowing coherent phase to be achieved. An I/Q modulator provides accurate
phase modulation. I/Q baseband signals are mixed up to RF by two double-balanced mixers driven by the same frequency but
separated in phase by 90 degrees. The outputs of the two mixers are then summed to provide the modulated output.
Quadrature Demodulation
The quadrature demodulator is designed for IF/RF operation and has very low power consumption. Input frequencies in the
range 20 to 300 MHz are allowed. The demodulator system has two gain stages, one before and one after the I/Q
down-converters and the gain elements can be independently controlled. This flexible architecture allows users to optimise
characteristics depending on their system requirements. Optimum noise figure can be achieved by maximising the gain in first
gain stage and minimising gain in the second stage. Intermodulation performance can be optimised by decreasing gain in the
first or second stages. A lower gain in the second gain stage will tend to reduce dc offsets in the output I/Q signal. Further de-
tailed information on the effects of controlling gain stages is available in the CMX973 datasheet.
Typical system application using:
CMX973—Quadrature Modulator/Demodulator
CMX7861—Programmable Baseband Interface IC
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