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High quality, all-digital RF frequency modulation generation with the ADSP-2181 DSP and the AD9850 direct digital synthesizer

Posted: 20 Jan 2010 ?? ?Print Version ?Bookmark and Share

Keywords:frequency modulation? RF modulation? audio frequency?

This application note describes the implementation of a high quality all-digital FM RF generator for use with audio frequency input signals. The FM RF signal output is in the broadcast FM band and can be monaural or stereo.

FM modulators have been with us since the time of Major Edwin H. Armstrong (sometimes known as the father of FM) and his classic 1936 paper on the subject. He first put down on paper the fundamental concept of a fixed center frequency that is varied per a constant deviation ratio that is independent of the frequency of the modulating signal. FM became very popular because much of the noise generated in nature exhibits AM characteristics.

The SNR of an FM signal will be found to exceed that of an AM signal due to the improvement ratio related to the deviation ratio. We are all familiar with the wideband FM broadcast band and its superior sound quality and noise immunity when compared with AM broadcast.

Analog FM modulators typically employ an oscillator with a varactor diode in the tuned circuit. The voltage to the varactor is varied by the modulating signal, which changes its capacitance, therefore changing the resonant point of the oscillator's tuned circuit, which shifts the frequency. As noted above, constant center frequency and deviation ratio are essential to quality FM. Much work has been done to embellish the basic oscillator circuit to provide the needed stability. Even though complex circuitry can be added to automatically compensate for power supply and temperature variations, these circuits still require periodic recalibration to deal with component aging.

View the PDF document for more information.

Click here to view related datasheets.

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