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In the last few years, attention has been paid to direct conversion receivers for engineering use. In direct conversion, a vector product of the received signal and the orthogonal oscillator output of the same frequency (orthogonal transformation) is used for direct conversion and demodulation to the baseband. As compared with current superheterodyne techniques, direct conversion has the following advantages: Resonant circuits for radio-frequency stages are not required. Integration will be possible in the future. Because of these reasons, direct conversion will play an important role in the development of compact and adjustment-free tuners. To demodulate analog modulated signals by direct conversion, highly accurate orthogonal transformation and signal processing sections are required. For this reason, practical application of direct conversion has been difficult. Our objective is to develop a direct conversion receiver for automotive use which enables AM and FM broadcast reception.

Automobile radio receivers must perform well under varying reception conditions, and increasingly sophisticated mobile audio gear requires better sound quality for AM and FM reception. Fujitsu Ten have developed a mobile AM tuner IC to meet these requirements. The IC features an RF-AGC circuit which enables the receiver to tune in remote signals while rejecting strong local signals, a variable IF bandwidth control circuit to demodulate broader bandwidths, and a new tracking circuit for the RF tuning circuits and local oscillator. These features enable the received frequency range to be expanded (to 1710 kHz) and to improve the quality of AM stereo reception. The techniques developed for the new mobile AM tuner IC are described below.