1. Field of the Invention
The present invention relates to a processor for processing an FM signal such as an FM stereo signal.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of the entire arrangement of an FM stereo radio receiver. Reference numeral 331 represents an antenna. Reference numeral 332 represents an RF amplifier. Reference numeral 333 represents an mixer for mixing an oscillation signal of a local oscillation circuit 334 and an RF signal from the RF amplifier 332 to output an IF signal. Reference numeral 335 represents an IF filter. Reference numeral 336 represents an IF amplifying circuit. Reference numeral 337 represents a limiter. Reference numeral 339 represents a 90.degree. phase shifting circuit. Reference numeral 340 represents a multiplier serving as an FM detector. Reference numeral 341 represents a multiplexer. Reference numeral 342 represents a power amplifying circuit. Reference numerals 343 and 344 represent left and right speakers.
The IF amplifier 336, the limiter 337, the 90.degree. phase shifting circuit 339, the multiplier 340 and the multiplexer 341 are formed in a one-chip IC device 338. To the 90.degree. phase shifting circuit 339, an resonance circuit 345 including a coil L and a capacitor C is externally attached, and to the multiplexer 341, a circuit 347 constituting a part of a voltage controlled oscillator (VCO) 346 is externally attached. The circuit includes a resistor and a capacitor.
In this FM radio receiver, the frequency of a received FM radio signal is converted into an intermediate frequency (IF) by the IF circuit 336, and the amplitude thereof is made constant by the limiter 337. Thereafter, the signal is transmitted through a line a directly to the multiplier 340 and is also transmitted through a line b to the multiplier 340 by way of the phase shifting circuit 330 where its phase is shifted by 90.degree.. These two signals are multiplied at the multiplier 340. The phase shifting circuit 339 shifts the phase by the capacitor C and the coil L. The VCO 346 used in the multiplexer 341 is designed to regulate a free-running oscillation frequency by a CR time constant of a circuit externally attached to the IC device 338.
Referring to FIG. 2, there is shown another conventional FM radio receiver where ceramic resonators 350 and 351 are externally attached instead of the resonance circuit 345 and the circuit 347 of FIG. 1. In a phase shifting circuit 339 using the ceramic resonator 350, as shown in FIG. 3, three resistors R and the ceramic resonator 350 form a Wheatstone bridge, and the phase of the signal is shifted by using a reactance component of the ceramic resonator 350. The oscillation frequency of the VCO 346 depends on the ceramic resonator 351.
However, in the phase shifting circuit 339 of FIG. 1, it is necessary to adjust a free-running oscillation frequency by the capacitor or the coil, and the regulation which is a mechanical regulation is liable to deterioration with age. In addition, since the coil cannot be formed in an integrated circuit (IC) device, more number of terminal pins are required and an externally-connected part is required. The VCO 346 has similar disadvantages since the free-running oscillation frequency is regulated by a CR time constant of a circuit externally attached to the IC device.
When the ceramic resonators 350 and 351 are externally attached as shown in FIG. 2, not only an externally-attached part is required but also the part (ceramic resonator) is not available in some countries since it is a special part. In addition, the ceramic resonator is inferior to the former method in property, and difficult to match with an IC device. Further, since the free-running frequency of the VCO 346 depends on only the property of the resonator 351, to obtain a comparatively low frequency such as 19 kHz and 38 kHz necessary to process a composite signal, a number of frequency dividers are required for dividing (into 24 and into 12) 456 kHz of the ceramic resonator 351. This increases the chip area of the IC device.