Clear channel selection system for a cordless telephone

DETAILED DESCRIPTION OF THE EMBODIMENT Referring to FIG.
1, a cordless telephone base unit 100 includes a Duplexer 105 for receiving radio frequency (RF) signals from, and applying RF signals to, an antenna 106.
Duplexer 105 may be a type DPX 46/49-B10 duplexer manufactured by Soshin Electric Ltd.
Signals received from duplexer 105 are applied to a receiver circuit 110 for detection, processing and amplification.
Receiver circuit 110 provides baseband audio signals to a compander 120 for spectral expansion, which in turn applies the audio signals to an audio amplifier 130.
Audio amplifier 130 applies amplified audio signals to the tip (T) and ring (R) terminals of a telephone network via a telephone interface unit 140.
Incoming telephone signals are received by telephone interface unit 140 and applied to compander 120 via audio amplifier 130.
Compander 120 compresses the amplitude of the audio signals to increase their noise immunity and applies the compressed signals to a transmitter circuit 175.
Transmitter 175 modulates the audio signals onto an RF carrier and applies the RF signals to duplexer 105 for transmission via antenna 106.
Interfacing to the telephone network, dialing operations, channel selection and communication with a handset 101 of the cordless telephone apparatus is under control of a microcontroller 160.
Microcontroller 160 may be a microprocessor or a microcomputer or a dedicated controller integrated circuit.
Microcomputer 100 controls a phase-locked loop (PLL) circuit 165 for frequency selection, and receives data input by user via a keyboard 170.
Elements of handset 101 of FIG.
1 bearing similar reference numerals to elements of base unit 100 FIG.
1 serve the same function and need not be explained again.
Audio amplifiers 130' of handset 101 receive input signals from a mouthpiece microphone 180' and provide output audio signals to an earpiece speaker unit 190'.
U.
S.
patent application Ser.
No.
08/171,353 discloses that carrier detect function can be performed by the microcontroller in the base unit by causing that microcontroller to examine the frequency content of the detected baseband audio signal