Radio telephone suitable for portable and vehicle-mounted use
BACKGROUND OF THE INVENTION 
     The present invention relates to a radio apparatus and, more particularly, to a radio telephone to be used as a portable or a vehicle-mounted telephone. 
     A conventional radio telephone is energized by a battery in portable use and by a vehicle battery in vehicle-mounted use. In both cases, a DC voltage is applied to a receiver section through a voltage stabilizer. The voltage to be applied to the receiver section is minimized to save the battery power in the portable use. As a result, in the vehicle-mounted use, receiver performance such as an intermodulation (IM) characteristic and reception sensitivity is sacrificed for the power saving. On the contrary, if the voltage is increased to improve the receiver performance, the life of the battery for portable use will be shortened. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is, therefore, to provide a radio telephone suitable for both portable and vehicle-mounted use. 
     Another object of the present invention is to provide a radio telephone capable of saving power in portable use while improving the receiver performance in vehicle-mounted use. 
     Yet another object of the present invention is to provide a radio telephone having an improved IM characteristic and reception sensitivity in its vehicle-mounted use. 
     A further object of the present invention is to provide a radio telephone consuming less power in its portable use. 
     According to the present invention, there is provided a portable radio telephone comprising, a receiver section which includes at least an RF amplifier and a first frequency mixer. The telephone also comprises a battery which supplies a first voltage to the receiver section when the telephone is in portable use. When the telephone is mounted on a vehicle, a vehicle battery applies a second voltage which is higher than the first voltage, to the RF amplifier and to the frequency mixer. By this higher voltage supply, the intermodulation characteristic and reception sensitivity of the receiver section can be improved in the vehicle-mounted use. On the other hand, battery saving can be achieved by the lower voltage supply in the portable use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other object, features and advantages of the present invention will become more apparent from the following description referring to the accompanying drawings, in which: 
     FIG. 1 is a block diagram showing a radio telephone embodying the present invention; 
     FIG. 2 is a block diagram illustrating a power supply source to be connected to the FIG. 1 telephone; 
     FIG. 3 shows a graph demonstrating a IM characteristic and reception sensitivity with respect to a supply voltage which were obtained from the telephone shown in FIG. 1; and 
     FIG. 4 is a schematic circuit diagram showing an RF amplifier and a first frequency mixer in the FIG. 1 telephone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a radio telephone 10 includes an antenna 11 picking up a radio frequency (RF) signal and radiating an RF signal. The picked-up RF signal is supplied to a receiver section 20 through an antenna duplexer 12. The receiver section 20, which will be described in detail later, demodulates the supplied RF signal to provide an audio frequency (AF) signal to an earpiece receiver or speaker 21. In response to the AF signal, the speaker 21 produces an audible signal. 
     An audible signal which is picked up by a mouthpiece microphone 41 is supplied to a transmitter section 40 at which the audible signal is subject to modulation and frequency conversion to become an RF signal. The RF signal from the transmitter section 40 is transmitted through the antenna duplexer 12 and antenna 11 to a mobile base station (not shown), for example. The earpiece receiver 21 and the microphone 41 may be accommodated in the earpiece and mouthpiece portions of a handset (not shown) of the telephone 10. 
     The radio telephone 10 further includes a rechargeable battery 15 from which a direct current (DC) voltage V 0  is to be supplied through a power supply switch 13 and a power source switch 14 to both the receiver section 20 and the transmitter section 40. The battery 15 may be attached to or built in the radio telephone 10. The power source switch 14 is also connected to a terminal 7 to which a DC voltage V 1  is to be applied from a DC/DC converter 35 connected to a vehicle battery 34 (see FIG. 2). The switch 14 connects the terminal 7 to the power supply switch 13 when the radio telephone 10 is mounted on a vehicle for vehicle-mounted use. On the other hand, the switch 14 connects the battery 15 to the power supply switch 13 when the telephone 10 is removed from the vehicle for portable use. The switch 14 may manually or automatically be controlled. To achieve the automatic control, a connection detector may be provided which detects an electrical coupling between the radio telephone 10 and a vehicle battery. Such a connection detector is disclosed in U.S. Pat. No. 4,654,882 which is assigned to the same assignee as the present application and incorporated herein. 
     When the radio telephone 10 is mounted on a vehicle, a voltage V 2  is applied from the vehicle battery 34 (FIG. 2) to the receiver section 20 though a resistor 38. The voltage V 2  is also applied to the rechargeable battery 15 through a resistor 16 and a diode 17 in order to charge the battery 15. The battery 15 is also charged by a DC voltage V 3  which is applied through a terminal 9 and a diode 18. The voltage V 3  may be obtained from a commercial alternating current (AC) voltage using an AC/DC converter 36 to which an AC plug 37 is connected (see FIG. 2). The voltage V 2  is higher than the voltage V 0 , V 1  and V 3  and may be 13.8 volts, for example. The voltages V 0 , V 1  and V 3  may be substantially equal to each other and V 0  and V 1  may be 7 volts and V 3 , 7.7 volts. 
     The receiver section 20 is a double superheterodyne type receiver, which is well known in the art and thus will be briefly described below. The receiver section 20 comprises an RF amplifier 22 for amplifying an RF signal which is supplied from the antenna 11 through the antenna duplexer 12. The RF amplifier 22 supplies the amplified RF signal to a first frequency mixer or converter 23 to which a first local oscillation signal is also supplied from a first local oscillator 24. The first mixer 23 frequency mixes the RF signal and the first local oscillation signal to produce a first intermediate frequency (IF) signal. 
     The first mixer 23 supplies the first IF signal to a second frequency mixer or converter 25 to which a second local oscillation signal is also supplied from a second oscillator 26. The second mixer 25 frequency mixes the first IF signal and the second local oscillation signal to produce a second IF signal. The second mixer 25 supplies the second IF signal to an IF amplifier 29 which amplifies the second IF signal and supplies the amplified signal to a demodulator 28. The demodulator 28 demodulates the second IF signal to produce an audio frequency (AF) signal and supplies the AF signal to an AF amplifier 29. The amplifier 29 amplifies the AF signal and supplies the amplified AF signal to the earpiece receiver 21 from which the AF signal is outputted as an audible signal. 
     The receiver section 20 further includes a voltage stabilizer 30 to which one of the voltages V 0  and V 1  is applied through the switches 13 and 14. The stabilizer 30 supplies a stabilized voltage of 5 volts, for example, to the RF amplifier 22 and the first mixer through a diode 32 and to the other elements 24 to 29 directly. To the RF amplifier 22 and the first mixer 21, the vehicle battery voltage V 2  is to be applied through a diode 31. Thus, when the radio telephone 10 is mounted on a vehicle, the higher voltage V 2  is applied to the RF amplifier 22 and the first mixer 23. In this case, the lower voltage V 1  is applied to the other elements 24 to 29. When the radio telephone 10 is demounted from the vehicle for portable use, the lower battery voltage V 0  is applied to all the receiver section elements 22 to 29. 
     FIG. 3 shows an IM characteristic with respect to a voltage Vcc applied to the RF amplifier 22 and the first mixer 23. FIG. 3 also shows a reception sensitivity (RS) vs. voltage Vcc characteristic. These characteristics have been measured using a mock-up set. It can be seen from the IM vs. voltage Vcc characteristic 51 that as the voltage Vcc increases, the IM characteristic is improved. Similarly, it can be seen from the RS vs. voltage Vcc characteristic 52 that as the voltage Vcc increases, the reception sensitivity also increases. Thus, when the higher voltage is applied to the RF amplifier 22 and the first mixer 23, both the IM and RS characteristics can be improved. 
     It should be noted that to improve the IM and RS characteristics the higher voltage may be applied to only one of RF amplifier 22 and first mixer 23. In this case, applying the higher voltage to the first mixer 23 gives a better result than to the RF amplifier 22. 
     In FIG. 4, the RF amplifier is composed of impedance matching circuits 221 and 225, a bias circuit 223 and an NPN transistor 224. The matching circuit 221 includes a capacitor 101 one of whose ends is connected to a terminal 226 to which the antenna duplexer 12 is connected. The other end of capacitor 101 is grounded through an inductor 103. The junction of the capacitor 101 and the inductor 103 is connected to one end of capacitor 102 whose other end is connected to the base of transistor 224. The collector of transistor 224 is connected to the terminal 33 through a resistor 104 of bias circuit 223. A resistor 105 of bias circuit 223 is connected between the collector and base of transistor 224. The emitter of transistor 224 is grounded. 
     The matching circuit 225 includes a capacitor 106 one of whose ends is connected to the collector of transistor 224. The other end of capacitor 106 is connected to ground through an inductor 108. The junction of the capacitor 106 and the inductor 108 is connected to one end of capacitor 107 whose other end is connected to the output of RF amplifier 22. 
     The first mixer 23 is composed of two band-pass filters (BPFs) 231 and 232, impedance matching circuits 233 and 236, a bias circuit 234 and an NPN transistor 235. The input of BPF 231 is connected to the output of RF amplifier 22. The input of BPF 232 is connected to the output of first local oscillator 24. The outputs of BPFs 231 and 232 are connected to the matching circuit 233. The matching circuit 233 includes a capacitor 301 one of whose ends is connected to the outputs of BPFs 231 and 232. The other end of capacitor 301 is connected to ground through an inductor 303. The junction of the capacitor 301 and the inductor 303 is connected to one end of capacitor 302 whose other end is connected to the base of transistor 235. The collector of transistor 235 is connected to the terminal 33 through a resistor 304 of bias circuit 234. A resistor 305 of bias circuit 234 is connected between the collector and base of transistor 235. 
     The matching circuit 236 includes a capacitor 306 one of whose ends is connected to the collector of transistor 235. The other end of capacitor 306 is connected to ground through a capacitor 309. The junction of the capacitors 306 and 309 is connected to one end of inductor 308 whose other end is grounded through a capacitor 310. The other end of inductor 308 is also connected through a capacitor 307 to a terminal 238 which in turn is connected to the input of second mixer 25. 
     In operation, an RF signal is applied from the antenna duplexer 12 to the base of transistor 224 through the terminal 226 and the matching circuit 221. The transistor 224 amplifies the RF signal and outputs the amplified RF signal at its collector. The amplified RF signal is then supplied to BPF 231 through the matching circuit 225. Operating and bias voltages are applied to the collector and base of transistor 224 through the terminal 33 and the bias circuit 223. As the applied voltages increase, the IM and RS characteristics are improved, as shown in FIG. 3. 
     BPF 231 filters the amplified RF signal to pass and supply a desired RF signal to the base of transistor 235 through the matching circuit 233. BPF 232 filters the first local oscillation signal to pass and supply a desired local oscillation signal to the base of transistor 235 through the matching circuit 233. The transistor 235 frequency mixes the supplied RF and local oscillation signals with its nonlinear characteristic to provide a first IF signal at its collector. The first IF signal is supplied to the second frequency mixer 25 through the matching circuit 236 and the terminal 238. Operating and bias voltages are applied to the collector and base of transistor 235 through the terminal 33 and the bias circuit 234. As the applied voltages increase, the IM and RS characteristics are improved, as shown in FIG. 3.