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In a radio communication device of a CDMA system selectively operable in high and low power modes to receive a reception analog signal and to produce a demodulated signal, an A/D converter is controlled to reduce power consumption in the low power mode in comparison with the high power mode. The red

In a radio communication device of a CDMA system selectively operable in high and low power modes to receive a reception analog signal and to produce a demodulated signal, an A/D converter is controlled to reduce power consumption in the low power mode in comparison with the high power mode. The reduction of power consumption can be established by partially operating the A/D converter or by lowering a sample rate in the low power mode. A despreading portion connected to the A/D converter is also controlled in a manner similar to the A/D converter. Each of the high and the low power modes is indicated by a mode control signal produced by a CPU controller.

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1. A method of controlling electric power in a radio communication device which has an analog-to-digital (A/D) converter and which is used in a code division multiple access (CDMA) system, the radio communication device being operable in a selected one of a high power mode and a low power mode, the

1. A method of controlling electric power in a radio communication device which has an analog-to-digital (A/D) converter and which is used in a code division multiple access (CDMA) system, the radio communication device being operable in a selected one of a high power mode and a low power mode, the method comprising the steps of:detecting each of the high and low power modes by monitoring the perching channel to determine if the receiver is in standby mode to produce a detection signal representative of either one of the high and the low power modes; and operating the A/D converter in the low power mode in response to the detection signal, as compared with the high power mode; wherein the A/D converter is operated in lower power mode when the receiver is in standby mode. 2. A method as claimed in claim 1, the A/D converter converting a reception analog signal into a first digital signal of a first bit number in the high power mode, wherein the step of operating the A/D converter in the low power mode comprises the step of:reducing, in the low power mode, the first bit number of the first digital signal to a second bit number of a second digital signal smaller than the first bit number so as to decrease the electric power in the radio communication device. 3. A method as claimed in claim 1, the A/D converter converting a reception analog signal into a first digital signal at a first sample rate in the high power mode, wherein the step of operating the A/D converter in the low power mode comprises the step of:changing the first sample rate to a second sample rate lower than the first sample rate in the low power mode so as to decrease the electric power in the radio communication device. 4. A radio communication device responsive to a reception analog signal and operable in a selected one of high and low power modes to demodulate the reception analog signal into a demodulated signal, the device comprising:an analog-to-digital (A/D) converter for converting the reception analog signal into a digital signal; means for processing the digital signal into the demodulated signal; and a control circuit for controlling the A/D converter so that electric power consumption of the A/D converter is reduced in the lower power mode as compared with the high power mode when a receiver is determined to be in standby mode by monitoring a perching and a paging channel for control signals. 5. A radio communication device as claimed in claim 4, the A/D converter producing, as the digital signal, a first digital signal of a first bit number in the high power mode, wherein the A/D converter comprises:reducing means for reducing the first bit number of the first digital signal to a second bit number of a second digital signal in the low power mode so as to decrease the electric power consumption in the low power mode as compared with the high power mode, and means for selectively producing the first and second digital signals as the digital signal. 6. A radio communication device as claimed in claim 5, wherein the second bit number is equal to half of the first bit number.7. A radio communication device as claimed in claim 4, wherein the A/D converter converting the reception analog signal into a first digital signal at a first sample rate in the high power mode, wherein the A/D converter comprises:rate changing means for changing the first sample rate to a second sample rate of a second digital signal lower than the first sample rate in the low power mode so as to decrease the electric power in the radio communication device; and means for selectively producing, as the digital signal, the first and the second digital signals. 8. A radio communication device for demodulating a reception analog signal into a demodulate signal, comprising:an analog-to-digital (A/D) converter for converting the reception analog signal into a digital signal so that a bit number of the digital signal is variable; wherein the A/D converter has low and high significant bit parts both of which are operable to produce a first part and a second part of the digital signal, respectively; the A/D converter selectively producing a selected one of the first and the second parts and a combination of the first and the second parts so as to vary the bit number of the digital signal. 9. A radio communication device as claimed in claim 8, operable in a high power mode and a low power mode and comprising a controller for supplying the A/D converter with a mode control signal representative of either the high power mode or the low power mode;the A/D converter producing the combination of the first and the second parts of the digital signal when the high power mode is indicated by the mode control signal and, otherwise, producing only the selected one of the first and the second parts of the digital signal. 10. A radio communication device as claimed in claim 9, further comprising a sampling signal generator for supplying the A/D converter with a sampling signal of a predetermined frequency which is kept unchanged in the high and low power modes.11. A radio communication device as claimed in claim 9, further comprising a rake receiver which is connected to the A/D converter and which is supplied with the digital signal variable in the bit number to carry out a predetermined calculation and to produce an calculation result signal; anddemodulating means for demodulating the calculation result signal into the demodulated signal. 12. A radio communication device as claimed in claim 11, wherein the demodulating means comprises:combining means for combining the calculation result signal into a combined signal; and means for processing the combined signal into the demodulated signal. 13. A radio communication device as claimed in claim 12, wherein each of the finger receivers comprises:a despreading portion responsive to the digital signal variable in the bit number for carrying out a despreading operation in accordance with the variable bit number of the digital signal, to produce a despreading result signal; and means for calculating the despreading result signal to obtain the calculation result signal. 14. A radio communication device as claimed in claim 13, wherein the despreading portion comprises:a despreading code generator supplied with the mode control signal from the controller for selectively generating first and second despreading codes corresponding to the bit numbers of the high and the low power modes indicated by the mode control signal, respectively; and a despreading calculation portion supplied with the digital signal variable in the bit number and a selected one of the first and the second despreading codes for carrying out a despreading calculation in accordance with each of the bit numbers and the selected despreading code to produce the despreading result signal varied in bit number determined by each of the bit numbers of the digital signal and the selected despreading code. 15. A radio communication device as claimed in claim 11, wherein the rake receiver is structured by a plurality of finger receivers.16. A radio communication device operable in response to a reception analog signal to produce a demodulated signal, comprising:an A/D converter for converting the reception analog signal into a digital signal; and a controllable sampling signal generator for selectively supplying the A/D converter with a first sampling signal of a first sample rate and a second sampling signal of a second sample rate different from the first sample frequency; the A/D converter being operable in response to either one of the first and the second sampling signals and the reception analog signal to produce the digital signal which has the bit number determined by either one of the first and the second samples rates, wherein the first rate is used when operating in transmission/reception mode and the second sample ratio is used in standby mode and where the standby mode is determined by monitoring the perching and paging channels for control signals instead of speech signals. 17. A radio communication device as claimed in claim 16, wherein the first sample rate is higher than the second sample rate.18. A radio communication device as claimed in claim 17, further comprising a controller for supplying the sampling signal generator with a mode control signal representative of either a high power mode or a low power mode;the sampling signal generator delivering, to the A/D converter, either one of the first and the second sampling signals indicated by the mode control signal to make the A/D converter produce the digital signal which has the-bit number determined by either one of the first and the second sample rates. 19. A radio communication device as claimed in claim 18, further comprising a rake receiver operable in response to the mode control signal to carry out calculation of the digital signal in accordance with either one of the first and the second sample rates to produce a calculation result signal.20. A radio communication device as claimed in claim 19, further comprising demodulating means for demodulating the calculation result signal into the demodulated signal.21. A radio communication device as claimed in claim 20, wherein the demodulating means comprises:a combination circuit for combining the calculation result signal into a combined signal; and processing means for processing the calculation result signal into the demodulated signal. 22. A radio communication device as claimed in claim 19, wherein the rake receiver is structured by a plurality of finger receivers.23. A radio communication device as claimed in claim 22, wherein each of the finger receivers comprises:a despreading portion responsive to the digital signal variable in the bit number for carrying out a despreading operation of the digital signal in accordance with either one of the first and the second samples rates, to produce a despreading result signal; and means for calculating the despreading result signal to obtain the calculation result signal. 24. A radio communication device as claimed in claim 16, further comprising;means for manually or automatically varying the bit number of the digital signal produced from the A/D converter. 25. A radio communication device as claimed in claim 24, wherein the reception analog signal is a CDMA signal.