A transmit amplifier has a problem that the gain deteriorates due to changes in temperature and changes with time. The characteristic deterioration caused by the changes in temperature is improved by installing a temperature-dependent characteristic changing element on a circuit such as a thermistor as a representative example.
Further, in order to cope with the changes with time, the amplification gain of the transmit power is compensated using a variable gain amplifier or variable attenuator installed on a signal processing line.
As disclosed in Japanese Laid-Open Patent Publication HEI11-27233, a conventional transmission apparatus uses as a reference signal a signal generated by converting a modulated signal into a signal with a transmit frequency, compares a level of the reference signal with a transmit power level, and controls the amplification gain so that both power levels are matched with each other.
FIG. 1 is a block diagram illustrating a configuration of the conventional transmission apparatus. In the apparatus, respective transmit data is spread in spreading sections 11-1 to 11-n (n: natural number), and is multiplexed in a code multiplexing section 12. The multiplexed transmit data is rectified with a root-Nyquist filter 13, the resultant digital signal is converted into an analog signal in a D/A conversion section 14, and the analog signal is amplified in a variable amplifying section 15 so as to provide transmit output data. The transmit output data is distributed in a distributor 16 to be transmitted to communication partners and provided to a detection section 17 for detection. The detected analog signal is converted into a digital signal in an A/D conversion section 18.
Difference value calculating section 19 calculates an output average value resulting from averaging amplitude values of transmit output data, which is the converted digital signal, accumulated during a predetermined period of time, and also calculates average value information resulting from averaging amplitude values of code-multiplexed transmit data accumulated during a predetermined period of time. Difference value calculating section 19 further calculates a difference value between a target amplitude value based on the average value information and the output average value. The transmit output data or code-multiplexed transmit data is averaged because the amplitude values are not constant with time.
Gain control section 20 generates based on the difference value a gain control signal for controlling variable amplifying section 15 so that the output average value is equal to the target amplitude value. The gain control signal is converted from the digital signal to the analog signal in D/A conversion section 21, and the analog signal is output to variable amplifying section 15.
Thus, the conventional transmission apparatus controls variable amplifying section 15 using the gain control signal based on the amplified transmit output data and the code-multiplexed transmit data, and thereby performs the gain compensation.
However, since the conventional transmission apparatus needs to average amplitude values of the amplified transmit output data and amplitude values of the code-multiplexed transmit data accumulated during a predetermined period of time, the apparatus has a problem that the calculation amount increases. Further, in the conventional transmission apparatus, when the number of codes to be multiplied increases, root-nyquist filter 13 undergoes a limiter limitation with a predetermined value with respect to the amplitude of the multiplexed transmit data, and thereby a problem occurs that accurate gain compensation becomes difficult.