(1) Field of the Invention
The present invention relates to an automatic level control circuit and particularly to an automatic level control circuit used in a transmitter.
(2) Description of the Related Art
In present digital radio transmission systems, multi-value modulation such as 64 QAM or 256 QAM has been introduced rapidly with improvement in effective use of frequency and in transmission capacity. Introduction of a multivalue modulation method requires, in the transmitter, an automatic level control (ALC) circuit for always maintaining the transmitting output at a constant level in view of suppressing a quantity of distortion generated in a high output amplifier by obtaining a large a saturation power as shown in FIG. 1. A ratio of the saturation power and operation power (back off) and by not generating an error in an adjacent channel or in the same channel.
FIG. 2 schematically shows an ordinary automatic level control circuit. In this circuit, a band-limited modulated signal wave is amplified by a high output amplifier 1. A part of the output of the amplifier 1 is branched by a directional coupler 2. The branched output is detected by a detector 3 and is converted to a DC output. The DC output is compared with a reference value. A gain control voltage of the amplifier 1 is controlled by the difference between the DC output and the reference value. Thus, the transmitting output is kept at a constant level.
Such automatic level control circuit uses, as shown in FIG. 3, a 1/4 wavelength (.lambda.) side couple type coupler as a directional coupler 2. This coupler uses the branching function explained. The structure includes microstrip lines 2a and 2b provided on a dielectric substrate (not illustrated) made of teflon. A termination resistor r of characteristic impedance (50.OMEGA.) is connected to the one end of the microstrip line 2b, while a diode 3 as a detector is connected to the other end of line 2b.
The high output amplifier 1 used for such automatic level control circuit has a problem that it searches a saturated power value for evaluating the performance thereof but it cannot realize automatic level control. This is because the back-off shown in FIG. 1 is set large and an excessive power is input to the diode 3 from the microstrip line 2b. This high output may harm the diode 3.