| 摘要 |
PURPOSE:To control very exactly a reception level, based on a digital value which has been set multi-stepwise, by controlling the reception level by controlling a current control type attenuator by a current controlling circuit. CONSTITUTION:A digital value D1 which has been set by a CPU 1 in relation to a desired reception level is converted to an analog voltage E1 by a D/A converter 2. A current i1 flowing to an FET Q1 in a current controlling circuit 3 is denoted by i1=(V-E1)/R1. Accordingly, the current i1 is controlled exactly and stably by the voltage E1. A current control type attenuator 4 has a variable impedance circuit in which the impedance of a point P is varied by the current i1 flowing to diodes d1, d2, therefore, a ratio of a receiving signal S which has been inputted to the attenuator 4 and a receiving signal S' which is outputted is controlled exactly by the impedance of the point P, namely, a value of the current i1. A level of a receiving signal is controlled multi-stepwise by said consecutive operations.
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