AUTOMATIC GAIN CONTROL CIRCUIT
Circuits (an AGC amplifier 8 and a second A/D converting circuit 9 ) for detecting an antenna level of a disturbing wave are provided on an output of a frequency converting circuit 4, and levels of antenna ends of a desirable wave and the disturbing wave are calculated by a DSP 10 and a gain of an antenna damping circuit 2, an LNA 3 or the frequency converting circuit 4 is adjusted corresponding to the respective levels, thereby enabling an optimum gain distribution for an RF stage to be set corresponding to the levels of the desirable wave and the disturbing wave.
1 . An automatic gain control circuit comprising:
a gain adjusting portion for adjusting a gain of a received signal;
a first level detecting portion for detecting a level of a desirable wave frequency contained in the received signal;
a second level detecting portion for detecting a level of a disturbing wave frequency contained in the received signal;
a table information storing portion for storing table information causing the levels detected by the first and second level detecting portions to correspond to the gain of the received signal which is to be adjusted by the gain adjusting portion; and
a control portion for referring to the table information based on the level of the desirable wave frequency which is detected by the first level detecting portion and the level of the disturbing wave frequency which is detected by the second level detecting portion, thereby controlling the adjustment of the gain of the received signal through the gain adjusting portion.
2 . The automatic gain control circuit according to claim 1 , wherein the table information storing portion stores first table information for causing the level of the desirable wave frequency which is detected by the first level detecting portion to correspond to the gain of the received signal which is to be adjusted by the gain adjusting portion, and second table information for causing the level of the desirable wave frequency which is detected by the first level detecting portion and the level of the disturbing wave frequency which is detected by the second level detecting portion to correspond to the gain of the received signal which is to be adjusted by the gain adjusting portion, and
the control portion refers to either the first table information or the second table information based on the level of the desirable wave frequency which is detected by the first level detecting portion and the level of the disturbing wave frequency which is detected by the second level detecting portion, thereby controlling the adjustment of the gain of the received signal through the gain adjusting portion.
3 . The automatic gain control circuit according to claim 2 , wherein the control portion controls the adjustment of the gain of the received signal through the gain adjusting portion by referring to the first table information when the level of the disturbing wave frequency which is detected by the second level detecting portion is lower than a predetermined value and referring to the second table information when the level of the disturbing wave frequency which is detected by the second level detecting portion is equal to or higher than the predetermined value.
4 . The automatic gain control circuit according to claim 1 , wherein the first level detecting portion, the second level detecting portion and the control portion are constituted by a digital signal processing portion.
5 . The automatic gain control circuit according to claim 4 , further comprising:
an amplifying circuit for amplifying a broadband intermediate frequency signal output from a frequency converting circuit for carrying out a frequency conversion processing for a received signal having a gain adjusted by the gain adjusting portion; and
an A/D converting circuit for converting the broadband intermediate frequency signal amplified by the amplifying circuit into a digital signal and supplying the digital signal to the digital signal processing portion.
6 . The automatic gain control circuit according to claim 1 , further comprising:
a decoder for decoding control data output from the control portion to control the adjustment of the gain of the received signal through the gain adjusting portion; and
an analog switch for controlling change-over based on an output of the decoder,
the gain of the received signal through the gain adjusting portion being controlled by changing over the analog switch.
7 . The automatic gain control circuit according to claim 1 , further comprising:
a D/A converting circuit for converting, into an analog signal, control data output from the control portion to control the adjustment of the gain of the received signal through the gain adjusting portion; and
a threshold determining circuit for determining a threshold of a gain control in the gain adjusting portion based on a signal output from the D/A converting circuit,
the threshold being changed to control the gain of the received signal through the gain adjusting portion.