Automatic gain control circuit

1. An automatic gain control circuit for generating a gain control signal for controlling the gain of a variable gain amplifier block in accordance with an output of the variable gain amplifier block, the automatic gain control circuit comprising:

a count control signal generating block for generating a count control signal according to the output of the variable gain amplifier block;

an up-down counter for upcounting an upcount clock signal or downcounting a downcount clock signal according to the count control signal;

a gain control signal generating block for generating the gain control signal corresponding to a count value of the up-down counter; and

a downcount clock signal generating block for generating the downcount clock signal whose frequency corresponds to the count value of the up-down counter, wherein:

the gain control signal generating block includes a digital-to-analog converter for converting the count value of the up-down counter to a voltage, and

the downcount clock signal generating block is a voltage controlled oscillator whose oscillation frequency is controlled by an output of the digital-to-analog converter.

2. The automatic gain control circuit of claim 1 , wherein the gain control signal generating block includes a gain control signal amplifier for amplifying an output of the digital-to-analog converter.

3. The automatic gain control circuit of claim 1 , wherein the downcount clock signal generating block has a selector switch for selecting, as the downcount clock signal, one of a plurality of clock pulses of different frequencies in accordance with the count value of the up-down counter.

4. The automatic gain control circuit of claim 3 , wherein the downcount clock signal generating block has an analog-to-digital converter for generating digital data corresponding to an output of the digital-to-analog converter; and

the selector switch is controlled by an output of the analog-to-digital converter.

5. The automatic gain control circuit of claim 1 , wherein the count control signal generating block has a rectifier circuit for rectifying the output of the variable gain amplifier block; and

a threshold voltage comparator for comparing an output of the rectifier circuit with a threshold voltage to output a result of the comparison as the count control signal.

6. The automatic gain control circuit of claim 5 , wherein the threshold voltage is determined based on the count value of the up-down counter.

7. The automatic gain control circuit of claim 6 , further comprising a threshold voltage changer for changing an output of the digital-to-analog converter to the threshold voltage.

8. An automatic gain control circuit for generating a gain control signal for controlling the gain of a variable gain amplifier block in accordance with an output of the variable gain amplifier block, the automatic gain control circuit comprising:

a count control signal generating block for generating a first count control signal according to the output of the variable gain amplifier block;

a first up-down counter for upcounting a first upcount clock signal or downcounting a first downcount clock signal according to the first count control signal;

a second up-down counter for upcounting a second upcount clock signal or downcounting a second downcount clock signal according to a second count control signal;

a gain control signal generating block for generating the gain control signal and the second count control signal in accordance with a count value of the first up-down counter and a count value of the second up-down counter; and

a downcount clock signal generating block for generating the first downcount clock signal whose frequency corresponds to a difference between the count value of the first up-down counter and the count value of the second up-down counter, wherein:

the gain control signal generating block generates the second count control signal in accordance with a result of a comparison between the count value of the first up-down counter and the count value of the second up-down counter, and generates the gain control signal according to the count value of the first up-down counter, and

the gain control signal generating block includes:

a first digital-to-analog converter for converting the count value of the first up-down counter to a voltage to generate the gain control signal; and

an arithmetic circuit for performing a comparison operation between the count value of the first up-down counter and the count value of the second up-down counter, thereby generating the second count control signal.

9. The automatic gain control circuit of claim 8 , wherein the downcount clock signal generating block has a selector switch for selecting, as the first downcount clock signal, one of a plurality of clock pulses of different frequencies in accordance with an output of the arithmetic circuit.

10. An automatic gain control circuit for generating a gain control signal for controlling the gain of a variable gain amplifier block in accordance with an output of the variable gain amplifier block, the automatic gain control circuit comprising:

a count control signal generating block for generating a first count control signal according to the output of the variable gain amplifier block;

a first up-down counter for upeounting a first upcount clock signal or downcounting a first downcount clock signal according to the first count control signal;

a second up-down counter for upcounting a second upcount clock signal or downcounting a second downcount clock signal according to a second count control signal;

a gain control signal generating block for generating the gain control signal and the second count control signal in accordance with a count value of the first up-down counter and a count value of the second up-down counter; and

a downcount clock signal generating block for generating the first downcount clock signal whose frequency corresponds to a difference between the count value of the first up-down counter and the count value of the second up-down counter,

wherein the gain control signal generating block includes a first digital-to-analog converter for converting the count value of the first up-down counter to a voltage, and a second digital-to-analog converter for converting the count value of the second up-down counter to a voltage; and

the downcount clock signal generating block is a voltage controlled oscillator whose oscillation frequency is controlled by a voltage difference between an output of the first digital-to-analog converter and an output of the second digital-to-analog converter.