Automatic gain control circuit capable of compensating variation of reception level with wide range

An RF AGC control circuit (47) gives a first initial voltage signal generated by a first initial level generating circuit (44) to an RF amplifier circuit (22) during a predetermined time duration T, when the channel selection signal was updated or when power supply is turned to on. After lapse of the predetermined time duration T, the RF AGC control circuit gives to the RF amplifier circuit a level difference signal representative of a level difference between output of an optimum level generating circuit (46) and output of a comparator (43). On the other hand, an IF AGC control circuit (48) gives a second initial voltage signal generated by a second initial level generating circuit (45) to an IF amplifier circuit 31 during the predetermined time duration T, when the channel selection signal was updated or when the power supply is turned to on. The IF AGC control circuit gives the output of the comparator to the IF amplifier circuit after lapse of the predetermined time duration T.

BACKGROUND OF THE INVENTION 
This invention relates to an automatic gain control circuit and a reception 
front end device comprising the automatic gain control circuit. The 
automatic gain control circuit is particularly suitable for a television 
receiver which receives a CATV (Cable Television) signal or a wire-less 
television signal. 
The television receiver comprises a tuner comprising an RF (Radio 
Frequency) amplifier circuit, a visual detector circuit, and a control 
unit for controlling the tuner and the visual detector circuit. The tuner 
receives an RF (Radio Frequency) signal and selects a specific frequency 
signal of a specific channel, namely, a reception channel, designated by a 
channel switching signal. Furthermore, the tuner amplifies the specific 
frequency signal into an amplified frequency signal and then converts the 
amplified frequency signal into an IF (Intermediate Frequency) signal of 
the specific channel. The visual detector circuit converts the IF signal 
into a composite video signal. 
The control unit receives a channel selection signal designating the 
reception channel from an outer unit and delivers the channel switching 
signal to the tuner. Furthermore, the control unit controls the RF 
amplifier circuit by supplying an RF AGC (Automatic Gain Control) voltage 
control signal to the RF amplifier circuit. The RF amplifier circuit is 
controlled in amplification degree. For this purpose, the control unit 
comprises a memory unit memorizing a plurality of AGC voltage control 
information in each of a plurality of channels. By the use of the 
plurality of AGC voltage control information, the control unit controls 
the tuner so that the tuner produces the IF signal having an appropriate 
level. However, each of the plurality of AGC voltage control information 
is a fixed value in each of the plurality of channels and is determined 
taking level variation of the RF signal into consideration. This means 
that the television receiver has the following disadvantage. 
As will later be described, there is a case that the RF signal has a larger 
level variation larger than it had expected. In this case, it is 
impossible to compensate the level variation of the RF signal by only 
controlling the amplification degree of the RF amplifier circuit. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide an automatic gain 
control circuit which is capable of compensating level variation of a 
reception RF signal with a wide range. 
It is another object of this invention to provide a reception front end 
device which comprises the automatic gain control circuit of the type 
mentioned above. 
According to one aspect of this invention, an automatic gain control 
circuit comprises a level detection circuit for detecting a reception 
level by utilizing an output signal produced from a demodulation circuit 
of a television signal. The automatic gain control circuit produces an 
automatic gain control voltage signal according to the reception level and 
further comprises: a reference level generating circuit for generating a 
reference voltage signal having a predetermined reference level; a 
comparator for comparing the reception level with the predetermined 
reference level to produce a first level difference signal representative 
of a first level difference between the reception level and the 
predetermined reference level; a first initial level generating circuit 
for generating a first initial voltage signal having a first initial 
level; a second initial level generating circuit for generating a second 
initial voltage signal having a second initial level; an optimum level 
generating circuit supplied with a channel selection signal designating a 
reception channel for generating a channel reference level signal having a 
preselected level that corresponds to the reception channel designated by 
the channel selection signal; an RF AGC control circuit connected to the 
comparator, the first initial level generating circuit, and the optimum 
level generating circuit for delivering the first initial voltage signal, 
as an RF AGC voltage which serves as a part of the automatic gain control 
voltage signal, during a predetermined time duration on reception of the 
channel selection signal or when power supply is turned to on. The RF AGC 
control circuit delivers a second level difference signal, as the RF AGC 
voltage, representative of a second level difference between the 
preselected level and the first level difference after lapse of the 
predetermined time duration. The automatic gain control circuit still 
further comprises an IF AGC control circuit connected to the comparator 
and the second initial level generating circuit for delivering the second 
initial voltage signal, as an IF AGC voltage which serves as another part 
of the automatic gain control voltage signal, during the predetermined 
time duration on reception of the channel selection signal or when the 
power supply is turned to on. The IF AGC control circuit delivers the 
first level difference signal, as the IF AGC voltage, after lapse of the 
predetermined time duration. 
According to another aspect of this invention, a reception front end device 
comprises an automatic gain control circuit which comprises a level 
detection circuit for detecting a reception level by utilizing an output 
signal produced from a demodulation circuit of a television signal. The 
automatic gain control circuit produces an automatic gain control voltage 
signal according to the reception level and further comprises: a reference 
level generating circuit for generating a reference voltage signal having 
a predetermined reference level; a comparator for comparing the reception 
level with the predetermined reference level to produce a first level 
difference signal representative of a first level difference between the 
reception level and the predetermined reference level; a first initial 
level generating circuit for generating a first initial voltage signal 
having a first initial level; a second initial level generating circuit 
for generating a second initial voltage signal having a second initial 
level; an optimum level generating circuit supplied with a channel 
selection signal designating a reception channel for generating a channel 
reference level signal having a preselected level that corresponds to the 
reception channel designated by the channel selection signal; an RF AGC 
control circuit connected to the comparator, the first initial level 
generating circuit, and the optimum level generating circuit for 
delivering the first initial voltage signal, as an RF AGC voltage which 
serves as a part of the automatic gain control voltage signal, during a 
predetermined time duration on reception of the channel selection signal 
or when power supply is turned to on. The RF AGC control circuit delivers 
a second level difference signal, as the RF AGC voltage, representative of 
a second level difference between the preselected level and the first 
level difference after lapse of the predetermined time duration. The 
automatic gain control circuit still further comprises an IF AGC control 
circuit connected to the comparator and the second initial level 
generating circuit for delivering the second initial voltage signal, as an 
IF AGC voltage which serves as another part of the automatic gain control 
voltage signal, during the predetermined time duration on reception of the 
channel selection signal or when the power supply is turned to on. The IF 
AGC control circuit delivers the first level difference signal, as the IF 
AGC voltage, after lapse of the predetermined time duration. The reception 
front end device further comprises: a tuner supplied with a radio 
frequency signal and comprising an RF amplifier circuit controlled by the 
RF AGC voltage. The tuner selects the reception channel designated by the 
channel selection signal and converts the radio frequency signal into an 
intermediate frequency signal having an intermediate frequency. The 
reception front end device further comprises an IF circuit supplied with 
the intermediate frequency signal and comprising an IF amplifier circuit 
controlled by the IF AGC voltage to produce an amplified intermediate 
frequency signal and the demodulation circuit for demodulating the 
amplified intermediate frequency signal into the output signal.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, description will be made with regard to a conventional 
reception front end device included in a television receiver in order to 
facilitate a better understanding of the present invention. Such a 
reception front end device is disclosed in Japanese Unexamined Patent 
Publication No. Tokkai Hei 4-271585 (271585/1992). 
The reception front end device comprises a tuner 11, a visual detector 
circuit 12, a visual process circuit 13, a display unit 14, and a control 
unit 15. The tuner 11 comprises a tuning circuit, an RF (Radio Frequency) 
amplifier circuit, and a frequency converting circuit all of which are not 
shown. The tuning circuit is supplied with an RF (Radio Frequency) signal 
through an input terminal 11-1 and selects a specific frequency signal of 
a specific channel, namely, a reception channel, designated by a channel 
switching signal. The RF amplifier circuit amplifies the specific 
frequency signal into an amplified frequency signal. The frequency 
converting circuit converts the amplified frequency signal into an IF 
(Intermediate Frequency) signal. 
The visual detector circuit 12 converts the IF signal into a composite 
video signal. The visual detector circuit 12 feeds an RF AGC voltage to 
the tuner 11. The visual process circuit 13 converts the composite video 
signal into an R (Red) signal, a G (Green) signal, and a B (Blue) signal 
or a luminance (or brightness) signal and a color-difference signal. The 
display unit 14 displays outputs of the visual process circuit 13 as a 
visual image. 
The control unit 15 is implemented by a microcomputer and is supplied with 
a channel selection signal from an outer unit (not shown). The channel 
selection signal is for designating one of a plurality of channels as the 
reception channel. Responsive to the channel selection signal, the control 
unit 15 delivers the channel switching signal to the tuner 11. The control 
unit 15 controls the RF AGC voltage by the use of an RF AGC (Automatic 
Gain Control) voltage control signal. The RF amplifier circuit is 
controlled in amplification degree by the RF AGC voltage controlled by the 
control unit 15. For this purpose, the control unit 15 comprises a memory 
unit (not shown) memorizing a plurality of AGC voltage control information 
in each of the plurality of channels. By the use of the plurality of AGC 
voltage control information, the control unit 15 controls the RF amplifier 
circuit so that the IF signal has an appropriate level. Thus, the tuner 11 
is controlled in gain thereof by one of the plurality of AGC voltage 
control information corresponding to the reception channel that is 
designated by the channel selection signal. The tuner 11 produces the IF 
signal having the appropriate level. 
The memory unit included in the control unit 15 further memorizes control 
information in each of the plurality of channels for the visual process 
circuit 13. The control information are for controlling brightness and 
image quality. By the use of the control information, the control unit 15 
controls the visual process circuit so that an appropriate visual image 
can be obtained. 
As mentioned above, the control unit 15 controls the tuner 11 in gain by 
the use of the plurality of AGC voltage control information stored in the 
memory unit in accordance with the reception channel. In other words, only 
the amplification degree of the tuner 11 is controlled. 
The input terminal 11-1 is often connected to a CATV (Cable Television) 
cable and supplied with the RF signal through the CATV cable. In this 
event, there is a case that the CATV cable is provided with an amplifier 
because the CATV cable has no uniform frequency characteristic and so as 
to compensate a loss caused by the CATV cable. The frequency 
characteristic of the amplifier changes in accordance with a frequency 
band to be received. In other words, the frequency characteristic is 
different in each of the amplifiers. This means that there is a case the 
RF signal supplied with the tuner 11 has a large level variation. 
When the tuner 11 receives a wire-less RF signal, it occasionally causes 
the case that the RF signal has the large level variation. 
Since each of the plurality of AGC voltage control information is a fixed 
value in each of the plurality of channels, it is impossible to compensate 
the large level variation of the RF signal by only controlling the 
amplification degree of the RF amplifier circuit, when the tuner 11 
received the RF signal has the large level variation larger than it had 
expected. 
Referring to FIG. 2, the description will proceed to a reception front end 
device according to a preferred embodiment of this invention. The 
reception front end device is particularly suitable for a television 
receiver and comprises a tuner 2, an IF (Intermediate Frequency) circuit 
3, and an automatic gain control circuit 4. 
The tuner 2 comprises a tuning circuit 21, an RF amplifier circuit 22, and 
a frequency converting circuit 23. The tuning circuit 21 is supplied with 
an RF (Radio Frequency) signal and selects a specific frequency signal of 
a specific channel, namely, a reception channel, designated by a channel 
selection signal. The RF amplifier circuit 22 is controlled in 
amplification degree by an RF AGC voltage supplied from the automatic gain 
control circuit 4 and amplifies the specific frequency signal into an 
amplified RF signal. The frequency converting circuit 23 converts the 
amplified RF signal into an IF (Intermediate Frequency) signal having a 
predetermined intermediate frequency. 
The IF circuit 3 comprises an IF amplifier circuit 31 and an IF 
demodulation circuit 32. The IF amplifier circuit 31 is controlled in 
amplification degree by an IF AGC voltage supplied from the automatic gain 
control circuit 4 and amplifies the IF signal into an amplified IF signal. 
The IF demodulation circuit 32 demodulates the amplified IF signal into a 
composite TV (Television) signal of the reception channel and produces the 
composite TV signal as an output signal. 
The automatic gain control circuit 4 comprises a level detection circuit 
41. The level detection circuit 41 detects a mean level of the output 
signal as a detected level. The detected level is regarded as a reception 
level of the RF signal. A reference level generating circuit 42 is for 
generating a reference voltage signal having a predetermined reference 
level. A comparator 43 is for comparing the detected level with the 
predetermined reference level to produce a first level difference signal 
representative of a first level difference between the detected level and 
the predetermined reference level. A first initial level generating 
circuit 44 is for generating a first initial voltage signal having a first 
initial level. A second initial level generating circuit 45 is for 
generating a second initial voltage signal having a second initial level. 
An optimum level generating circuit 46 is supplied with a channel 
selection signal designating a reception channel and generates a channel 
reference level signal having a preselected level that corresponds to the 
reception channel designated by the channel selection signal. 
An RF AGC control circuit 47 is connected to the comparator 43, the first 
initial level generating circuit 44, and the optimum level generating 
circuit 46. The RF AGC control circuit 47 delivers the first initial 
voltage signal, as the RF AGC voltage, during a predetermined time 
duration T on reception of the channel selection signal, or when the 
channel selection signal was updated, or when power supply is turned to 
on. Furthermore, the RF AGC control circuit 47 delivers a second level 
difference signal, as the RF AGC voltage, representative of a second level 
difference between the preselected level and the first level difference 
after lapse of the predetermined time duration T. 
An IF AGC control circuit 48 is connected to the comparator 43 and the 
second initial level generating circuit 45. The IF AGC control circuit 48 
delivers the second initial voltage signal, as the IF AGC voltage, during 
the predetermined time duration T on reception of the channel selection 
signal, or when the channel selection signal was updated, or when the 
power supply is turned to on. The IF AGC control circuit 48 delivers the 
first level difference signal, as the IF AGC voltage, after lapse of the 
predetermined time duration T. 
Referring to FIG. 2, the optimum level generating circuit 46 comprises a 
channel table memory 46-1 and a channel decision unit 46-2. The optimum 
level generating circuit 46 memorizes a plurality of channel reference 
levels corresponding to a plurality of channels, respectively. The channel 
decision unit 46-2 is supplied with the channel selection signal and 
decides the channel designated by the channel selection signal as the 
reception channel. On reference to the channel table memory 46-1, the 
channel decision unit 46-2 delivers the channel reference level signal 
having one of the plurality channel reference levels, as the preselected 
level, that corresponds to the reception channel. 
Referring to FIGS. 3 and 4 together with FIG. 1, operation of the reception 
front end device will be described. Generally, the television receiver is 
provided with a channel selection unit which is for designating the 
reception channel. For example, the channel selection unit is implemented 
by a plurality of selection buttons. When one of the plurality of 
selection buttons is pushed, the channel selection signal is generated. 
The channel selection signal designates the channel corresponding to one 
of the plurality of selection buttons that was pushed. The channel 
selection signal is received by the optimum level generating circuit 46, 
the RF AGC control circuit 47, and the IF AGC control circuit 48. 
The RF AGC control circuit 47 selects the first initial voltage signal, as 
the RF AGC voltage, generated by the first initial level generating 
circuit 44, each time when the channel selection signal is updated. The 
first initial voltage signal is supplied to the RF amplifier circuit 22 
during the predetermined time duration T as shown in FIG. 3. The RF AGC 
control circuit 47 carries out the same operation when the power supply 
for the automatic gain control circuit 4 was turned to on. In other words, 
the RF AGC control circuit 47 decides that the channel selection signal 
was updated, when the power supply was turned to on. 
On the other hand, the optimum level generating circuit 46 decides the 
reception channel designated by the channel selection signal and then 
selects the channel reference level, as a selected channel reference 
level, from the plurality of channel reference levels which are stored in 
the channel table memory 46-1 (FIG. 2). The selected channel reference 
level corresponds to the reception channel and is delivered to the RF AGC 
control circuit 47. 
As shown in FIG. 3, the RF AGC control circuit 47 delivers the second level 
difference signal, as the RF AGC voltage, representative of the second 
level difference between the preselected level and the first level 
difference after lapse of the predetermined time duration T. 
As illustrated in FIG. 4, the IF AGC control circuit 48 selects the second 
initial voltage signal produced from the second initial level generating 
circuit 45 each time when the channel selection signal was updated. The IF 
AGC control circuit 48 delivers the second initial voltage signal, as the 
IF AGC voltage, to the IF amplifier circuit 31 during the predetermined 
time duration T. 
On the other hand, the level detection circuit 41 detects the mean level of 
the output signal as the detected level. The comparator 43 compares the 
detected level with the predetermined reference level generated by the 
reference level generating circuit 42 and produces the first level 
difference signal representative of the first level difference between the 
detected level and the predetermined reference level. After a lapse of the 
predetermined time duration T, the IF AGC control circuit 48 selects the 
first level difference signal produced from the comparator 43 and delivers 
the first level difference signal, as the IF AGC voltage, as shown in FIG. 
4. The IF AGC control circuit 44 controls the gain of the IF amplifier 
circuit 31 so that the first level difference becomes to zero. 
As mentioned above, when the power supply is turned to on, or when the 
channel selection signal was updated, the automatic gain control circuit 4 
gives the first initial level and the second initial level to the RF 
amplifier circuit 22 and the IF amplifier circuit 31 as the RF AGC voltage 
and the IF AGC voltage, respectively, during the predetermined time 
duration T. As a result, the gains of the RF amplifier circuit 22 and the 
IF amplifier circuit 31 are kept at a constant value regardless of the 
reception channel. After a lapse of the predetermined time duration T, the 
RF amplifier circuit 22 is supplied from the RF AGC control circuit 47 
with the RF AGC voltage that is the second level difference between the 
channel reference level corresponding to the reception channel and the 
first level difference produced from the comparator 43. Even if the IF 
signal has a lower level as a result of a lower reception level of the 
reception channel, it is possible to keep the output of the IF amplifier 
circuit 31 at a constant value by increasing the gain of the IF amplifier 
circuit 31 under the control of the IF AGC voltage.