JPH09163265A - Television signal receiver and its automatic gain control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extract and process only a desired signal from a sum signal in which two signals or over whose level differs from each other are subject to frequency division multiplex processing. SOLUTION: The receiver adopts a basic building block of a double super heterodyne system receiver, an output of a 2nd IF amplifier 80 of a pre-stage of a 2nd IF filter 90 is detected by a detection circuit 170 and the detected signal is given to an AGC changeover circuit 190 via a limiter circuit 180 to select a gain of a variable gain amplifier 30 and a 2nd IF variable gain amplifier 100 depending of the magnitude of disturbance so as to reduce the disturbance. As an input signal level to the receiver increases, when no disturbance is in existence, after the gain attenuation of the 2nd IF variable gain amplifier 100 is maximized, the gain of the variable gain amplifier 30 is controlled. When any disturbance is in existence, the gain of the variable gain amplifier 30 is controlled before the gain attenuation of the 2nd IF variable gain amplifier 100 is maximized so as to reduce the input signal level of the receiver equivalently thereby reducing the disturbance. The disturbance in the receiver caused by a disturbing signal whose level is higher than a desired signal is reduced so as to allow the receiver to receive the desired signal in an excellent way.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to two different levels.
The present invention relates to a television receiver for extracting and processing only a desired signal from a sum signal obtained by frequency-division-multiplexing two or more signals, and an automatic gain control device thereof. In particular, the present invention is a high-definition television signal receiving device having a function of suppressing reception interference between channels when broadcasts having different signal levels are simultaneously broadcast, such as a high-definition television signal and a standard television signal. It is suitable for application to.

[0002]

2. Description of the Related Art In recent years, with the expansion of television broadcasting, CATV broadcasting, and the like, multichannel broadcasting has been promoted. Along with this, a double superheterodyne receiver is being used in order to reduce the deviation (tilt) characteristic in the channel band and image interference even when receiving multiple channels, and to reduce the interference due to leakage of the local oscillation signal. Has become.

FIG. 4 shows a conventional double super-heterodyne television receiver. As shown in FIG.
The signal input from the television signal input terminal 10 includes an input filter 20, a variable gain amplifier 30, a first mixer (first mixing circuit) 40, a first IF filter (first intermediate frequency filter) 50, and a first IF amplifier (first IF filter). Intermediate frequency amplifier circuit) 60, second mixer 70, second IF amplifier 80, second
The signals are sequentially processed by the IF filter 90 and the second IF variable gain amplifier 100, and output from the output terminal 110. A first local oscillator 120 is connected to the first mixer 40.
The first local oscillator 120 has a PLL (Phase Locked Loo
p) The reference oscillator 140 is connected via the circuit 130. A second local oscillator 150 is connected to the second mixer 70. In addition, the second IF variable gain amplifier 100
The output signal of 1 is branched and input to the detection circuit 200. An AGC switching circuit 190 is connected to the detection circuit 200. The reference voltage circuit 210 is also connected to the AGC switching circuit 190. AGC (Automatic gain control: Auto
The output of the matic gain control) switching circuit 190 is input to the variable gain amplifier 30 and the second IF variable gain amplifier 100. The input signal from the tuning signal input terminal 230 is input to the control circuit 220. The output of the control circuit 220 is input to the PLL circuit 130 and the input filter 20.

The operation of the conventional double super-heterodyne television receiver shown in FIG. 4 will be described.

R modulated by AM with a standard television signal
The F (Radio Frequency) signal is input from the input terminal 10, the band is divided by the input filter 20, and only the band including the desired channel is selectively input to the variable gain amplifier 30. By being amplified by the variable gain amplifier 30, the noise figure of the television receiver is reduced. A tuning signal is input to the tuning signal input terminal 230.

The input filter 20 is variably controlled by the control circuit 220 so as to have an appropriate pass band in accordance with the selected channel. The variable gain amplifier 30 amplifies or attenuates the signal band-limited by the input filter 20 so as to have an appropriate reception level, and the first mixer 40.
Enter

The first local oscillator 120 is controlled by the control circuit 22 so as to oscillate at a frequency corresponding to a desired channel according to a tuning signal input from the tuning signal input terminal 230.
0, controlled by the PLL circuit 130. PLL circuit 1
Reference numeral 30 compares the frequency-divided oscillation signal of the stable frequency from the reference oscillator 140 with the frequency-divided oscillation signal of the first local oscillator 120, so that the first error is zero. The oscillation frequency of the local oscillator 120 is controlled.
By appropriately changing the frequency division ratio by the control circuit 220, the first local oscillator 120 can oscillate at the frequency corresponding to the desired channel.

The first mixer 40 mixes the signal from the variable gain amplifier 30 and the local oscillation signal from the first local oscillator 120, and outputs a first IF signal (first intermediate frequency signal) 41. The first IF filter 50 selectively passes only the desired channel of the first IF signal 41. The first IF amplifier 60 amplifies this signal and inputs it to the second mixer 70. The second mixer 70 mixes the signal from the first IF amplifier 60 and the local oscillation signal from the second local oscillator 150, and outputs the second IF signal 71. Second IF amplifier 80
Amplifies this signal, and SAW (surface acoustic wave: Surface Ac
It is input to the second IF filter 90 including an oustic wave) filter or the like. The second IF filter 90 uses the second IF
Only the desired channel of signal 71 is selectively passed. The second IF signal 71 that has passed through the second IF filter is
The signal is amplified by the F variable gain amplifier circuit 100 and output from the output terminal 110.

A gain control signal for controlling the gains of the variable gain amplifier 30 and the second IF variable gain amplifier 100 is generated by detecting a signal obtained by branching the output signal of the second variable gain amplifier 100 by the detection circuit 200. The AGC switching circuit 190 sends this gain control signal to the variable gain amplifier 30,
Either of the second IF variable gain amplifiers 100 is added.

That is, the output signal of the second IF gain amplifier 100 is detected by the detection circuit 200 and becomes a gain control signal. If the output signal of the second IF gain amplifier is large, A
The GC switching circuit 190 first outputs this gain control signal to the second
IF variable gain amplifier 100 is added. Even when the gain attenuation amount of the second IF variable gain amplifier 100 is maximum, when the signal level is high, the AGC switch 190 adds the gain control signal to the variable gain amplifier 30. Whether or not the gain attenuation amount of the second IF variable gain amplifier 100 has become maximum is detected by comparing the output of the detection circuit 200 and the output of the reference voltage 210.

When the level of the output signal of the second IF variable gain amplifier is low, the AGC switching circuit 190 adds the gain control signal to the variable gain amplifier 30 to increase the gain of the variable gain amplifier 30. Then, when the signal level is low even after the gain of the variable gain amplifier 30 is maximized, the gain control signal is added to the second IF variable gain amplifier 100,
The gain of the IF variable gain amplifier 100 is increased. Whether or not the gain of the variable gain amplifier 30 has become maximum is detected by comparing the output of the detection circuit 200 and the output of the reference voltage 210.

In this way, when it is necessary to reduce the gain of the variable gain amplifier 30 or 100 in order to keep the signal level output from the output terminal 110 constant, as much as possible, the second stage of the receiving device is provided. 2IF variable gain amplifier 1
00 gain, and if this is not sufficient, by reducing the gain of the variable gain amplifier 30 in the preceding stage,
The gain of the variable gain amplifier 30 in the previous stage can be kept as large as possible. Therefore, it is possible to suppress the deterioration of the noise figure of the entire receiving device.

In general, in order to maintain a good noise figure (NF), that is, a small value, it is necessary to increase the gain of the amplifier in the front stage as compared with the amplifier in the rear stage. Therefore, when controlling the gains of the front-stage amplifier and the rear-stage amplifier, when the gain of the amplifier is attenuated, first, the gain of the rear-stage amplifier is attenuated, and when the output signal is still large, the gain of the front-stage amplifier is attenuated. Have control. On the contrary, when increasing the gain of the amplifier, the gain of the amplifier at the front stage is first increased, and when the gain is still insufficient, the gain of the amplifier at the rear stage is increased.

The gain control signal to the variable gain amplifier 30 is set to a relatively long time constant, and the gain control signal to the second IF variable gain amplifier 100 is set to a relatively short time constant. Thus, when the signal level changes in a short period such as flutter caused by an airplane or the like, the second IF variable gain amplifier 100 follows the change.

[0015]

However, the above-mentioned conventional receiving apparatus is effective when receiving a television signal in which the received power between the channels is substantially uniform.
When broadcasts with standard television signals such as SC (National Television System Committee) broadcasts and high-definition television signal broadcasts with digital transmission are broadcast at the same time by frequency division multiplexing, the filter 20 and It is difficult for the filter 50 to remove the signal of the adjacent channel. Therefore, the signals of the adjacent channels are taken into the receiving device together with the desired signal to be received. Only the desired signal can be selected by the filter 90 for the first time. This is because the frequency of the signal passing through the filters 20 and 50 is high, and it is currently difficult to fabricate a filter having a narrow pass bandwidth capable of passing only a desired signal. is there. Therefore, each circuit processes the desired signal to be received and the signal of the adjacent channel until the signal passes through the filter 90 that can use the SAW filter. However, the NT of adjacent channels
The SC broadcast signal has a higher level than the digital desired signal, exceeds the dynamic range of each circuit in the preceding stage of the filter 90, and may cause interference.

The interference due to the difference in the received power between the respective channels is also caused by a special terrain or a weather change during the transmission by the ground wave.

It is an object of the present invention to provide an automatic gain control circuit having a function of suppressing interference generated when a signal of an adjacent channel is taken in when receiving a frequency division multiplexed signal, and a receiver thereof. To do.

[0018]

In order to achieve the above object, according to the present invention, a terminal for inputting a signal to be received, and a signal input to the input terminal are amplified to a set level. Or a first variable gain amplifying section for attenuating,
Of the signals output from the first variable gain amplifier, a filter that selectively passes a signal in a predetermined band and a second filter that amplifies or attenuates the signal that has passed the filter to a set level It has a variable gain amplification section and a control section for setting a gain in the first and second variable gain amplification sections, wherein the control section has a signal input to the filter equal to or lower than a predetermined level, and When the output of the second variable gain amplifying section is higher than a predetermined level, the second variable gain amplifying section is made to correspond to the output of the second variable gain amplifying section.
If the output of the second variable gain amplifying section is larger than the predetermined level even if the gain of the second variable gain amplifying section is decreased and the gain of the second variable gain amplifying section reaches the predetermined minimum value. Reduces the gain of the first variable gain amplifier,
When the signal input to the filter is larger than a predetermined level and the output of the second variable gain amplifying section is larger than the predetermined level, the output of the second variable gain amplifying section is made to correspond to the output of the second variable gain amplifying section. An automatic gain control circuit characterized by decreasing the gain of the second variable gain amplifying section and decreasing the gain of the first variable gain amplifying section in accordance with the signal level input to the filter. Provided.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described.

FIG. 1 shows a block diagram of a high definition television signal receiving apparatus according to an embodiment of the present invention.

As shown in FIG. 1, the signal input from the television signal input terminal 10 has an input filter 20, a first variable gain amplifier 30, a first mixer (first mixing circuit) 40, and a first IF arranged in order. Filter (first intermediate frequency filter) 50, first IF amplifier (first intermediate frequency amplification circuit) 6
0, the second mixer 70, the second IF amplifier 80, the second IF filter 90, and the second variable gain amplifier 100, and output from the output terminal 110. A first local oscillator 120 is connected to the first mixer 40. The first local oscillator 120 includes a PLL (Phase Locked Loop) circuit 130.
The reference oscillator 140 is connected via. A second local oscillator 150 is connected to the second mixer 70.

The output signal of the second variable gain amplifier 100 is branched and input to the detection circuit 200. Also,
The output signal of the second IF amplifier 80 is branched into the amplifier 1
60, the detection circuit 170, and the detection circuit 180 are sequentially input. The output of the detection circuit 200 and the output of the detection circuit 180 are AGC (automatic gain control).
It is input to the switching circuit 190. The output of the AGC switching circuit 190 is input to the first variable gain amplifier 30 and the second variable gain amplifier 100. The input signal from the tuning signal input terminal 230 is input to the control circuit 220. The output of the control circuit 220 is the PLL circuit 130 and the input filter 2
Input to 0.

A high-definition television signal, which is digitized information, is data-compressed and VSB (Vestigial Sideband Modulation) QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Modulation), OFDM (Orthogonal Frequency Multiplexing) ) Is used for multi-level digital modulation and has the same bandwidth as the bandwidth of a normal television signal. This high-definition television signal is simultaneously transmitted as an NTSC signal and a terrestrial wave using a frequency division multiplexing system. Here, the NTSC signal represents a television signal according to the NTSC system.

This signal is input from the antenna to the television signal input terminal 10, and the input filter 20 outputs VHF.
The band is divided into a band and a UHF band (further, the VHF band may be divided into a low band, a middle band, and a high band), and only a band including a desired channel is selectively input to the variable gain amplifier 30.

The pass band of the input filter 20 is controlled by the control circuit 220 to have an appropriate pass band based on the tuning signal input to the tuning signal input terminal 230. By reducing the number of channels input to the variable gain amplifier 30 by the input filter 20, it is possible to reduce intermodulation interference between input signals generated in the subsequent circuits, but the characteristics of the filter 20 are completely reduced. Since it is not possible to extract only the necessary channels, the signals of adjacent channels are also input.

The variable gain amplifier 30 includes the input filter 20.
The signal whose band is limited by is amplified so as to have an appropriate reception level and is input to the first mixer 40. The gain attenuation amount of the variable gain amplifier 30 is set by the automatic gain control signal supplied from the AGC switching circuit 190. The gain attenuation amount to be set will be described later in detail.

The first local oscillator 120 is controlled by the control circuit 22 so as to oscillate at the frequency corresponding to the desired channel by the tuning signal input from the tuning signal input terminal 230.
0, controlled by the PLL circuit 130. PLL circuit 1
Reference numeral 30 compares the frequency-divided oscillation signal of the stable frequency from the reference oscillator 140 with the frequency-divided oscillation signal of the first local oscillator 120, so that the first error is zero. The oscillation frequency of the local oscillator 120 is controlled.
By appropriately changing the frequency division ratio by the control circuit 220, the first local oscillator 120 can oscillate at the frequency corresponding to the desired channel.

The first mixer 40 mixes the signal from the variable gain amplifier 30 and the local oscillation signal from the first local oscillator 120, and outputs a first IF signal 41.

The frequency of the first local oscillator 120 is equal to the first I
The frequency of the F signal 41 is set to be higher than the upper limit frequency of the terrestrial transmission band of the NTSC television signal or the CATV transmission band in order to reduce image interference and prevent reception interference due to leakage of the local oscillation signal. For example, in the domestic and US frequency allocation, the 960MHz band, 1200MHz
Band, 1700MHz band, 2600MHz band, 3000M
Set to the Hz band, etc.

The first IF filter 50 selectively passes the band including the desired channel of the first IF signal and outputs the first IF signal.
Input to the amplifier 60. The first IF filter 50 uses a bandpass filter (dielectric filter, SAW filter, or the like) having in-band flatness characteristics and low group delay deviation characteristics that do not deteriorate demodulation of high-definition television signals. Since the frequency of the input signal is very high, it is difficult to pass only the desired channel,
The signals of adjacent channels also pass. First IF amplifier 6
0 amplifies this signal and inputs it to the second mixer 70.

The second mixer 70 mixes the signal from the first IF amplifier 60 and the local oscillation signal from the second local oscillator 150 and outputs a second IF signal 71.

The frequency of the second local oscillator 150 is equal to the second I
The frequency of the F signal 71 is set to be the same as when the current standard television signal is received. Specifically, in the case of the United States, it is set to the same 44 MHz band as when receiving the NTSC signal. In the case of Japan, the frequency band is 57 MHz.

The second IF signal 71 is sent to the second IF amplifier 80.
Amplified by Since the frequency of the second IF signal 71 is low,
The second IF filter 90 can use a filter that can selectively pass only a desired channel,
Only then is the adjacent channel signal removed.
The second IF filter 90 needs to have an in-band flatness and a low group delay deviation that do not deteriorate the demodulation characteristics of the high definition television signal, and at the same time, can pass only the channel selected as described above. For example, SAW
It is composed of filters.

Then, the second IF variable gain amplifier 100 amplifies or attenuates to a desired signal level,
It is output from the output terminal 110. The signal output from the output terminal 110 is input to a synchronous detection circuit (not shown).

When an NTSC signal having a higher level than the high definition television signal which is the desired signal is arranged in the adjacent and adjacent channels of the desired signal, as described above, this signal is filtered by the filters 20 and 50. Since it cannot be removed, it is input from the signal input terminal 10 together with the desired signal. Therefore, when an NTSC signal having a high level is input together with the desired signal, the level of the sum signal of the desired signal and the NTSC signal may exceed the dynamic range. In this case, interference occurs in the circuit blocks up to the stage before the second IF filter 90. In particular, when the SAW filter is used for the second IF filter 90 as described above, S
Since the loss of the AW filter is large, the second mixer 70 and the second IF amplifier 80 are set so that the signal level of the desired signal is considerably high. Therefore, when the second IF signal includes the NTSC signal of the adjacent channel, the dynamic range of the amplifier in the preceding stage of the second IF filter 90 is deviated and a large disturbance is generated. That is, when the signal of the adjacent channel is large and goes out of the dynamic range of the amplifier, the waveform of the desired signal is damaged or the information of the desired signal is lost. Also, the higher the level of the NTSC signal, the greater the interference.

In order to suppress this interference, the signal input to the second IF filter 90 is branched, amplified by the amplifier 160, and detected by the detection circuit 170, so that an NTSC signal with a high level in the adjacent channel is input. Detect whether or not there is an interference.

Further, the detection circuit 200 detects the output signal (second IF signal) of the second IF variable gain amplifier 100.

The AGC switching circuit 190 includes a detection circuit 17
The output of 0 is input through the detection circuit 180, and at the same time, the output of the detection circuit 200 is also input.

The AGC switching circuit 180 is constructed so as to operate as follows.

The AGC switching circuit 180 includes a detection circuit 170.
When the output level of 1 is a level indicating that there is no interference, it operates in the same manner as the conventional double super heterodyne type receiving device. First, the output signal of the detection circuit 200 is applied as a gain control signal to the second IF variable gain amplifier 100. The gain control signal is added such that the larger the output of the detection circuit 200, the larger the gain attenuation amount. Then, the second IF variable gain amplifier 9
If the output of the detection circuit 200 does not decrease even when the gain attenuation amount of 0 becomes maximum (or a predetermined attenuation amount), the AGC switching circuit 190 changes the output of the detection circuit 200 as a gain control signal. In addition to the gain amplifier 30,
The gain of the variable gain amplifier 30 is reduced.

When the output of the detection circuit 200 decreases due to such control, the AGC switching circuit 190 first adds a gain control signal to the variable gain amplifier 30 so as to reduce the gain attenuation amount, and this amplifier is added. Increase the gain of 30. Then, after the gain of the variable gain amplifier 30 reaches the maximum (or a predetermined gain), the second I
The gain control of the F variable gain amplifier 100 is performed so as to reduce the gain attenuation amount, and the gain of the second IF variable gain amplifier 100 is increased.

Further, the gain control signal to the variable gain amplifier 30 is set to a relatively long time constant, and the gain control signal to the second IF variable gain amplifier 100 is set to a relatively short time constant, so that an aircraft or the like can be used. When the signal level changes in a short period like flutter, the second IF variable gain amplifier 100 can follow the change. The method of setting these time constants is a well-known technique in the automatic gain control of the receiver, and therefore its explanation is omitted.

When the AGC switching circuit 190 detects that the output of the detection circuit 170 is larger than a predetermined value and there is interference in the circuit preceding the second IF filter 90, the second IF variable gain amplifier 90 is detected. The gain of the variable gain amplifier 30 is reduced before the maximum gain attenuation amount (or the predetermined gain attenuation amount) is reached. At this time, the timing for reducing the gain of the variable gain amplifier 30 is determined in accordance with the output of the detection circuit 170. That is, the larger the interfering signal, the earlier the gain of the variable gain amplifier 30 is reduced. If the disturbance is small, the gain of the variable gain amplifier 30 is reduced after the attenuation amount of the second IF variable gain amplifier 100 becomes close to the maximum.

That is, when there is an interference, the second variable gain amplifier 1 responds to the output of the second variable gain amplifier 100.
00, the gain attenuation amount of the variable gain amplifier 30 is started to increase according to the disturbing signal level.

As the amount of interference increases, if the gain attenuation of the variable gain amplifier 30 is infinitely increased and the input level is decreased, the CN ratio of the desired signal becomes insufficient. Further, as the interference decreases, if the gain attenuation amount of the variable gain amplifier 30 is infinitely decreased and the input level is increased, the dynamic range of the variable gain amplifier 30 will be exceeded. Detection circuit 180
Defines an upper limit and a lower limit of the input level of the detection circuit 170 in order to prevent such a situation.

By doing so, the signal level input from the television signal input terminal 10 is equivalently reduced, and the signal level exceeds the dynamic range in the circuit before the second IF filter 90, causing interference. Can be prevented. Further, as the gain attenuation amount of the variable gain amplifier 30 increases or decreases, the gain attenuation amount of the second IF variable gain amplifier 100 also increases or decreases. Therefore, the output level of the desired signal output from the output terminal 100 is the presence or absence of interference. It can be kept constant regardless of the value.

Also, for reasons other than interference, the output terminal 110
When it is necessary to reduce the gain of the variable gain amplifier 30 or 100 in order to keep the signal level output from the variable gain amplifier 30 or 100 constant, the second IF variable gain amplifier 100 in the latter stage of the receiving device is as much as possible, as in the conventional case. The gain of the variable gain amplifier 30 is reduced, and when this is insufficient, the gain of the variable gain amplifier 30 in the preceding stage is reduced, so that the gain of the variable gain amplifier 30 in the preceding stage can be kept as large as possible. Therefore, it is possible to suppress the deterioration of the noise figure of the entire receiving device.

AGC switching circuit 19 which operates in this way
0 and an example of a specific embodiment of the detection circuit 180 will be described in detail with reference to FIG.

In FIG. 2, parts performing the same operations as in FIG. 1 are assigned the same numbers as in FIG. 1 and their explanations are omitted. As shown in FIG. 2, the detection circuit 180 includes a PNP transistor 540, a variable resistor 55, and a comparator 510, which are sequentially connected.
0, a resistor 560, and a reference voltage power supply 570.
The detection circuit 1 is connected to the non-inverting input terminal of the comparator 510.
The output of 70 is input. A reference voltage power supply 500 is connected to the inverting input terminal. Comparator 51
The output terminal of 0 is connected to the base electrode of the PNP transistor 540 via the resistor 530. Also, PN
The voltage source 5 is connected to the emitter electrode of the P-transistor 540.
80 and one end of the resistor 520 are connected. Resistance 52
The other end of 0 is connected to the base terminal of the PNP transistor 540. Further, an output signal line 590 is drawn out from between the variable resistor 550 and the resistor 560.

The AGC switching circuit 190 includes a comparator 6
00 and 620 and a reference voltage power supply 610. The reference voltage power supply 610 is connected to the inverting input terminal of the comparator 600, and the output of the detection circuit 200 is input to the non-inverting input terminal. In addition, the comparator 6
The output of 00 is input to the second IF variable gain amplifier 100, is branched, and is also input to the inverting input terminal of the comparator 620. The output signal line 590 of the detection circuit 180 is connected to the non-inverting input terminal of the comparator 620. The output of the comparator 620 is input to the variable gain amplifier 30.

The voltage of the reference voltage power source 500 is detected by the detection circuit 1
When the output level of 70 is a signal level that does not interfere with the signal in the previous stage of the second IF filter 90, the comparator 51
The output of 0 is set to 0.

When the interference is small, the output level of the detection circuit 170 is small, the output of the comparator 510 is small, and the PNP transistor 540 is turned on. When there is no interference, the output of the comparator 510 is 0, the power supply voltage 580 is Vcc, the reference voltage 570 is VL,
When the variable resistor 550 is VR and the resistor 560 is R, the output signal line 590 of the detection circuit 180 is connected to the comparator 620.
The non-inverting input Vcont to Vcont is Vcont = (Vcc-VL) R / (R + VR) + VL (1).

As the interference increases, the PNP transistor 540 is turned off. When the interference increases to a certain extent or more, Vcont = VL (2).

Further, Vcont takes a value between the expressions (1) and (2) depending on the magnitude of the disturbance.

In this way, the value of Vcont changes according to the magnitude of the disturbance. Further, the maximum value of Vcont is represented by the equation (1), and the minimum value thereof is represented by the equation (2), and does not take a value larger or smaller than these. Therefore, the detection circuit 180 acts as a limiter that outputs a signal having an upper limit and a lower limit based on the output of the detection circuit 170. As is clear from the above equation, the detection circuit 180 is configured to take a smaller value as the interference of Vcont increases.

The AGC switching circuit 190 is the detection circuit 200.
Is compared with the voltage of the reference voltage power supply 610 by the comparator 600, and when the output of the detection circuit 200 is larger than the voltage of the reference voltage power supply 610, the gain control signal is set to the first value in correspondence with the output of the detection circuit 200. Input to the 2IF variable gain amplifier 100. That is, if the output of the second IF variable gain amplifier 100 is large, a high level gain control signal is added to the second IF variable gain amplifier 100.
Since the second IF variable gain amplifier 100 has a polarity such that the gain attenuation amount increases as the level of the gain control signal increases, the second IF variable gain amplifier 100 uses
The larger the output level of itself, the larger the gain attenuation amount is set.

The gain control signal output from the comparator 600 is branched and input to the inverting input terminal of the comparator 620. Here, the gain control signal is input to the inverting input terminal of the comparator 620 because the polarity of the variable gain amplifier 30 is opposite to that of the second IF variable gain amplifier. The comparator 620 compares the gain control signal with the output Vcont of the detection circuit 180. As described above, Vcont corresponds to the degree of interference of the circuit in the preceding stage of the filter 90. Comparator 62
When the gain control signal is greater than Vcont, 0 outputs the gain control signal to the variable gain amplifier 30 and increases the gain attenuation amount of the variable gain amplifier 30. Here, when there is no interference, the second IF variable gain amplifier 100
In order to prevent the gain control signal from being output to the variable gain amplifier 30 until the gain attenuation amount of Vmax becomes maximum (or reaches a predetermined value), Vcont = (Vcc-VL) in the absence of interference R / (R + VR)
Vcc of the detection circuit 180, so that the level of + VL becomes the same level as the output level of the comparator 600 for setting the gain attenuation amount of the second IF variable gain amplifier 100 to the maximum (or a predetermined value), VL, R,
Set VR.

With such a configuration, when the signal level of the output signal of the second IF variable gain amplifier 100 increases, the gain control signal is obtained when Vcont has the value shown in the equation (1) (when there is no interference). The second IF variable gain amplifier 1
In addition to 00, the gain attenuation amount of the second IF variable gain amplifier 100 becomes maximum (predetermined value),
The gain control signal is applied to the variable gain amplifier 30. Also, V
When cont is other than the value shown in the equation (1) (when interference exists), the signal level of the output signal of the second IF variable gain amplifier 100 increases, and the second I
The maximum amount of gain attenuation of the F variable gain amplifier 100 (or
The gain control signal is applied to the variable gain amplifier 30 before it becomes a predetermined value).

In this way, by detecting the signal level of the front stage of the second IF filter 90 by the detection circuit 170,
When the signal level is high, it can be detected that there is interference by an adjacent channel with a level higher than the signal level of the channel to be received. And this detection circuit 17
A comparator 6 which outputs a signal corresponding to the detected interference level of 0 to the variable gain amplifier 30 as a gain control signal.
By inputting to 20, it is possible to change the time when the gain of the variable gain amplifier 30 starts to be attenuated according to the magnitude of the disturbance. As a result, the variable gain amplifier 30
It is possible to reduce interference input to the subsequent circuits. Therefore, it is possible to effectively reduce the interference caused by the input of a signal having a level higher than that of the reception channel from the adjacent channel. As a result, even if a signal with a high level of NTSC broadcasting exists in a channel adjacent to the channel of high-definition television broadcasting by digital transmission, and the NTSC signal from the adjacent channel is input together with the desired signal, it is variable. Since the gain is attenuated by the gain amplifier 30, interference is reduced without fear of exceeding the dynamic range of the circuit preceding the filter 90. Therefore,
High-definition television broadcasting can be received with a low error rate.

Further, since Vcont takes a value between the equations (1) and (2) depending on the magnitude of the disturbance, not only the disturbance can be suppressed but also the decrease in the CN ratio can be minimized.

When there is no interference, the output signal level is maintained by reducing the gain of the second IF variable gain amplifier 100 as much as possible. Therefore, as in the conventional case,
The noise figure of the receiving device can be reduced.

As described above, the high-definition television receiving apparatus adopting the automatic gain control circuit shown in this embodiment can prevent the interference due to the NTSC signal of the adjacent channel and effectively reduce the noise figure of the receiving apparatus. Moreover, the output level of the desired signal to be received, which is output from the output terminal 100, has a constant value regardless of the presence or absence of interference, and the desired signal can be received at a stable level.

The receiving apparatus of the present embodiment can suppress the interference from the adjacent channels as described above, but in addition to this, it receives a television signal which is modulated with a different required CN ratio. It is also suitable for Although the present embodiment has been described with respect to a high definition television receiver, the present invention is not restricted to this and can be applied to a receiver that is susceptible to interference from adjacent channels and other interference.

Next, a high definition television signal receiving apparatus as a second embodiment of the present invention will be described with reference to FIG.

In FIG. 3, parts performing the same operations as in FIG. 1 are assigned the same numbers as in FIG. 1 and their explanations are omitted. The receiving apparatus of FIG. 3 is different from the receiving apparatus of FIG. 1 in that the second IF amplifier 8
The output of 0 is not detected, but a signal of a level corresponding to Vcont in FIG. 1 is digitally generated, and this signal is converted into an analog signal and input to the AGC switching circuit 190. The Vcont digital signal is internally digitally generated by the high definition television signal demodulator 240. The demodulator 240 sets the level of Vcont to the expression (1) and the expression (2) when the demodulation cannot be performed.
Scan between levels represented by the formula and output. The digital-analog converter 250 converts this scanning signal into an analog signal and inputs it to the non-inverting input terminal of the comparator 620 of the AGC switching circuit 190.

More specifically, the demodulator 240 performs analog-to-digital conversion on the output of the second variable gain amplifier 100, and then processes the demodulator 240 with a digital filter having a cosine root roll-off characteristic. Furthermore, by performing waveform equalization,
Performs carrier recovery and correction of in-band characteristics. If carrier recovery cannot be performed, a signal indicating this is generated. This signal is supplied to the microcomputer 260. Although this microcomputer 260 is shown separately from the demodulator 240 for convenience,
When the demodulator 240 uses a microcomputer, the microcomputer can be used. When the carrier recovery impossible signal is supplied to the microcomputer 260, the microcomputer 26
Software is incorporated so that 0 outputs a digital signal representing a sawtooth wave. The output signal of the microcomputer 260 is supplied to the digital-analog converter 250, and the converter 250 outputs a sawtooth wave. This sawtooth wave is added to the reference voltage 270 and supplied to the AGC switching circuit 190. This sawtooth wave is input to the non-inverting input terminal of the comparator 620. With this configuration, when the interfering wave is mixed in the demodulator 240 and the carrier recovery cannot be performed, the sawtooth wave is supplied to the non-inverting input of the comparator 240, and thus the second variable gain amplifier 100 outputs the first Since the switching point of the gain control to the variable gain amplifier 30 can be changed, the disturbance caused by the distortion of the television signal is reduced by exceeding the dynamic range in the circuit in the previous stage of the filter 90, and carrier recovery is reduced. Is possible.

That is, the NT having a higher level than the desired signal is adjacent to or adjacent to the high definition television signal which is the desired signal.
When the SC signal is arranged, interference occurs in the receiving device and the high-definition television signal demodulator 240 cannot demodulate. There are various other factors that make it impossible to demodulate, and it is difficult for the demodulator to detect only this interference. However, if the cause of the inability to demodulate is this interference, if Vcont is scanned when it becomes impossible to demodulate, it can be demodulated at any value of Vcont. Therefore, the high-definition television signal demodulator 240 shown in FIG. 3 is configured to scan Vcont when the demodulation cannot be performed. If the demodulation is still not possible, it can be inferred that the cause is other than the above-mentioned interference, and other means are taken.

Further, in the configuration of FIG. 3, it is possible to change the range of change of Vcont depending on the required CN ratio of the received signal.

In the embodiment shown in FIG. 3, the analog detection circuit, the limiter circuit, etc. are unnecessary as in the embodiment shown in FIG. 1, and the demodulator IC can be built in, so that the number of parts is increased. Can be reduced.

Although the embodiments of FIGS. 1, 2 and 3 are the receivers using the double super heterodyne system, the present invention can be applied to the receivers of the single super heterodyne system.

[0071]

According to the present invention, an automatic gain control method and apparatus capable of extracting and processing only a desired signal by suppressing interference from a sum signal in which two or more signals having different levels are frequency division multiplexed. It is possible to provide.

Further, by using this apparatus, the broadcasting of standard television signals such as NTSC broadcasting having different transmission levels and the broadcasting of different transmission levels such as high definition television signal broadcasting by digital transmission are simultaneously frequency division multiplexed. It is possible to configure a high-definition television signal receiving device having a function of suppressing an interference generated in the receiving device due to a difference in received power between channels caused by special terrain, weather, etc. when transmitted by terrestrial waves. . However, the high-definition receiver has been described as an example of the present invention, but the present invention is not necessarily limited to this, and it is applicable to a receiver in which interference occurs in the receiver as described above. Therefore, excellent effects can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a receiving device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a detailed configuration of an AGC switching circuit and a limiter circuit of FIG.

FIG. 3 is a block diagram showing a configuration of a receiving device according to another embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a conventional receiving device.

[Explanation of symbols]

10 ... Television signal input terminal, 20 ... Input filter, 30 ... Variable gain amplifier, 40 ... First mixer, 41 ...
First IF signal, 50 ... First IF filter, 60 ... First I
F amplifier, 70 ... second mixer, 71 ... second IF signal, 8
0 ... 2nd IF amplifier, 90 ... 2nd IF filter, 100
... 2nd IF variable gain amplifier, 110 ... Output terminal, 120
... 1st local oscillator, 130 ... PLL circuit, 140 ... reference oscillator, 150 ... 2nd local oscillator, 160 ... amplifier, 1
70 ... Detection circuit, 180 ... Limiter circuit, 1190 ... A
GC switching circuit, 200 ... Detection circuit, 210 ... Reference voltage,
220 ... Control circuit, 230 ... Channel selection signal input terminal, 240
... High-definition television signal demodulator, 250 ... Digital-analog converter, 260 ... Microcomputer, 270 ...
Reference voltage, 500 ... Reference voltage, 510 ... Comparator,
520 ... Resistor, 530 ... Resistor, 540 ... PNP transistor, 550 ... Variable resistor, 560 ... Resistor, 570 ... Reference voltage, 580 ... Power supply voltage, 590 ... Output signal line, 600
Comparator, 610 ... Reference voltage, 620 ... Comparator.

Continuation of the front page (72) Inventor Toshio Nagashima 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi, Ltd. Multimedia system development headquarters

Claims (12)

[Claims] 1. A terminal for inputting a signal to be received, a first variable gain amplifying section for amplifying or attenuating a signal input to the input terminal to a set level, and the first variable gain. A filter that selectively passes a signal in a predetermined band among signals output from the amplification unit; a second variable gain amplification unit that amplifies or attenuates the signal that has passed through the filter to a set level; A second demodulator for demodulating an output of the variable gain amplifying section; and a gain switching section for switching the gain control of the first variable gain amplifying section and the second variable gain amplifying section. An automatic gain control circuit, wherein a switching point of the gain switching unit is changed by using a control signal when a received signal input to the demodulator does not match a demodulation format. 2. A terminal for inputting a signal to be received, a first variable gain amplifier for amplifying or attenuating a signal input to the input terminal to a set level, and the first variable gain. A filter that selectively passes a signal in a predetermined band among signals output from the amplification unit; a second variable gain amplification unit that amplifies or attenuates the signal that has passed through the filter to a set level; A gain switching unit that switches the gain control of the first variable gain amplifying unit and the second variable gain amplifying unit is provided, and the gain switching unit sets the level of the signal input to the filter to a predetermined level. When it exceeds, the automatic gain control circuit is characterized in that the switching point of the gain control from the second variable gain amplifying section to the first variable gain amplifying section is changed according to the input signal to the filter. 3. The gain switching unit according to claim 1, wherein the gain switching unit first reduces the gain of the variable gain amplifying unit in accordance with an output signal of the second variable gain amplifying unit. The gain of the second variable gain amplifier is reduced,
When the gain attenuation amount of the gain amplifying unit reaches a predetermined value, the gain of the first variable gain amplifying unit is decreased and the gain of the variable gain amplifying unit is increased. An automatic gain, wherein the gain of the variable gain amplification section is increased, and when the gain of the first variable gain amplification section reaches a predetermined value, the gain of the second gain amplification section is increased. Control circuit. 4. The gain switching unit according to claim 1, wherein the gain switching unit controls the gain attenuation amount of the second variable gain amplifying unit when the signal supplied to the filter is equal to or lower than a predetermined level. When the predetermined value is reached, the gain of the first variable gain amplifying unit is reduced, and when the signal supplied to the filter exceeds a predetermined value, the level of the signal is changed according to the level of the signal. An automatic gain control circuit, wherein the gain of the first variable gain amplifying section is decreased before the gain attenuation amount of the second variable gain amplifying section reaches a predetermined value. 5. A terminal for inputting a signal to be received, a first variable gain amplifier for amplifying or attenuating a signal input to the input terminal to a set level, and the first variable gain. A filter that selectively passes a signal in a predetermined band among signals output from the amplification unit; a second variable gain amplification unit that amplifies or attenuates the signal that has passed through the filter to a set level; A gain switching unit that sets a switching point for gain control of the first and second variable gain amplifying units, the gain switching unit branches the output signal of the second variable gain amplifying unit, and A first detection section for detecting, a second detection section for branching a signal input to the filter and detecting the same, and a second variable gain amplification corresponding to a signal level detected by the first detection section. First to set the divisional gain A gain setting unit; a detection unit that outputs a signal corresponding to the signal level of the second detection unit when the signal level of the second detection unit exceeds a predetermined value; and an output signal of the detection unit and a signal level of the first detection unit. And a second gain setting section that sets the gain of the first variable gain amplifying section in correspondence with a difference signal between the automatic gain control circuit and the automatic gain control circuit. 6. The comparator according to claim 5, wherein the detector outputs a signal corresponding to the signal from the second detector when the signal from the second detector exceeds a predetermined value, and the comparator. An automatic gain control circuit comprising an active element that changes from cutoff to saturation depending on the output of the device. 7. The first gain setting section according to claim 5,
Comprised of a comparator and a reference power supply, the first of the comparator
Is connected to the reference power source, the second input terminal is connected to the output of the first detection section, and the output terminal of this comparator is connected to the second variable gain amplification section. Automatic gain control circuit 8. The second gain setting section according to claim 5,
The output of the first gain setting section is input to the first input terminal of the comparator, the output of the detection section is input to the second input terminal, and the output of the comparator is the first variable gain amplifier. An automatic gain amplifier, characterized in that it is connected to a section. 9. A reception signal input terminal, a first variable gain amplifying section for amplifying a signal supplied to the input terminal, and a frequency conversion for frequency converting an output of the first variable gain amplifier. Section, a filter for passing a signal to be received from the output signal of the frequency conversion section, a second variable gain amplification section for amplifying the output of the filter, and the first and second variable gains. A gain switching circuit for switching control of the gain of the amplifying unit, wherein the gain switching circuit has a signal input to the filter higher than a predetermined level, and the output of the second variable gain amplifying unit is preset. When the level is higher than a predetermined level, the gain of the second variable gain amplifying section is decreased corresponding to the output of the second variable gain amplifying section, and
A television receiving apparatus, wherein gain control is switched from the second variable gain amplifying section to the first variable gain amplifying section in accordance with a signal level input to the filter. 10. The frequency conversion section according to claim 9, wherein the first frequency conversion section converts the output of the first variable gain amplification section into a first intermediate frequency, and the first frequency conversion section converts the output. A second converted frequency signal to a second intermediate frequency
A television signal receiving device comprising: a frequency converter. 11. A terminal for inputting a signal to be received, a first variable gain amplifier for amplifying or attenuating a signal input to the input terminal to a set level, and the first variable gain. A filter that selectively passes a signal in a predetermined band among signals output from the amplification unit; a second variable gain amplification unit that amplifies or attenuates the signal that has passed through the filter to a set level; A demodulator that extracts only the signal to be received from the output of the second variable gain amplifier and demodulates it; a gain switching unit that switches the gain control of the first and second variable gain amplification units; A control unit that controls switching of the gain switching unit according to an output, and the gain switching unit changes a switching point of gain control from the second variable gain amplification unit to the first variable gain amplification unit according to the output of the control unit. A television signal receiving device characterized by being converted. 12. The demodulator according to claim 10, wherein the demodulator has a signal output terminal for outputting a signal indicating whether the received signal meets the demodulation format, and the control section is supplied with the signal at the output terminal. A microcomputer and a converter for converting a digital signal from the microcomputer into an analog signal and generating a sawtooth wave when the signal from the signal output terminal does not match the format, and by the output of this converter, A television signal receiving device, characterized in that the gain switching unit is controlled.