JP3435794B2 - TV tuner - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television tuner,
In particular, it relates to the AGC. 2. Description of the Related Art A conventional television tuner will be described with reference to FIG. First, the configuration of the tuner 1 will be described. An amplitude-modulated video signal (high-frequency video signal), which is a signal received from the antenna 3, is supplied to a high-frequency amplifier 5 through an input single tuning circuit 4, amplified, and then transmitted to a frequency mixer 7 through an interstage double tuning circuit 6. Supply. The input single tuning circuit 4 and the interstage double tuning circuit 6 work together so as to exhibit a band-pass characteristic such that the gain is substantially constant for a signal having a bandwidth of 6 MHz or more around the carrier frequency. It is configured. The band pass center frequency is switched in conjunction with the switching of the oscillation frequency of the local oscillator 9 according to the channel switching. A local oscillation signal from a local oscillator 9 whose oscillation frequency has been switched according to the channel switching is supplied to a frequency mixer 7, and an intermediate frequency amplitude-modulated video signal obtained from the frequency mixer 7 is converted into an intermediate frequency. Through the amplifier 8,
It is supplied to the intermediate frequency amplifier 10 of the video intermediate frequency circuit 2. The frequency mixer 7 and the local oscillator 9 constitute a frequency converter. Next, the configuration of the video intermediate frequency circuit 2 will be described. The intermediate frequency signal from the intermediate frequency amplifier 10 is supplied to an amplitude detector (AM detector) 11 for amplitude detection. The obtained video signal is supplied from an output terminal 15 to a video circuit (not shown). AGC for intermediate frequency amplifier
The signal is supplied to a signal generation circuit (IF-AGC signal generation circuit) 12. The signal from the IF-AGC signal generation circuit 12
The C signal is supplied to the intermediate frequency amplifier 10 to control the gain of the IF signal and to output the IF-AGC signal
2 is supplied to an AGC signal generation circuit for high frequency amplifier (RF-AGC signal generation circuit) 13. The AGC signal from the RF-AGC signal generation circuit 13 is supplied to the high frequency amplifier 5, and the gain is controlled.
The operation start input electric field of the AGC signal supplied to the high frequency amplifier 5 is adjusted by a potentiometer 14 connected between the power supply + B and the ground. FIG. 2 shows the relationship between the AGC attenuation amount dB and the noise figure dB of the tuner for the high frequency amplifier 5 of the conventional television tuner by a broken line. The characteristic that the noise figure dB of the tuner increases as the AGC attenuation increases. FIG. 3 shows the relationship between the input electric field dBm of such a conventional television tuner and the S / N dB of the video signal by a broken line. As the input electric field of the tuner increases, the S / N of the video signal linearly increases, and the input electric field of the tuner starts, for example, from -50 dB. An increase in signal S / N is suppressed. [0008] Recently, with the progress of multi-channel, strict characteristics against cross-modulation interference, inter-modulation interference and the like are required of television tuners. At the same time, the bandwidth of the video signal is also widened, so that the S / N
Therefore, a TV tuner is required to reduce noise. Since the intermodulation occurs at the frequency mixer, the operation start point of the AGC for the high frequency amplifier should be earlier, that is, the input electric field at which the AGC starts operation should be lowered. S / N deteriorates. As described above, since the improvement of the interference characteristics of the intermodulation and the intermodulation and the improvement of the S / N of the video signal are contradictory requirements, conventionally, a compromise between the interference characteristics and the S / N of the video signal is found. The start point of the AGC operation for the high-frequency amplifier was adjusted. In view of the above, the present invention is to propose a television tuner in which the noise figure of the tuner with respect to the AGC attenuation is less deteriorated than the conventional example. Another object of the present invention is to propose a television receiver capable of increasing the S / N of a video signal as compared with the conventional example. Further, the present invention is to propose a television tuner capable of improving the interference characteristics of intermodulation interference and intermodulation interference as compared with the conventional example. The present invention relates to a high frequency amplifier 5 to which AGC is applied, a frequency mixer 7 to which a modulated video signal from the high frequency amplifier 5 is supplied, and a connection to the high frequency amplifier 5. And a local oscillator 9 for supplying a local oscillation signal to the frequency mixer 7 in the television tuner having the selected signal selecting tuning circuits 4 and 6 .
Detector 11 for detecting video signal by detecting intermediate frequency signal
And the video signal from the detector 11 is supplied to the AGC
First AGC signal generating circuits 12 and 13 for forming signals
And A from the first AGC signal generation circuits 12 and 13.
Based on the GC signal, the input electric field becomes higher than the first level.
When this occurs, a first AGC signal is generated, and the input electric field is
When the second level is lower than the first level
A second AGC signal generation circuit for generating a second AGC signal
16 for supplying the first AGC signal to the high-frequency amplifier 5.
And a second AGC signal supplied to the frequency mixer 7 . According to the present invention, the high-frequency amplifier 5, the frequency mixer 7 to which the modulated video signal from the high-frequency amplifier 5 is supplied, and the high-frequency amplifier 5 In a television tuner having a connected signal selection tuning circuit and a local oscillator 9 for supplying a local oscillation signal to the frequency mixer 7, the AGC is applied to the high frequency amplifier 5 and the frequency mixer 7
Apply GC. Referring to FIG. 1, a television tuner according to an embodiment will be described. In FIG. 1, parts corresponding to FIG.
The description is given with the same reference numerals. This embodiment differs from the conventional example of FIG. 4 in that the AGC signal from the high-frequency amplifier AGC signal generation circuit 13 in the video intermediate frequency circuit 2 is converted to an AGC control circuit (AGC signal generation circuit) of the tuner 1. 16. The high frequency amplifier AGC signal from the AGC control circuit 16 is supplied to the high frequency amplifier 5, and the frequency mixer AGC signal from the AGC control circuit 16 is supplied to the frequency mixer 7. In this case, the starting point of the AGC operation of the high-frequency amplifier 5 is determined as shown in FIG.
It is set at 50 dBm, and the starting point of the AGC operation of the frequency mixer (having the function of an amplifier and capable of varying its gain) 7 is set to a predetermined value at which the input electric field is lower than -50 dBm. In this case, as compared with the conventional example, the AM detector 1
The S / N of the video signal obtained from 1 increases. The start point of the AGC operation of the high-frequency amplifier 5 is set at a position where the input electric field is, for example, a predetermined value lower than -50 dBm, and a frequency mixer (having the function of an amplifier and having a variable gain is provided). 7) The start point of the AGC operation is set to a predetermined value equal to or lower than the predetermined value. In this case, the S / N of the video signal obtained from the AM detector 11 is substantially equal to that of the conventional example, but the interference characteristics of cross-modulation interference and intermodulation interference can be improved as compared with the conventional example. Next, other configurations will be described. First, the configuration of the tuner 1 will be described. An amplitude-modulated video signal (high-frequency video signal), which is a signal received from the antenna 3, is supplied to a high-frequency amplifier 5 through an input single tuning circuit 4 and amplified, and then transmitted to a frequency mixer 7 through an interstage double tuning circuit 6. Supply. The input single tuning circuit 4 and the interstage double tuning circuit 6 cooperate to exhibit a band-pass characteristic such that the gain is substantially constant for a bandwidth signal of 6 MHz or more around the carrier frequency. Have been. The band-pass center frequency is switched in conjunction with the switching of the oscillation frequency of the local oscillator 9 according to the channel switching. The local oscillation signal from the local oscillator 9 whose oscillation frequency is switched according to the channel switching is supplied to the frequency mixer 7, and the intermediate frequency amplitude-modulated video signal obtained from the frequency mixer 7 is converted into the intermediate frequency Through the amplifier 8,
It is supplied to the intermediate frequency amplifier 10 of the video intermediate frequency circuit 2. The frequency mixer 7 and the local oscillator 9 constitute a frequency converter. Next, the configuration of the video intermediate frequency circuit 2 will be described. The intermediate frequency signal from the intermediate frequency amplifier 10 is supplied to an amplitude detector (AM detector) 11 for amplitude detection. The obtained video signal is supplied from an output terminal 15 to a video circuit (not shown). AGC for intermediate frequency amplifier
The signal is supplied to a signal generation circuit (IF-AGC signal generation circuit) 12. AG from IF-AGC signal generation circuit 12
The C signal is supplied to the intermediate frequency amplifier 10 to control the gain of the signal and to output the IF-AGC signal to the IF-AGC signal generation circuit 1.
2 is supplied to an AGC signal generation circuit for high frequency amplifier (RF-AGC signal generation circuit) 13. As described above, the AGC signal from the RF-AGC signal generation circuit 13 is transmitted to the AGC control circuit (AGC signal generation circuit) 1
6. The operation start input electric field of the AGC signal supplied to the high frequency amplifier 5 is adjusted by a potentiometer 14 connected between the power supply + B and the ground. FIG. 2 shows the relationship between the AGC attenuation amount dB and the noise figure dB of the tuner for the high frequency amplifier 5 of the television tuner of this embodiment by a solid line. The characteristic that the noise figure dB of the tuner increases as the AGC attenuation increases. As can be seen from FIG. 2, the decrease in the noise figure of the tuner of the embodiment is smaller than that of the conventional example. This is not only high frequency amplifier 5,
This is because AGC is applied to the frequency mixer 7. FIG. 3 shows the relationship between the input electric field dBm of the television tuner of this embodiment and the S / N dB of the video signal by broken lines. As the input electric field of the tuner increases, the S / N of the video signal linearly increases, and the input electric field of the tuner starts, for example, from -50 dB. An increase in signal S / N is suppressed. As is clear from FIG. 3, the S / N of the video signal of the embodiment is better than the S / N of the video signal of the conventional example. Conversely, when the S / N of the video signal of the embodiment is made substantially the same as the S / N of the video signal of the conventional example, the AGC
Can be started from a place where the input electric field is lower, so that the interference characteristics of cross-modulation interference and inter-modulation interference can be improved. According to te Rebichuna embodiment, AGC
, A frequency mixer 7 to which the modulated video signal from the high-frequency amplifier 5 is supplied, and signal selection tuning circuits 4 and 6 connected to the high-frequency amplifier 5.
(Input single tuning circuit 4 provided in front of high-frequency amplifier 5)
And a local oscillator 9 that supplies a local oscillation signal to the frequency mixer 7. More intermediate frequency signal
A detector 11 for detecting and obtaining a video signal, and the detector 11
Video signal is supplied to form an AGC signal.
AGC signal generation circuits 12 and 13 and the first AGC signal
Based on the AGC signals from the signal generation circuits 12 and 13,
A first AGC when an input electric field becomes higher than a first level;
Generate a signal whose input electric field is below its first level.
A second AGC signal is issued when the
And a second AGC signal generating circuit 16 for generating
The AGC signal is supplied to the high frequency amplifier 5 and the second AGC signal
Signal to the frequency mixer 7,
The input electric field at which the AGC to the amplifier 5 starts operating is:
AGC is applied to the high frequency amplifier 5, but A is applied to the frequency mixer 7.
Even if it is the same as the case without GC,
Therefore, the S / N of the video signal can be increased,
The S / N of the video signal and AGC to the high frequency amplifier 5
As in the case where AGC is not applied to the frequency mixer 7
When compared to the high-frequency amplifier 5,
The input electric field at which the AGC operation starts can be set low.
Therefore, the interference characteristics of intermodulation interference and intermodulation interference
Can be improved. According to the present invention, a high frequency amplifier to which AGC is applied, a frequency mixer to which a modulated video signal from the high frequency amplifier is supplied, In a television tuner having a signal selection tuning circuit connected to an amplifier and a local oscillator for supplying a local oscillation signal to a frequency mixer, an intermediate frequency signal from the frequency mixer is detected and a video signal is detected.
A detector that obtains the signal and the video signal from the detector are supplied
AGC signal generating circuit for generating AGC signal
And the AGC signal from the first AGC signal generation circuit
When the input electric field is higher than the first level,
1 AGC signal, and the input electric field is applied to the first signal.
The second AG when it goes above the second level below the bell
A second AGC signal generation circuit for generating a C signal;
A first AGC signal is supplied to a high-frequency amplifier, and a second AGC signal is supplied to the high-frequency amplifier.
The C signal is supplied to the frequency mixer.
The input electric field at which the AGC for the wave amplifier starts operating is
AGC is applied to the high frequency amplifier, but AGC is applied to the frequency mixer.
Even if it is the same as when no
The S / N of the image signal can be increased, and conversely,
AGC is applied to the S / N of the signal,
When the same as when the AGC is not applied to the number mixer,
The AGC operation for the high-frequency amplifier is
Since the starting input electric field can be set lower,
It is possible to improve the interference and intermodulation interference characteristics.
Wear. [0034]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a television tuner according to the present invention. FIG. 2 is a characteristic curve diagram showing a relationship between AGC and noise figure of the tuner according to the embodiment and a conventional example. Characteristic curves showing the relationship between the input electric field of the tuner and the S / N of the video signal according to the embodiment and the conventional example. FIG. 4 is a block diagram showing the conventional example of a television tuner. Circuit 3 Antenna 4 Input single tuning circuit 5 High frequency amplifier 6 Interstage double tuning circuit 7 Frequency mixer 8 Intermediate frequency amplifier 9 Local oscillator 10 Intermediate frequency amplifier 11 AM detector 12 Intermediate frequency amplifier AGC signal generation circuit 13 High frequency amplifier AGC Signal generation circuit 14 AGC1 start point adjustment potentiometer 15 Video signal output terminal

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/38-5/62 H04B 1/16 H04B 1/26

Claims (1)

(57) Claims 1. A high-frequency amplifier to which AGC is applied, a frequency mixer to which a modulated video signal from the high-frequency amplifier is supplied, and a signal selector connected to the high-frequency amplifier In a television tuner having a tuning circuit and a local oscillator for supplying a local oscillation signal to the frequency mixer, an intermediate frequency signal from the frequency mixer is detected and a video signal is detected.
A detector for obtaining a signal and an image signal supplied from the detector to form an AGC signal
A first AGC signal generating circuit for performing the operation, and an AGC signal from the first AGC signal generating circuit.
Therefore, when the input electric field becomes higher than the first level, the first A
Generate a GC signal and the input electric field is lower than the first level.
A second AGC signal is issued when the
A second AGC signal generating circuit for generating the AGC signal, supplying the first AGC signal to the high-frequency amplifier,
A television tuner, wherein the second AGC signal is supplied to the frequency mixer .