JP3610282B2 - Intermediate frequency circuit for television signal transmitter. - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an intermediate frequency circuit, and more particularly to an intermediate frequency circuit suitable for a case where a plurality of types of television signals are selectively input and AGC is applied to the television signals for amplification.
[0002]
[Prior art]
On the transmission side of the CATV system, general terrestrial television signals are retransmitted, entertainment programs such as movies and sports, and original programs closely related to the local community are transmitted. These programs are transmitted to the CATV system. A transmitter is used to transmit on the cable over the channel assigned to. In the transmitter, the output level of the intermediate frequency circuit is adjusted to a predetermined level by controlling the gain of the intermediate frequency circuit by the AGC circuit.
[0003]
Hereinafter, an intermediate frequency circuit used in a conventional transmitter will be described with reference to FIG. The intermediate frequency circuit receives a television signal frequency-converted to an intermediate frequency band from the input terminal 21. The input television signal is amplified to a predetermined level by the intermediate frequency amplifier 22 and then input to the mixer 23 and also input to the AGC circuit 27. The television signal input to the mixer 23 is frequency-converted to the frequency band output from the transmitter and output from the output terminal 26. The mixer 23 receives a local oscillation signal for frequency conversion from the local oscillator 24 together with the television signal. The oscillation frequency of the local oscillator 24 is controlled by the PLL circuit 25.
[0004]
When a normal analog television signal is input to the intermediate frequency circuit, the switch 30 connects the operational amplifier 31 and the holding circuit 29. In this case, the output of the intermediate frequency amplifier 22 is input to the mixer 23 and also to the AGC circuit 27. The AGC circuit 27 is a circuit that outputs an AGC voltage in order to control the gain of the intermediate frequency amplifier 22, and includes a detection circuit 28, a holding circuit 29, and an operational amplifier 31. The television signal amplified by the intermediate frequency amplifier 22 is detected by the detection circuit 28, the peak voltage of the detected television signal is held by the holding circuit 29, and the peak voltage passes through the switch 30 and passes through the switch 30. Input to non-inverting input terminal. In the case of a normal analog television signal, since the horizontal sync signal has the largest amplitude, the level of the horizontal sync signal is continuously input to the non-inverting input terminal of the operational amplifier 31. The operational amplifier 31 amplifies the peak voltage and applies it to the intermediate frequency amplifier 22 as an AGC voltage. Since the amplification factor of the intermediate frequency amplifier 22 varies depending on the magnitude of the AGC voltage, the intermediate frequency amplifier 22 is automatically controlled so that the output level of the intermediate frequency amplifier 22 becomes a predetermined level.
[0005]
On the other hand, scrambled analog television signals and digital television signals cannot be subjected to AGC according to the level of the horizontal synchronization signal because there is no horizontal synchronization signal. Therefore, when a scrambled analog television signal or digital television signal is input to the intermediate frequency circuit, the switch 30 is switched so that the reference voltage source 32 is connected to the non-inverting input terminal of the operational amplifier 31. The gain of the intermediate frequency amplifier 22 is adjusted by connecting and manually adjusting the voltage level of the reference voltage source 32. In this case, after the adjustment, the gain of the intermediate frequency amplifier 22 does not change, so that AGC is not applied.
[0006]
[Problems to be solved by the invention]
As described above, in the conventional intermediate frequency circuit, AGC cannot be applied to a scrambled analog television signal or digital television signal, and the television signal can be kept at a predetermined level. could not.
[0007]
Therefore, the present invention applies not only a normal analog television signal but also an scrambled analog television signal and a digital television signal to AGC according to the type of the television signal, and the optimum output. The purpose is to realize an intermediate frequency circuit capable of leveling.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, an intermediate frequency circuit of the present invention includes an intermediate frequency amplifier that amplifies a plurality of television signals in an intermediate frequency band, and an AGC that outputs an AGC voltage for controlling the gain of the intermediate frequency amplifier The AGC circuit includes a detection circuit that detects a peak voltage of the television signal output from the intermediate frequency amplifier, and a holding circuit that holds the detected peak voltage, and the holding circuit The AGC voltage level is made different depending on the type of the television signal by adding a different voltage depending on the type of the television signal to the peak voltage held by the above .
[0009]
The intermediate frequency circuitry of the present invention is the AGC circuit, switches the level of the AGC voltage by adding the level of the voltage the peak voltage corresponding to the type of the inputted said television signal to said peak voltage An MPU and a memory for storing a voltage at a level corresponding to the type of the television signal are provided.
[0011]
The intermediate frequency circuitry of the present invention, the AGC circuit has a op amp which the peak voltage outputs one of input to the input terminal AGC voltage, to the other input terminal of said operational amplifier said television A plurality of levels of reference voltages were alternatively input according to the type of signal.
[0012]
In addition, the types of television signals of the intermediate frequency circuit of the present invention include two or more types of normal analog television signals, scrambled analog television signals, or digital television signals. .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the intermediate frequency circuit of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram of an intermediate frequency circuit according to the present invention, which is used for a transmitter of a CATV system. The intermediate frequency circuit receives from the input terminal 1 a television signal frequency-converted to the intermediate frequency band. The input television signal is amplified by the intermediate frequency amplifier 2 and then input to the mixer 3 and also input to the AGC circuit 7. The television signal input to the mixer 3 is frequency-converted to a predetermined frequency band and output from the output terminal 6. The mixer 3 receives a local oscillation signal for frequency conversion from the local oscillator 4 together with the television signal. The oscillation frequency of the local oscillator 4 is controlled by the PLL circuit 5.
[0014]
The output of the intermediate frequency amplifier 2 is input to the mixer 3 and also to the AGC circuit 7. The AGC circuit 7 is a circuit that outputs an AGC voltage for controlling the gain of the intermediate frequency amplifier 2, and includes a detection circuit 8, a holding circuit 9, an A / D converter 10, an MPU 11, and a non-volatile circuit. A memory (PROM) 12, a D / A converter 13, and an operational amplifier 14 are included. The television signal amplified by the intermediate frequency amplifier 2 is detected by the detection circuit 8, and the peak voltage of the detected television signal is held by the holding circuit 9. The peak voltage held by the holding circuit 9 is converted into a digital signal by the A / D converter 10 and input to the MPU 11. The MPU 11 is connected to the memory 12. The memory 12 stores the voltage level to be added to the peak voltage according to the type of the television signal as shown in FIG. 2, and the MPU 11 sets the level stored in the memory 12 to the peak voltage level. Addition and output to the D / A converter 13. Note that the memory 12 has a free space, and when a new modulation system appears in the future, it can be easily handled by simply adding a new level difference and storing it.
[0015]
Here, when the normal analog television signal shown in FIG. 3 is inputted to the intermediate frequency circuit, a signal indicating that the normal analog television signal is inputted by manually switching a switch (not shown). It transmits to MPU11. Then, the memory 12 outputs 0 [dB] to the MPU 11 as a level to be added to the peak voltage level. Then, the MPU 11 outputs the level of the input peak voltage as it is from the MPU 11. In this case, the horizontal synchronizing signal 19 has a peak voltage.
[0016]
Further, in the scrambled analog television signal shown in FIG. 4, there is no horizontal synchronization signal, and the pedestal level 20 becomes the peak voltage. In the intermediate frequency circuit of this embodiment, the level difference between the horizontal synchronizing signal and the pedestal level in a normal analog television signal is 2.5 [dB], so that the intermediate voltage circuit increases so that the peak voltage increases accordingly. It is necessary to control the gain of the frequency amplifier 2. Therefore, when the scrambled analog television signal is input to the intermediate frequency circuit, a signal indicating that the scrambled analog television signal is input by manually switching a switch (not shown). Is transmitted to the MPU 11. Then, the memory 12 outputs 2.5 [dB] to the MPU 11 as a level to be added to the peak voltage level. Then, the MPU 11 adds 2.5 [dB] to the level of the input peak voltage and then outputs it from the MPU 11.
[0017]
Further, although the amplitude is constant in the digital television signal shown in FIG. 5, since the digital television signal is in a signal degradation state due to overmodulation, the gain of the intermediate frequency amplifier 2 needs to be reduced. In the intermediate frequency circuit of this embodiment, it is necessary to lower the amplitude level of the digital television signal by 3.0 [dB] than the level of the horizontal synchronizing signal in the normal analog television signal. Therefore, when a digital television signal is input to the intermediate frequency circuit, a switch (not shown) is manually switched to transmit a signal indicating that a digital television signal is input to the MPU 11. Then, the memory 12 outputs 3.0 [dB] to the MPU 11 as a level to be added to the peak voltage level. Then, the MPU 11 adds 3.0 [dB] to the peak voltage level, and then outputs it from the MPU 11.
[0018]
The D / A converter 13 converts the digital signal output from the MPU 11 into an analog signal and inputs the analog signal to the non-inverting input terminal of the operational amplifier 14. The operational amplifier 14 amplifies the input voltage and applies it to the intermediate frequency amplifier 2 as an AGC voltage. As the AGC voltage level increases, the gain of the intermediate frequency amplifier 2 decreases, so that the output level of the intermediate frequency amplifier 2 is maintained at a constant level.
[0019]
FIG. 6 is a circuit diagram showing another embodiment of the present invention. This different embodiment is different from the embodiment shown in FIG. 1 in that there is no MPU and the reference voltage input to the non-inverting input terminal of the operational amplifier 14 is switched. .
[0020]
In another embodiment shown in FIG. 6, the intermediate frequency amplifier 2, the mixer 3, the local oscillator 4, the detection circuit 8, and the holding circuit 9 are the same as those in the embodiment, and thus redundant description is omitted. To do.
[0021]
An inverting input terminal of the operational amplifier 14 is alternatively connected to the first reference voltage source 16, the second reference voltage source 17, or the third reference voltage source 18 via the switch 15. .
[0022]
When a normal analog television signal is input to the intermediate frequency circuit, the switch 15 is switched so that the operational amplifier 14 and the first reference voltage source 16 are connected. A voltage of 3 [V] is output from the first reference voltage source 16. Therefore, a voltage proportional to the difference between the level of the horizontal synchronization signal 19 and the level of the first reference voltage source 16 is output from the operational amplifier 14 as an AGC voltage.
[0023]
When the scrambled analog television signal is input to the intermediate frequency circuit, the switch 15 is switched so that the operational amplifier 14 and the second reference voltage source 17 are connected. When a scrambled analog television signal is input to the intermediate frequency circuit of this embodiment, the peak voltage level needs to be 2.5 [dB] lower than that of a normal analog television signal. Therefore, the second reference voltage source 17 outputs a voltage that lowers by 0.5 [dB]. A voltage proportional to the difference between the pedestal level 20 and the level of the second reference voltage source 17 is output from the operational amplifier 14 as an AGC voltage.
[0024]
When a digital signal is input to the intermediate frequency circuit, the switch 15 is switched so that the operational amplifier 14 and the second reference voltage source 17 are connected. When a digital television signal is input to the intermediate frequency circuit of this embodiment, the level of the peak voltage needs to be 3.0 [dB] lower than that of a normal analog television signal. The second reference voltage source 17 is configured to output a voltage that lowers 0 [dB]. A voltage proportional to the difference between the amplitude level of the digital television signal and the level of the second reference voltage source 17 is output as an AGC voltage from the operational amplifier.
[0025]
【The invention's effect】
As described above, the intermediate frequency circuit of the present invention includes the intermediate frequency amplifier that amplifies a plurality of television signals in the intermediate frequency band, and the AGC circuit that outputs the AGC voltage for controlling the gain of the intermediate frequency amplifier. The AGC circuit has a detection circuit for detecting the peak voltage of the television signal output from the intermediate frequency amplifier, and a holding circuit for holding the detected peak voltage, and the television set has a peak voltage held by the holding circuit. Since different voltages are added according to the signal type and the AGC voltage level is made different according to the type of television signal, the output level of the intermediate frequency amplifier is kept at a predetermined value for each type of television signal. Can do.
[0026]
The intermediate frequency circuitry of the present invention, the peak voltage MPU, by adding the level corresponding to the type of television signal, since switching the AGC voltage, the appropriate size according to the type of the television signal It is possible to apply an AGC voltage to the intermediate frequency amplifier.
[0027]
The intermediate frequency circuitry of the present invention, AGC circuit has an operational amplifier the peak voltage outputs one of input to the input terminal AGC voltage, the other input terminal of the operational amplifier according to the type of the television signal Since a plurality of levels of reference voltages are alternatively input, it is easy to switch the AGC voltage.
[0028]
The intermediate frequency circuitry of the present invention, the type of television signal, including conventional analog television signal, or the analog television signal that is scrambled, or two or more of the digital television signal Therefore, the level of these television signals output from the intermediate frequency amplifier can be kept at a predetermined value.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of an intermediate frequency circuit of the present invention.
FIG. 2 is a table showing contents stored in a memory of the present invention.
FIG. 3 is a graph showing a normal analog television signal.
FIG. 4 is a graph showing a scrambled analog television signal.
FIG. 5 is a graph showing a digital television signal.
FIG. 6 is a circuit diagram showing another embodiment of the intermediate frequency circuit of the present invention.
FIG. 7 is a circuit diagram showing a conventional intermediate frequency circuit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Input terminal 2 Intermediate frequency amplifier 3 Mixer 4 Local oscillator 5 PLL circuit 6 Output terminal 7 AGC circuit 8 Detection circuit 9 Holding circuit 10 A / D converter 11 MPU
12 memory 13 D / A converter 14 operational amplifier 15 switch 16 first reference voltage source 17 second reference voltage source 18 third reference voltage source 19 horizontal synchronization signal 20 pedestal level

Claims (4)

An intermediate frequency amplifier for amplifying a plurality of television signals in the intermediate frequency band; and an AGC circuit for outputting an AGC voltage for controlling a gain of the intermediate frequency amplifier, wherein the AGC circuit is output from the intermediate frequency amplifier. A detection circuit for detecting the peak voltage of the television signal , and a holding circuit for holding the detected peak voltage, and the peak voltage held by the holding circuit depends on the type of the television signal. An intermediate frequency circuit of a television signal transmitter, wherein different voltages are added to change the level of the AGC voltage according to the type of the television signal . The AGC circuit receives the peak voltage, adds a voltage of a level corresponding to the type of the television signal to the peak voltage, and switches the level of the AGC voltage. According to the type of the television signal 2. An intermediate frequency circuit for a television signal transmitter according to claim 1, further comprising a memory for storing a voltage of a different level. The AGC circuit includes an operational amplifier that outputs the AGC voltage when the peak voltage is input to one input terminal, and the other input terminal of the operational amplifier has a plurality of levels of reference voltages according to the type of the television signal. 2. An intermediate frequency circuit for a television signal transmitter according to claim 1, wherein: 4. The types of television signals include two or more types of normal analog television signals, scrambled analog television signals, or digital television signals. An intermediate frequency circuit of the television signal transmitter according to any one of the above.

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