JPH06252706A - Input tuning circuit for tv tuner - Google Patents

Abstract

PURPOSE:To improve image interference by forming an optimum image trap for each of channels receiving a low band and a high band. CONSTITUTION:In the reception of a high band, diodes 4,5 are conductive, and a series connection circuit comprising a tuning coil 8 and a varactor diode 10 and a combined capacitance of capacitors 6,7 provide parallel resonance to form an image trap. Moreover, in the reception of a low band, the diodes 4,5 are nonconductive, and a series connection circuit comprising tuning coils 3, 8 and the varactor diode 10 and the capacitance of the capacitor 6 provide parallel resonance to form an image trap. Then a channel selection voltage is applied to a varactor diode 13 to implement channel selection to shift a trap frequency formed by a parallel resonance circuit with a capacitance of the varactor diode 10 for each reception channel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input tuning circuit of a TV tuner which has a switching between high band and low band and is provided with an image trap as a countermeasure against reception interference by an image frequency signal.

[0002]

2. Description of the Related Art In a superheterodyne receiver, a signal at a frequency higher than the local oscillation frequency by an intermediate frequency (this frequency is called an image frequency) is
Since it is converted to the same intermediate frequency as the reception frequency by the frequency mixing circuit, it causes reception interference (this reception interference is called image interference). Therefore, a trap circuit is provided in the input tuning circuit of the TV tuner to remove the image frequency signal to prevent image interference. For example,
An input tuning circuit of a conventional TV tuner for receiving Japanese channels is composed of a circuit as shown in FIG.

In FIG. 4, reference numeral 1a is an antenna, 1 is an antenna filter, 2 is a matching coil for matching the antenna filter 1 with a subsequent input tuning circuit, and 8 and 17 are tuning coils with high inductance and small inductance. And
Reference numerals 3 and 15 are tuning coils of large inductance for low band, and 22 is a capacitor for low band image trap. Reference numerals 4 and 5 are switching diodes, which are turned on when the output voltage of the band selection power supply 21 is applied to VH during high band reception and turned off when the output voltage of the band selection power supply 21 is applied to VL during low band reception. Is configured to be.

Next, 7 is a capacitor for a high band image trap, 13 is a tuning variable capacitance diode, and 10 is a tuning variable capacitance diode.
Is a coupling variable capacitance diode. A tuning voltage is supplied from a tuning power supply 20 to these diodes 10 and 13 via a resistor 30. Reference numeral 12 is a high frequency amplifier circuit, and its output signal is supplied from an output end 12a to a frequency mixing circuit (not shown). Also resistor 1
8, 30, 32, 33, and 34 are for bias supply and for blocking high-frequency components, and capacitors 9, 11, 1
Reference numerals 6 and 19 are large-capacity capacitors for blocking direct current. The resistor 14 is for grounding the anode of the variable capacitance diode 10, and the resistor 31 is for grounding the anode of the diodes 4, 5.

Next, the operation of the conventional input tuning circuit of the TV tuner having the above configuration will be described. First, during high band reception, the diodes 4 and 5 are turned on, the coil 3 and the capacitor 22 are short-circuited at both ends, and the series-connected coil 15 and the capacitor 16 are both short-circuited. Further, since the capacitor 16 has a large capacity value, both ends of the coil 15 are short-circuited. Since the capacitors 9 and 19 also have large capacitance values, the equivalent circuit during high band reception is as shown in FIG. That is, at the time of high band reception, the coils 8 and 17 and the variable capacitance diode 13 form a high band tuning circuit.
Then, the reception signal selected by this tuning circuit is supplied to the high frequency amplifier circuit 12 via the variable capacitance diode 10. The coil 8 and the variable-capacitance diode 10 connected in series with respect to the image signal are configured to have an inductive reactance, so that a parallel resonance circuit of this inductive circuit and the capacitor 7 is formed. It
Here, the resonant frequency of the parallel resonant circuit is the tuning power source 20.
However, in this case, it is formed as a trap that is tuned near the image frequency on the high side of the received signal, and as a result, this trap is formed as an image trap for each received channel. Interference is improved.

Next, at the time of low band reception, the diodes 4 and 5 of the circuit shown in FIG.
5, 17 and the capacitor 19 are grounded in a series connection circuit. Further, since the capacitor 19 has a large capacitance value, it is grounded by the coil 26 which is the sum of the inductances of the coils 15 and 17. Since the capacitor 9 also has a large capacity, the coil 8 and the variable capacitance diode 10 are connected in series. Then, a parallel connection circuit in which the capacitor 7 is connected in parallel is formed in this series connection circuit. Since this parallel connection circuit has the same configuration as the parallel resonance circuit forming the image trap at the time of high band reception, at the time of low band reception In, the capacitor 7 has a large reactance value and becomes a negligible value, and thus can be omitted. Therefore, the equivalent circuit is as shown in FIG. That is, the tuning circuit for low band is formed by the coils 3, 8, and 26 and the variable capacitance diode 13 during low band reception. Then, the reception signal selected by this tuning circuit is supplied to the high frequency amplifier circuit 12 via the variable capacitance diode 10. Further, the resonance frequency of the parallel resonance circuit of the capacitor 22 and the coil 3 is formed as a fixed image trap that is tuned to the image frequency on the high side of the received signal.

The image trap frequency is, for example, 9 in the low band in the tuner capable of receiving the CATV channel of the Japanese channel, and the low band in the low band is higher than that in the general TV broadcast receiving tuner.
Since it is widened by the channel (about 54 MHZ), the image frequency of the upper band of the low band is set so as not to disturb the frequency characteristic in the low band.

[0008]

Therefore, in such an input tuning circuit of a conventional TV tuner, image interference during low band reception is deteriorated.

The solid line graph in FIG. 7 shows the selectivity characteristic in the input tuning circuit of the conventional TV tuner. FIG. 7 (1) shows the selectivity characteristic during high band reception, and FIG. 7 (2). ) Is the selectivity characteristic in low band reception, and shows the channel selectivity characteristics of the upper and lower ends of both high band and low band. That is, the graph on the left side of FIG. 7 (1) shows M1 in the Japanese channel.
0, named CATV channel (video carrier ftu = 165.25MHZ, local oscillator frequency flo = 22
4MHZ) reception characteristics, the graph on the right shows the reception characteristics of the S16 channel (image carrier ftu = 313.25MHZ, local oscillation frequency flo = 372MHZ).
The graph on the left side of (2) shows the J1 channel (video carrier ftu = 91.25MHZ, local oscillation frequency flo = 150).
MHZ) reception characteristics, the graph on the right shows reception characteristics of M9 channel (image carrier ftu = 157.25 MHZ, local oscillation frequency flo = 216 MHZ).

As can be seen from these characteristics, in the high band, the trap frequency ft is always in the vicinity of the image frequency fim of the receiving channel, so the image frequency band is attenuated. However, in the channel J1 at the lower end of the low band, the trap frequency ft is a frequency far from the image frequency fim, so that the image frequency band cannot be sufficiently attenuated. Similarly, the graph shown by the solid line in FIG. 8 shows the image interference ratio characteristic in the input tuning circuit circuit of the conventional TV tuner of the Japanese channel. The graph on the left shows the low band full band and the graph on the right shows the high band full band. It shows the image disturbance ratio characteristic across.

As can be seen from this characteristic, in the low band, the image interference ratio characteristic is poor in the lower end channel and shows a good characteristic toward the upper end. That is, it is understood that such a tendency has been caused because the low-band frequency band is wide as described above. Therefore, the input tuning circuit of the TV tuner of the present invention is made under such a background, and it is possible to form an image trap for each channel always received by the tuning voltage in the low band as well as the high band. It is intended to provide an input tuning circuit for a tuner.

[0012]

In the input tuning circuit of the TV tuner of the present invention, a low band tuning coil whose one end is connected to the input end and a low band tuning coil connected in parallel are non-conducting during low band reception. And a switching means that is brought into a conductive state in a high band receiving state, and a high band tuning coil whose one end is connected to the other end of the low band tuning coil and the other end of which is installed via a first variable capacitance diode. A second variable capacitance diode that connects the other end of the high-band tuning coil and the input end of the high-frequency amplification circuit, the input end of the low-band tuning coil, and the input end of the high-frequency amplification circuit. A first electrostatic capacitance means, and the other end of the low-band tuning coil,
A second capacitance means for connecting the input terminal of the high-frequency amplifier circuit, and the second variable capacitance diode applies a tuning voltage to the first variable capacitance diode, thereby applying a tuning voltage to the second variable capacitance diode. It is characterized in that the trap frequency moves due to a change in capacity.

[0013]

The above technical means operates as follows. By switching the reception band, the series connection circuit in which the second variable capacitance diode is connected in series with the tuning coil and the trap capacitor are connected in parallel to form a parallel resonance circuit, so that the first variable capacitance diode is selected. By applying the station voltage and performing tuning, the trap frequency formed by the parallel circuit is moved by the capacitance of the second variable capacitance diode.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An input tuning circuit for a TV tuner according to the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of an input tuning circuit of a TV tuner of the present invention. In the figure, parts corresponding to those of the input tuning circuit of the conventional TV tuner shown in FIG. The description is omitted. First, the configuration will be described. The embodiment of the input tuning circuit of the TV tuner of the present invention is different from the example of the input tuning circuit of the conventional TV tuner shown in FIG. 4 in the following two points. First
The point is that the low-band reception tuning capacitor 22 is removed, and the second point is that an image trap capacitor 6 is newly connected to the connection point between the coils 2 and 3 and the anode side of the variable capacitance diode 10. Is.

Next, the operation of the embodiment of the input tuning circuit of the TV tuner of the present invention having the above configuration will be described. First, at the time of high band reception, both ends of the coil 3 and both ends of the coil 15 are short-circuited by turning on the diodes 4 and 5 as in the conventional case. Further, since both ends of the coil 3 are short-circuited, the capacitor 6 is connected in parallel with the capacitor 7, so that the total capacitance value thereof becomes the capacitor 24 which is the sum of the capacitors 6 and 7. Therefore, this equivalent circuit is as shown in FIG. That is, FIG. 2 differs from the equivalent circuit FIG. 5 of the input tuning circuit of the conventional TV tuner in that the capacitor forming the image trap during high band reception is the capacitor 7 in FIG. 5 and the capacitor 24 in FIG. It is a point. Then, at the time of high band reception, an image trap is formed by a parallel resonance circuit in which the capacitor 24 is connected in parallel to the series connection circuit of the coil 8 and the variable capacitance diode 10. The capacitor 24 is the sum of the capacitors 6 and 7. Therefore, among them, the capacitor 7 corrects the resonance frequency of the parallel resonance circuit so as to be at the optimum position during high band reception.

Next, during low band reception, the diodes 4 and 5 of the circuit shown in FIG.
5, 17 and the capacitor 19 are grounded in a series connection circuit. Further, since the capacitor 19 has a large capacitance value and is short-circuited at a high frequency, it is grounded by the coil 26 which is the sum of the inductances of the coils 15 and 17.

Therefore, the equivalent circuit is as shown in FIG. The circuit shown in FIG. 3 is different from the circuit shown in FIG. 6 at the time of low band reception of an input tuning circuit of a conventional TV tuner in that a series circuit of tuning coils 3 and 8 and a variable capacitance diode 10 is connected in parallel with a capacitor 6. That is the point.
That is, during low band reception, the coils 3, 8,
A tuning circuit for low band is formed by 26 and the variable capacitance diode 13. Then, the reception signal selected by the tuning circuit is supplied to the high frequency amplifier circuit 12 via the variable capacitance diode 10.

During low band reception, since the coil 3 is further connected in series to the series circuit of the coil 8 and the variable capacitance diode 10, the series resonance frequency is lowered, and the coil 3 connected in series for the image signal. ,
8 and the variable capacitance diode 10 have an inductive reactance. A parallel resonance circuit is formed in the low-frequency image frequency band by the parallel circuit of the inductive reactance circuit and the capacitor 6. Here, the resonance frequency of the parallel resonance circuit changes depending on the tuning voltage of the tuning power supply 20, but in this case, it is formed as a trap variably tuned near the image frequency on the high side of the received signal. As a result, this trap is always formed as an image trap for each received channel to improve image interference.

Here, the broken line graph in FIG. 7 shows the selectivity characteristic in the input tuning circuit of the TV tuner of the present invention. FIG. 7 (1) shows the circuit characteristics at the time of high band reception,
FIG. 7 (2) shows the selectivity characteristic during low band reception. In the conventional circuit of the present invention, it can be seen that the image trap formed by the tuning voltage changes for each receiving channel and is tuned to the image frequency, so that the image disturbance is improved.

During high band reception, as shown in FIG. 7A, the input tuning circuit of the TV tuner of the present invention is
It is possible to obtain a selectivity characteristic which is substantially the same as that of the input tuning circuit of the conventional TV tuner. In addition, in low-band reception, FIG.
As shown in (2), it can be seen that the trap frequency ft is lower than that of the conventional circuit, and even in the case of the J1 channel, the trap frequency ft is close to the image frequency fim and the selectivity characteristic is improved.

FIG. 8 is a graph showing the image interference ratio characteristic. From this graph, it is understood that in the input tuning circuit of the TV tuner of the present invention, the image interference ratio characteristic is remarkably improved particularly during reception over the entire low band.

[0022]

As described above, according to the invention of the input tuning circuit of the TV tuner, the series connection circuit in which the second variable capacitance diode is connected in series with the tuning coil by switching the reception band and the trapping capacitor. Since they are connected in parallel, the optimum trap frequency can be set for the receiving channels in the entire receiving band, and as a result, the selectivity characteristics are improved and the image interference is improved. In addition, the selectivity characteristic of the low band channel is improved accordingly, so that there is an effect that the interference of the high band channel at the time of receiving the low band channel, that is, the high-low interference characteristic is also improved.

[0023]

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of an input tuning circuit of a TV tuner of the present invention.

FIG. 2 is a circuit diagram showing a configuration of an equivalent circuit at the time of high band reception of the embodiment of the input tuning circuit of the TV tuner of the present invention.

FIG. 3 is a circuit diagram showing a configuration of an equivalent circuit at the time of low band reception of the embodiment of the input tuning circuit of the TV tuner of the present invention.

FIG. 4 is a circuit diagram showing a configuration of an input tuning circuit of a conventional TV tuner.

FIG. 5 is a circuit diagram showing a configuration of an equivalent circuit of a conventional TV tuner input tuning circuit during high-band reception.

FIG. 6 is a circuit diagram showing a configuration of an equivalent circuit of a conventional TV tuner input tuning circuit during low-band reception.

FIG. 7 is a diagram showing selectivity characteristics of the input tuning circuit of the TV tuner according to the embodiment of the present invention and the input tuning circuit of the conventional TV tuner.

FIG. 8 is a diagram showing image interference ratio characteristics in the input tuning circuit of the TV tuner according to the embodiment of the present invention and the input tuning circuit of the conventional TV tuner.

[Explanation of symbols]

1 Antenna Filter 2 Matching Coil 3, 15 Coil (Low Band Tuning Coil) 8, 17 Coil (High Band Tuning Coil) 4, 5 Diode (Switch Means) 6 Capacitor (First Capacitance Means) 7 Capacitor (First Capacitance means 2) 9, 11, 16, 19 Capacitor (for direct current blocking) 10 Variable capacitance diode for coupling (second variable capacitance diode) 12 High frequency amplifier circuit 13 Variable capacitance diode for tuning (first variable capacitance) Diode) 18, 30, 32, 33, 34 Resistor (for bias and high frequency component blocking) 20 Tuning power supply 21 Band tuning power supply

Claims (2)

[Claims] 1. A low-band tuning coil whose one end is connected to an input end, and a switch means which is connected in parallel to the low-band tuning coil and is made non-conductive during low-band reception and conductive during high-band reception. And one end is connected to the other end of the low-band tuning coil,
A high-band tuning coil whose other end is grounded via a first variable-capacitance diode; and a second variable-capacitance diode connecting the other end of the high-band tuning coil and an input end of a high-frequency amplifier circuit. A first capacitance means for connecting the input end of the low-band tuning coil and the input end of the high-frequency amplifier circuit, the second variable capacitance diode being a tuning voltage for the first variable capacitance diode. The input tuning circuit of the TV tuner, wherein the trap frequency is moved by the change in the capacitance of the second variable capacitance diode when the voltage is applied. 2. The other end of the low-band tuning coil,
2. The input tuning circuit for a TV tuner according to claim 1, further comprising a second capacitance means that is connected to an input end of the high frequency amplifier circuit.