JPH0136365Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention relates to a television tuner for wideband reception that can also receive CATV bands.

Traditionally, general television tuners have been designed to receive radio waves from the antenna, that is, standard broadcasts, but recently in the United States and other countries, there has been a demand for television tuners that can also receive CATV bands at the same time.

<Prior Art> First, FIG. 1 shows an example of a VHF receiving section of a conventionally commonly used television tuner.

In FIG. 1, 1 is a VHF RF amplifier circuit, 2 is a VHF local oscillation circuit, and 3 is a VHF mixing circuit.

The operation of each of these circuits is well known and will not be discussed in detail here, but briefly describes the RF
In the amplifier circuit 1, Tr ₁ is a dual-gate MOS transistor for RF amplification, D ₁ and D ₂ are variable capacitance diodes for tuning, D ₃ to _{D 5} are band switching diodes, and L ₁ and L ₂ are low-band reception coils. , L ₃ ,
L ₄ is a high-band receiving coil, C ₁ and C ₂ are bypass capacitors, C ₃ and C ₄ are DC blocking capacitors, and C ₅ is a coupling capacitor. First, the antenna input signal from the antenna circuit is It is applied to the gate electrode G ₁ of the transistor Tr ₁ through the input circuit. Amplified by this transistor Tr ₁
The RF signal is supplied to the next mixing circuit 3 through a double-tuned circuit consisting of variable capacitance diodes D ₁ and D ₂ and a resonant circuit of each band receiving coil L ₁ to L ₄ , and further via a coupling capacitor C ₅ . be done.

At this moment, during low band reception, the low band reception power supply B _L is applied to the RF amplifier circuit 1,
At this time, the band switching diode _D5 is turned on and the other diodes _D3 and _D4 are cut off.
Therefore, during low band reception, the resonant circuit is equivalently constructed as shown in FIG. 2a, and a double-tuned circuit is formed by the variable capacitance diodes D ₁ and D ₂ and the coils L ₁ to L ₄ . Also, when receiving high band,
A high-band reception power source _BH is applied to the RF amplifier circuit 1, and at this time, diodes _D3 and _D4 are turned on and diode _D5 is cut off.In this case, the resonant circuit is equivalently shown in Figure 2b. It is composed like this.

In this way, in the above circuit, each band switching diode D ₃ to _{D 5} is turned on or off according to the band switching, and thereby the band receiving coil L ₁ to _{L 4} is turned on or off.
is switched appropriately, and at this time, in the RF signal supplied from the input circuit, especially each variable capacitance diode D ₁ ,
An RF signal of a desired channel corresponding to the tuning voltage V _T applied to D ₂ is selectively derived and applied to the mixing circuit 3 at the next stage. Here, in the mixing circuit 3, two transistors Tr ₂ and Tr ₃ are connected in a cascade type, and the above RF signal and the local oscillation signal obtained from the local oscillation circuit 2 via the coupling capacitor C ₆ are mixed, and the output terminal 4 A predetermined IF signal is derived. In addition, in the equivalent circuit shown in Fig. 2,
Cie is the input capacitance of the mixing transistor Tr ₂ , and Gie is the input conductance thereof.

However, when trying to receive wideband frequencies including the CATV band with the conventional TV tuner shown in Fig. 1 above, the MOS transistor Tr ₁
The output capacitance C _OS or output conductance G _OS is 50 ~
It changes more than twice between the 470MHz reception frequency,
In addition, since the Q value of variable capacitance diodes D ₁ and D ₂ similarly changes depending on the reception frequency and tuning voltage V _T ,
The primary side load Q _L1 of the double-tuned circuit changes significantly.
On the other hand, since the input impedances Cie and Gie of the mixing transistor Tr ₂ change from 200Ω to 40Ω,
The secondary side load Q _L2 of the double-tuned circuit also varies greatly between each band, and even within the same band, it varies depending on the receiving frequency. As a result, the voltage transmission ratio to the mixing circuit 3 changes depending on the reception frequency, so even within the same band, mismatches occur depending on the reception frequency, the bandwidth changes depending on the reception frequency, and the power gain and noise figure change. Characteristics were changing.

In other words, in the television tuner shown in Fig. 1, a coupling capacitor _C5 of as small a value as possible is normally used in order to obtain a large change in reception frequency due to the tuning voltage _VT , but even in this case, the reception channel within the same band is That is, as the receiving frequency becomes lower, the coupling impedance increases and the power gain decreases.

Therefore, a wideband reception television tuner as shown in FIG. 3 has been developed in order to compensate for characteristic changes due to reception frequency within the same band.

In the conventional circuit shown in FIG. 3, DC blocking capacitors C ₄ and C ₇ are connected in parallel to the coupling capacitor C ₅ of the interstage coupling circuit that couples the RF amplifier circuit 1 and the mixing circuit 3, respectively. A variable capacitance diode D ₆ for auxiliary coupling is inserted, the anode of this diode D ₆ is grounded via a resistor R ₁ , and the same tuning voltage V _T applied to the tuning variable capacitance diode D ₂ is connected to its cathode. By applying a tuning voltage, the coupling impedance between the RF amplifier circuit 1 and the mixing circuit 3 is varied according to changes in the receiving frequency.

That is, in this case, the tuning variable capacitance diode D ₁ ,
As the tuning voltage V _T applied to D ₂ becomes smaller and the reception frequency _becomes lower, the capacitance value of the coupling variable capacitance diode _{D 6} itself becomes larger. The combined coupling capacitance increases in proportion to the receiving frequency.

As a result, according to the above interstage coupling circuit,
As the reception frequency becomes lower, the capacitance component of the variable capacitance diode _D6 acts more effectively, suppressing an increase in coupling impedance.

However, in the conventional circuit shown in FIG. 3, although it is possible to compensate for changes in coupling impedance due to changes in reception frequency within each band by adding the auxiliary coupling variable capacitance diode _D5 , Since the coupling impedance of the interstage coupling circuit changes widely between high band reception and low band reception, it has not been possible to compensate for the change in coupling impedance between each band.

<Purpose of the invention> The present invention has been devised in view of these points, and provides a television tuner for broadband reception that can compensate for changes in coupling impedance even when switching bands.

<Embodiments> The present invention will be described below with reference to embodiments shown in the drawings. FIG. 4 shows a circuit diagram of an embodiment of the present invention,
Here, the same parts as in FIG. 1 are given the same reference numerals.

That is, here, in addition to the conventional circuit shown in FIG. 1, an auxiliary coupling capacitor _C8 is additionally inserted between the cathode of the band switching diode _D4 and the base of the mixing transistor _Tr2 .

Therefore, in the above circuit, during low band reception, the resonant circuit and interstage coupling circuit in the RF amplifier circuit ₁ are equivalently shown in FIG. It is small compared to ₄ and is almost ignored,
Therefore, in this case, the coupling capacitor C ₅
This means that the auxiliary coupling capacitor _C8 is connected in parallel with . As a result, the combined coupling capacitance of the interstage coupling circuit is the sum of both capacitors _C5 and _C8 , and has a relatively large value.

On the other hand, during high-band reception, the equivalent circuit of the resonant circuit and interstage coupling circuit in the RF amplifier circuit 1 becomes as shown in Figure 5b, and the auxiliary capacitor
One end of _C8 is grounded at high frequency via the band switching diode _D4 , and the parallel connection state to the coupling capacitor _C5 is released. In this case, the coupling capacitance of the interstage coupling circuit is determined only by the coupling capacitor _C5 , and the auxiliary coupling capacitor _C8 does not function as a coupling capacitance.Therefore, the coupling capacitance in this case is smaller than that for low-band reception mentioned above. Become.

In this way, in the interstage coupling circuit of the above embodiment, the effective coupling capacitance is changed in response to band switching, and as a result, even if the reception frequency changes widely due to band switching, the coupling impedance does not change much. become.

In the above embodiment, the case where the double tuning circuit of the RF amplifier circuit 1 is switched to two stages of VHF low band and high band for reception has been described, but the present invention is of course applicable to the case where the double tuning circuit of the RF amplifier circuit 1 is switched to three or more stages for reception. Of course, it can also be applied.

Furthermore, an auxiliary coupling capacitor according to the present invention
It goes without saying that if _C8 is inserted in the interstage coupling circuit of the conventional circuit shown in FIG. 3, the reception characteristics can be further improved.

<Effects> As described above, according to the TV tuner of the present invention,
In the interstage coupling circuit that couples the RF amplifier circuit and the mixing stage with a coupling capacitor, an auxiliary coupling capacitor is inserted in parallel with the coupling capacitor during low band reception in response to band switching, so band switching is possible. Therefore, even if the reception frequency changes widely, the value of the coupled impedance does not change that much, and there is no risk of changes in bandwidth or deterioration of characteristics such as power gain as in conventional circuits.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the VHF reception section of a conventional general TV tuner, Fig. 2 a and b are equivalent circuit diagrams of each band reception in the same TV tuner, Fig. 3 is a circuit diagram of a conventional TV tuner for wideband reception, FIG. 4 is a circuit diagram of an embodiment of a television tuner for wideband reception according to the present invention, and FIGS. 5a and 5b are equivalent circuit diagrams at the time of reception of each band in the same embodiment. 1...RF amplifier circuit, 2...Local oscillation circuit, 3
...Mixed circuit, C ₅ ...Coupling capacitor, C ₈ ...
Auxiliary coupling covalent capacitor, D ₄ ...Band switching diode.

Claims (1)

[Scope of utility model registration request] In a television tuner equipped with an interstage coupling circuit that couples an RF amplifier circuit and a mixing circuit using a coupling capacitor, the RF amplifier circuit has a band switching circuit that switches and controls a band switching circuit that includes a low band receiving coil and a high band receiving coil. an auxiliary coupling capacitor is connected between the connection point of the low band receiving coil, high band receiving coil and band switching diode on the output side and the input end of the mixing circuit, One end of the auxiliary coupling capacitor is connected to the connection point during low band reception when the band switching diode at the connection point is turned off, and one end of the auxiliary coupling capacitor is connected to the auxiliary coupling capacitor during high band reception when the band switching diode at the connection point is turned on. 1. A television tuner for broadband reception, characterized in that one end of the is grounded at a high frequency through the diode to release the parallel connection to the coupling capacitor.