JPS6030225A - High frequency circuit of television tuner - Google Patents

Abstract

PURPOSE:To obtain an excellent standing wave ratio over a wide band through simple circuit constitution by providing an input resistance which is much larger than the input terminal at an input terminal for a high frequency signal and decreases gradually according to an increase in frequency. CONSTITUTION:When a desired reception frequency is as high as the upper-limit frequency of the reception frequency band of the high frequency circuit, the capacity CV of a variable capacitor is much smaller than the capacity C1 of a capacitor C1, which is ignored. Then when the desired reception frequency is equal to the lower-limit frequency of the reception frequency band, the capacity CV increases almost to the capacity C1. In this case, the input impedance of an MOSFETQ becomes about {C1/(C1+C4)} times at a point P by the capacity division of the capacitors C1 and C4 and is sufficiently low. For the purpose, an impedance matching frequency f0 is set to the upper-limit frequency fH of the reception frequency band fL-fH to obtain the excellent standing wave ratio between fL and fH.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a high frequency circuit for a television tuner.

BACKGROUND ART AND PROBLEMS First, the high frequency circuit of a conventional television tuner will be explained with reference to FIG. t is an input terminal (antenna terminal) to which a high frequency signal is supplied. An input terminal t is connected to one gate of a double dart type MO8 field effect transistor Q through a first coil L and a DC blocking capacitor C2. The midpoint of the connection between the coil L1 and the capacitor C2 is grounded through the second coil L2. Furthermore, coil L
2 is a variable capacitor (variable capacitor diode) C
A series circuit of V and a fixed capacitor C1 are connected in parallel. A midpoint of the connection between the coil L1 and the capacitor C2 is connected through a resistor Rc to an input terminal T1 to which a capacitance control DC voltage is supplied to the variable capacitor Cv. Note that the other dart of the MOS field effect transistor is connected to the AGC signal input terminal T2.

Incidentally, the inductances of the first and second coils L1 and L2 are represented by the same symbols, , L2, and the capacitor Cv
.

The capacity of C1 is also expressed by the same symbol Cv.

Therefore, such a high-frequency circuit includes an impedance (resistance) of the input terminal t when looking to the left in the diagram, a matching circuit for matching between the input impedance Z1 of the MO8 field effect transistor Q, and an input tuning circuit. It also serves as Let R be the impedance (resistance) when looking at the right side of the input terminal t.

Next, an equivalent circuit of the high frequency circuit shown in FIG. 1 is shown in FIG. 2, and will be explained. In this equivalent circuit, MO8f
The input tempedance Z1 of the fi field effect transistor Q is represented by a parallel circuit of an input resistance Ri and an input zero #ct. Further, the series capacitance of the variable capacitor Cv and fixed capacitor C1 described above is represented by the parallel capacitance of capacitances Cx and C3.

Then, connect the input terminal of the frequency circuit to the side circuit 1,
The circuit on the Q side of the MO8 field effect transistor will be considered separately, and the circuit 1 and 2 will be combined into a circuit. still,
A capacitor having a capacitance fjl,' CXIC3IC1 shall be designated by the same symbol Cz v C5C1, and a resistor having a resistance Ri shall be designated by the same symbol R1. That is, in the circuit 1, there is an impedance path consisting of the coil L1 and the capacitor cx, and in the circuit 2, there is mainly a parallel circuit of the coil L2, the capacitors C5, Ci, and the resistor Ri, which cooperates with the impedance circuit 1. The input tuning circuit K is constructed by the following.

1 is extracted from the matching circuit in FIG. 3. To explain this, a resistor R8 is connected between the left side of the input terminal t and the ground.
1 is connected to the matching circuit on the right side of the input terminal t, and a resistor R2 is connected between the output terminal t' and ground. In this case, 1 in the matching circuit only functions under the condition of R8<R2. Further, the impedance G when looking at the right side of the input terminal t has a frequency characteristic as shown in FIG. 4, and exhibits a resistance value R2 when the frequency f is 0, and when the frequency f is f. When , a corresponding resistance value R8 is exhibited. That is, a matching state of 1 in this matching circuit corresponds to a specific frequency f. Only when. And this frequency f. In order to vary the inductance L1 and the capacitance C, it is necessary to change the inductance L1 and the capacitance C at the same time, but in most cases, the inductance L1 is usually fixed. Therefore, also in FIG. 2, R8=Rt
The frequency f. Setting column? India is the problem.

FIG. 5 shows the frequency f in the high frequency circuit of FIG.

This shows an example of the setting, where R1 indicates the frequency characteristic curve of the input resistance R4 of the MOS field effect transistor Q, which is sufficiently larger than the impedance (resistance) end, and whose value gradually decreases as the frequency increases. It shows. The frequency characteristic of the impedance (resistance) R,' is as shown in the curve R:,2 when the inductance L1 is small, and as shown in the curve R:,2 when the inductance L1 is large. 1 in Kyokuso R is the reception frequency band f! , ~fy, the impedance (resistance) R4 is made equal to R8 at the upper limit frequency fn of ~fy, and the song aRo2 is the case where the impedance (resistance) R is made equal to the reception frequency band f■, ~fn at an appropriate intermediate value fM. R6
This is a case where it is made to look like a day. However, the curve n
:1, the standing wave ratio near the upper limit frequency fn is good, but the standing wave ratio becomes poor near the lower limit frequency fL. In the case of curve R8'2, the intermediate frequency fM
Although the standing wave ratio is good at the lower limit and upper limit frequencies fL and f■, the standing wave ratio becomes poor. In this manner, when the standing wave ratio deteriorates, the 'ri+ and power gain in the vicinity thereof decrease, and the noise figure becomes materialized.

Purpose of the Invention In view of the above-mentioned points, the present invention provides a circuit decoder! Set it to simple η1,
This paper attempts to propose a high frequency circuit for a television tuner that can perform matching over a wide band.

Summary of the Invention A high frequency circuit for a television tuner according to the present invention includes a semiconductor active element having an input resistance that is sufficiently larger than an input resistance of a high frequency signal input terminal and that gradually decreases as the frequency increases;
It has first and second inductance elements and fixed and variable capacitance elements, one end of the first inductance element is connected to the high frequency signal input terminal, the other end is grounded through the variable capacitance element, and the fixed capacitance element is connected to the high frequency signal input terminal. One end of the element is connected to the connection midpoint between the first inductance element and the variable capacitance element, the other end is grounded through the second inductance element, and the connection midpoint between the fixed capacitance element and the second inductance element is connected to the semiconductor. It is connected to the input electrode of an active zero.

According to the present invention, it is possible to obtain a high frequency circuit for a television tuner that has a simple circuit configuration and can perform matching over a wide band.

Embodiment An embodiment of the present invention will be described below with reference to FIG. 6. In FIG. 6, parts corresponding to those in FIG. 1 are designated by the same reference numerals. The input terminal t is connected to one dart of the Dubcedar 1MO8 field effect transistor Q through the first coil L1, capacitor C1, and capacitor C2 in this order. The midpoint of the connection between the coil L1 and the capacitor C1 is connected to the cathode of a variable capacitor (variable capacitor diode) Cv, and its anode is grounded. The midpoint of the connection between the coil L1 and the capacitor C1 is connected to an input terminal T1 to which a DC voltage for capacitance control is supplied to the variable capacitance capacitor 9Cv through a resistor Rc. Furthermore, capacitor C1
The connection midpoint of pC2 is grounded through the second coil L2.

The other dart of transistor Q is the AGC signal input terminal T.
Connected to 2.

In the high frequency circuit shown in FIG. 6, if the desired reception frequency is equal to the upper limit frequency of the reception frequency band of this high frequency circuit, the capacitance Cv will be sufficiently smaller than the capacitance C1, and there will be almost no capacitance division by the capacitors C1 and Cv.
Capacitor C1 can be ignored.

Next, if the desired receiving frequency is equal to the lower limit frequency of the receiving frequency band, the capacitance Cv will deviate greatly and 1 capacitance Cv is C1
force approximately equal to . In this case, as shown in the equivalent circuit of FIG. 7, by dividing the capacitors C1 and C4, M
The input impedance (resistance) of the O8 field effect transistor Q is sufficiently low compared to the conventional high frequency circuit shown in FIG.

Incidentally, a capacitor with a capacitance of C4 is indicated by the same symbol C4, and a capacitor with a capacitance of i
c4 has the relationship c4=cv-cX.

Therefore, the impedance matching frequency f of the high frequency circuit of FIG. As shown in FIG. 8, the reception frequency band ft, -f
If selected equal to the upper limit frequency fH of m, the receiving frequency band f
A good standing wave ratio over L-fm can be obtained.

In FIG. 8, the curve Ro1 corresponds to the song fPARot in FIG. 5, and the song BRss is the input terminal t according to the present invention. The figure shows the frequency/i characteristic of impedance (resistance) R9 when looking at the right side of , and the value gradually decreases as the frequency increases, but the slope is extremely gentle and the amount of change is small.

Therefore, for impedance matching, if the frequency f is selected as the upper limit frequency fn of the reception frequency band fL-fn, a good standing wave ratio can be obtained over the reception frequency fL = 7g, and as a result, the power gain is large. In addition, the noise figure is also good.

Incidentally, as shown in FIG. 9, it is also possible to insert a filter F on the input side of the high frequency circuit, and in this case, the output impedance of the filter F becomes R8.

Effects of the Invention According to the present invention described above, it is possible to obtain a high frequency circuit for a television tuner that has a simple circuit configuration and can perform matching over a wide band.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the high frequency circuit of a conventional television tuner, Figure 2 is a circuit diagram showing an equivalent circuit of the high frequency circuit in Figure 1, and Figure 3 is a circuit diagram showing part of the circuit in Figure 2. , FIG. 4 and FIG. 5 are curve diagrams showing the frequency characteristics of the input resistance, FIG. 6 is a circuit diagram showing an embodiment of the high frequency circuit of the television tuner according to the present invention, and FIG. 7 is the high frequency circuit of FIG. 6. FIG. 8 is a curve diagram showing the frequency characteristics of the input resistance, and FIG. 9 is a circuit diagram showing another embodiment of the present invention. L 1 + L 2 are the first and second inductors (coils), C1 is a fixed capacitor, Cv is a variable capacitor, and Q is a double-type MO8 as a semiconductor active element.
It is a field effect transistor.

Claims (1)

[Claims] a semiconductor active element having an input resistance that is sufficiently larger than the input resistance of the high frequency signal input terminal and gradually decreases as the frequency increases, first and second inductance elements, and fixed and variable capacitance elements; One end of the first inductance element is connected to the high frequency signal input terminal, the other end is grounded through the variable capacitance element, and one end of the fixed capacitance element is connected to the first inductance element and the variable capacitance element. and the other end is grounded through a second inductance element, and the connection midpoint of the fixed capacitance element and the second inductance element is connected to the input electrode of the semiconductor active element. The high frequency circuit of the TV tuner is a feature.