JPH01282928A - Electronic tuner local oscillation output coupling circuit - Google Patents

Abstract

PURPOSE:To improve harmonic ratio of a local oscillator by connecting one terminal of a local oscillator to an input terminal of a local oscillating amplifier and connecting the other terminal to ground via a capacitive element through a line coupling the oscillating output of the local oscillator to the local oscillation amplifier. CONSTITUTION:A capacitor 15 is connected in parallel with an emitter resistor 6 of a transistor(TR) 10 connected to a coupling line 5 to extract an oscillation output of a UHF local oscillator 2 and connecting to ground. A series resonance point is formed between VHF local oscillation basic frequency and a UHF local oscillating frequency, i.e., at a harmonic band by the inductance of a coupling line 5 and the capacitance of the capacitor 15 to decrease the impedance at the harmonic band between the input terminal of the local oscillation amplifier 3 and ground. As a result, the local oscillating frequency component oscillated by the VHF local oscillator 1 is given to the input terminal of the local oscillation amplifier 3 via a coupling capacitor 4, the harmonic signal component is attenuated by a series resonance characteristic of the coupling line 5 and the capacitor 15 and the harmonic wave signal level is decreased by the input terminal of the local oscillating amplifier 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a local oscillation output coupling circuit for an electronic tuner used in a television receiver or a video tape recorder (hereinafter referred to as a VTR).

(Prior Art) In recent years, electronic tuners have become smaller and the mounting density of the components used has increased significantly. On the other hand, as a channel selection method for televisions and VTRs, a PLL method that allows direct channel selection has been increasingly adopted, in place of the conventional preset method using a potentiometer.

To use this PLL system, it is necessary to have a built-in prescaler circuit that divides the local oscillation frequency of the electronic tuner, or an output terminal for supplying the local oscillation frequency to an external prescaler circuit.

Furthermore, in order to stabilize the frequency division operation of the prescaler circuit, it is important that the input oscillation signal has a voltage higher than the input sensitivity of the prescaler IC and that harmonic spurious signals are small.

Here, FIG. 4 shows a conventional electronic tuner. In FIG. 4, the local oscillation frequency component oscillated by the VHF local oscillator 1 is applied to the emitter terminal of a transistor 10 constituting a local oscillation amplifier 3 via a coupling capacitor 4. Further, the local oscillation frequency component oscillated by the UHF local oscillator 2 is applied to the emitter terminal via a coupling line 5.

The local oscillation frequency component amplified by the local oscillation amplifier 3 is supplied from the collector terminal of the transistor 10 to the prescaler 14 via the DC blocking capacitor 13.

In the local oscillation amplifier 3, 12 is a voltage application terminal, 6 is an emitter resistor, and 7, 8.9 are transistors l.
A DC bias resistor of O and a capacitor 11 are grounding capacitors for grounding the base terminal of the transistor 10 at high frequencies.

(Problem to be Solved by the Invention) By the way, regarding the input of an oscillation signal with less harmonic spurious, in the frequency characteristics of the input sensitivity of the prescaler IC, the sensitivity is higher in the high signal frequency region than in the low signal frequency region, and In the case of an oscillation signal containing harmonics within 10 dB, normal frequency division operation may be inhibited. That is, it has the property of operating on harmonic signals.

In the conventional circuit, the VHF local oscillator 1 and the local oscillation amplifier 3 are coupled via a coupling capacitor 4, and the frequency characteristics of this line exhibit capacitance.

Passage characteristics are higher in the high signal frequency region than in the low signal frequency region,
Therefore, on the input side and output side of the coupling capacitor 4, there is a level difference between the fundamental signal and the harmonic signal (hereinafter referred to as harmonic ratio).
This will result in a decrease in

Furthermore, the harmonic ratio decreases due to conditions such as temperature changes, voltage fluctuations, and the frequency characteristics of the line connecting the first local oscillation amplifier 3 and the prescaler 14, resulting in the problem that a sufficient harmonic ratio cannot be secured and the prescaler malfunctions. There is a point. Although it is possible to provide a separate filter circuit here, this is not suitable for downsizing the device.

The present invention solves the above-mentioned conventional problems, and provides a local oscillation output coupling circuit for an electronic tuner that can improve the harmonic ratio of a local oscillation signal and obtain stable operation of a prescaler. .

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a line for coupling the oscillation output of one local oscillator to a local oscillation amplifier.

This is achieved by connecting one end to the input terminal of the local oscillation amplifier and grounding the other end via a Yongsha element.

(Function) With the above configuration, the impedance between the local oscillation amplifier input terminal and the ground becomes low in the harmonic signal band, so the harmonic signal level input to one local oscillation amplifier is attenuated. Thus, the above-mentioned conventional problems are solved.

(Example) An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention. In FIG. 1, circuit elements that are the same as those in the circuit shown in FIG. 4 are given the same reference numerals and redundant explanations will be omitted. 1st
The difference between the figure and Figure 4 is that the UHF local oscillator 2
This is because a capacitor 15 is connected in parallel to an emitter resistor 6 of a transistor 10 which is connected to a coupling line 5 for extracting an oscillation output and is grounded.

FIG. 2 shows the frequency characteristics of the line connecting the VHF local oscillator 1 and the local oscillation amplifier 3, where the solid line A and the broken line B represent the circuit of the present invention and the conventional circuit, respectively.

As can be seen from this figure, in this configuration, the inductance of the coupled line 5 and the capacitance of the capacitor 15
A series resonance point is formed between the F local oscillation fundamental frequency and the UHF local oscillation frequency, that is, in the harmonic band, and the impedance in the harmonic band between the input terminal of the local oscillation amplifier 3 and the ground is reduced. As a result, the local oscillation frequency component oscillated by the VHF local oscillator 1 is given to the input terminal of the local oscillation amplifier 3 via the coupling capacitor 4, but the harmonic signal component is generated by the series resonance between the coupling line 5 and the capacitor 15. Attenuated by characteristics. Therefore, the harmonic signal level at the input end of the local oscillation amplifier 3 can be lowered.

FIG. 3 shows the frequency characteristics of the output voltage of the local oscillation amplifier 3, where A is the fundamental signal component and B and C are the second harmonic signal components of the present invention and the conventional circuit, respectively.

The harmonic ratio of the local oscillation output signal can be improved by the simple configuration of adding the capacitor 15 to the capacitor 15 in this way.

(Effects of the Invention) As explained above, the present invention provides a line that connects a local oscillation amplifier, one end of which is connected to the input end of the local oscillation amplifier, and the other end of which is connected to the input end of the local oscillation amplifier via a capacitive element in parallel with the emitter resistor. The simple configuration of grounding provides the effect of stabilizing the operation of the prescaler without increasing the size.

[Brief explanation of the drawing]

FIG. 1 is a local oscillation output coupling circuit diagram of an embodiment of the present invention,
Fig. 2 is a frequency characteristic diagram of the line between the local oscillator and the local oscillation amplifier in the present invention and the conventional circuit; Fig. 3 is a frequency characteristic diagram of the output voltage of the local oscillation amplifier in the present invention and the conventional circuit;
The figure is a diagram of a conventional local oscillation output coupling circuit. 1... VHF local oscillator, 2... UHF local oscillator, 3... Local oscillation amplifier, 4... Coupling capacitor, 5... Coupling line, 6... Emitter resistor, 7, 8
．． 9... DC bias resistor, 10... Transistor, 11... Grounded capacitor, 12... Voltage application terminal, 13... DC blocking capacitor, 14... Prescaler, 15... Capacitor. Figure 1 2:==ko 2nd #+ So-called Nami-karioi l & 苧Figure 3 □Su￥n蓓しFigure 4

Claims (1)

[Claims] A local oscillation output coupling circuit for an electronic tuner, characterized in that, in a line for coupling the oscillation output of a local oscillator to a local oscillation amplifier, one end is connected to the input terminal of the local oscillation amplifier, and the other end is grounded via a capacitive element. .