JPS6313371B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an input coupling method for a mixing circuit using active elements such as transistors in a tuner.

Conventionally, in order to increase the load Q of the interstage circuit and local oscillation circuit, the mixing circuit input section of the tuner is connected to the interstage circuit and local oscillation circuit by connecting active elements such as transistors in the mixing circuit using a small capacitor. It was configured to be directly connected to the input terminal. However, with this method, the local harmonics generated in the local oscillator circuit are also injected into the mixing circuit, so the active elements of the mixing circuit such as transistors amplify the local harmonic components, causing the tuner's antenna terminal leakage voltage. This caused a significant increase in unnecessary radiation. In order to eliminate this drawback, conventional methods have often been adopted in which the oscillation power of the local oscillation circuit is reduced to suppress the injection of local harmonic components into the mixing circuit as low as possible. However, this method has the disadvantage that the locally oscillated fundamental wave component injected into the mixing circuit also decreases, resulting in deterioration of interference characteristics such as cross-modulation and beat caused by third-order distortion of the mixing circuit.

An object of the present invention is to reduce unnecessary local harmonic components injected into the mixing circuit without reducing the injection voltage of the local fundamental wave components necessary for the mixing operation of the tuner. An object of the present invention is to provide a tuner in which leakage voltage at a tuner antenna terminal and unnecessary radiation due to components are improved.

In the present invention, among the local frequency components generated in the local oscillator circuit, local harmonic components that are unnecessary for mixing operation are injected into the mixing circuit and amplified, thereby increasing tuner antenna terminal leakage voltage and unnecessary radiation. In order to prevent this, only the local harmonic components among the local frequency components injected into the mixing circuit are reduced.

Hereinafter, the present invention will be explained in detail by way of an example. An example of a method of coupling a mixing circuit, an interstage circuit, and a local oscillation circuit in a conventional tuner is shown in the first section.
As shown in the figure. In Fig. 1, 1 is an interstage circuit, 2 is a local oscillation circuit, 3 is a mixing circuit, 4 and 5 are a tuning coil and a capacitor of the interstage tuning circuit, respectively, and 6,
7 is a small capacitance coupling capacitor for increasing the load Q of the interstage circuit and the local oscillation circuit, and for injecting the RF signal and the local oscillation frequency into the mixing circuit, and 8 is a transistor of the mixing circuit. In the coupling method shown in Figure 1, the local harmonic components generated in the local oscillator circuit are also injected into the mixing circuit, so they are further amplified in the mixing circuit, radiated and transmitted around the mixing circuit, and leakage from the antenna terminal of the tuner occurs. voltage and unnecessary radiation deteriorate. In order to eliminate this drawback, the present invention inserts a low-pass filter into the input section of the mixing circuit to eliminate local harmonic components unnecessary for the mixing operation without reducing the local fundamental wave component injected into the mixing circuit. By reducing this, leakage voltage from the antenna terminal of the tuner and unnecessary radiation are improved.

FIG. 2 shows an embodiment according to the present invention. In FIG. 2, numerals 9 and 10 are a coil and a capacitor, respectively, for preventing local harmonic components from being injected into the mixing circuit. In Figure 2, the interstage circuit 2 is tuned to the RF frequency, so it becomes capacitive at frequencies higher than the RF tuning frequency, and the equivalent circuit in Figure 2 can then be represented by a capacitor 11 as shown in Figure 3. A low-pass filter is formed at the input section of the mixing circuit. Therefore, by selecting the constants of the coil 9 and capacitor 10 so as to have the passband characteristics as shown in FIG. 4, only the local harmonic components injected into the mixing circuit can be reduced.
It becomes possible to improve the tuner's antenna terminal leakage voltage and unnecessary radiation. Here, the frequency in Figure 4
_L indicates the local oscillation fundamental wave frequency generated by the local oscillation circuit.

As described above, according to the present invention, a low-pass filter can be formed by the equivalent capacitance 11 of the interstage circuit, and the coil 9 and capacitor 10 inserted into the input section of the mixing circuit. By appropriately selecting the constants, only the local harmonic components unnecessary for the mixing operation can be removed from the local oscillator components injected into the mixing circuit without reducing the local fundamental wave component necessary for the mixing operation. This makes it possible to reduce leakage voltage from the tuner's antenna terminal and improve unnecessary radiation.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a method of coupling a mixing circuit with an interstage circuit and a local oscillation circuit in a conventional tuner, and FIG. 2 is a circuit diagram showing an embodiment of the present invention.
FIG. 3 is an equivalent circuit diagram of FIG. 2, and FIG. 4 is a diagram showing the passband characteristics of a low-pass filter formed according to the present invention. 1... Interstage circuit, 2... Local oscillation circuit, 3...
Mixing circuit, 4... Inter-stage tuning coil, 5... Inter-stage tuning capacitor, 6-7... Coupling capacitor, 8...
...Mixing transistor, 9... Coil for forming a low-pass filter, 10... Capacitor for forming a low-pass filter, 11... Interstage circuit equivalent capacity.

Claims (1)

[Claims] 1. In a tuner having a mixing circuit that performs a mixing operation using an amplifying element, one end is connected to the connection point where the output end of the interstage tuning circuit and the output end of the local oscillation circuit are connected, and the other end is connected to the connection point where the output end of the interstage tuning circuit and the output end of the local oscillation circuit are connected. It consists of a coil connected to the input terminal of the amplification element, and a capacitor whose one end is grounded and whose other end is connected to the input terminal of the amplification element, and the interstage tuning circuit becomes capacitive at frequencies higher than the tuning frequency. Taking advantage of this fact, the interstage tuning circuit, the coil, and the capacitor constitute a π-type low-pass filter, and the cut-off frequency of the low-pass filter is higher than the fundamental oscillation frequency of the local oscillation circuit, and further lower than the desired harmonic frequency. The constants of the coil and capacitor are selected so that the fundamental wave component of the local oscillation frequency necessary for mixing operation is injected into the amplifier element of the mixing circuit, and the harmonic component of the local oscillation frequency unnecessary for mixing operation is injected into the amplifier element of the mixing circuit. By blocking or reducing
Due to the harmonic components injected and amplified by the amplification element of the mixing circuit, the antenna terminal leakage voltage of the tuner,
A tuner mixing circuit characterized by preventing unnecessary radiation from deteriorating.