JPH05335983A - Tuner - Google Patents

Abstract

PURPOSE:To surely prevent a leakage of a local oscillation signal to an intermediate frequency signal by forming a trap in the vicinity of a lowest frequency in the local oscillation signal based on a parallel resonance frequency comprising a coil and a capacitor. CONSTITUTION:Since an intermediate frequency signal subject to frequency conversion by a mixer circuit 1 is a signal at an intermediate frequency band, the signal passes through an intermediate frequency passing filter 2 and a low pass filter 4 and is amplified by an intermediate frequency amplifier circuit 3 and outputted from a tuner. The low pass filter 4 forms a trap in the vicinity of a lowest frequency in the local oscillation signal based on the parallel resonance frequency formed by a coil L2 and a capacitor C6 connected in parallel with the coil L2. Thus, even in the case of the low pass filter 4 with simple configuration, since the frequency characteristic is rapidly reduced around the trap frequency, the local oscillation signal with the lowest frequency is surely attenuated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a television receiver and a VTR.
The improvement of the intermediate frequency amplification circuit in the tuner of.

[0002]

2. Description of the Related Art As shown in FIG. 3, a mixing section and an intermediate frequency amplifying section in a tuner of a television receiver or the like are mixed circuits 1.
1, intermediate frequency pass filter 12, intermediate frequency amplifier circuit 13
And a low-pass filter 14. A high-frequency signal received by an antenna (not shown) is high-frequency amplified and then input to the mixing circuit 11 via the capacitor C1, and a local oscillation signal from the local oscillation circuit 15 is also input via the capacitor C2. ing. The mixing circuit 11 is a circuit that mixes the input high-frequency signal with the local oscillation signal and frequency-converts it into an intermediate-frequency signal (IF signal).

The output of the mixing circuit 11 is passed through the intermediate frequency pass filter 12 to extract only the intermediate frequency signal. Then, the intermediate frequency signal is amplified by the intermediate frequency amplifier circuit 13, and the high frequency components are removed through the low pass filter 14 to become the intermediate frequency signal output of the tuner.

[0004]

However, the mixing circuit 11 cannot completely prevent the local oscillation signal from the local oscillation circuit 15 from leaking to the output of the frequency-converted intermediate frequency signal. However, in the conventional tuner shown in FIG. 3, the intermediate frequency pass filter 12 that extracts only the intermediate frequency signal from the output of the mixing circuit 11 and the low pass filter 14 that blocks the high frequency of this intermediate frequency signal are complicated and expensive. Since the circuit configuration cannot be used, the frequency characteristic also changes relatively slowly and a steep characteristic cannot be obtained.

Here, the intermediate frequency signal has a video intermediate frequency of 45.75 MHz and an audio intermediate frequency of 41.25 MHz in the case of the US specification, for example, and a video intermediate frequency of 58.5 MHz and audio in the case of Japan. Intermediate frequency is 54.5
MHZ. On the other hand, the local oscillation signal with the lowest frequency is 2 channels of 101 MHz in the case of US specifications.
In Japan, one channel has a frequency of 150 MHz, which is not always a frequency sufficiently distant from the intermediate frequency band.

Therefore, the conventional tuner has the mixing circuit 1
Among the local oscillation signals leaked from 1, particularly the local oscillation signal having the lowest frequency and close to the intermediate frequency band cannot be reliably removed by the intermediate frequency pass filter 12 or the low pass filter 14 as described above. There has been a problem that it is impossible to completely prevent the local oscillation signal from being mixed in the frequency signal output and adversely affecting the circuit in the subsequent stage.

[0007]

In order to solve the above-mentioned problems, the present invention provides a signal obtained by mixing a high-frequency signal with a local oscillation signal for frequency conversion and performing intermediate frequency amplification, in series with a signal line. In a tuner that removes high-frequency components and outputs them by passing them through a low-pass filter that consists of a connected coil and the other end that is grounded and connected in parallel, place a capacitor in parallel with the coil of the low-pass filter. It is characterized in that a trap is formed near the lowest frequency in the local oscillation signal due to the parallel resonance frequency of the coil and the capacitor connected.

[0008]

With the above structure, the low-pass filter forms a trap near the lowest frequency in the local oscillation signal due to the parallel resonance frequency of the coil and the capacitor connected in parallel to the coil. Therefore, even with a low-pass filter having a simple configuration, the frequency characteristic sharply deteriorates in the vicinity of the trap frequency, so that the local oscillation signal having the lowest frequency can be reliably attenuated. Incidentally, the local oscillation signal of higher frequency can be sufficiently attenuated by the original high-frequency cutoff characteristic of the low-pass filter as in the conventional case.

As a result, according to the tuner of the present invention, the leakage of the local oscillation signal closest to the intermediate frequency band to the intermediate frequency signal output is prevented even though the low pass filter having a simple structure is used. It can be surely prevented.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

1 and 2 show an embodiment of the present invention. FIG. 1 is a circuit diagram of a mixing section and an intermediate frequency amplifying section in a tuner, and FIG. 2 is a leakage voltage of a local oscillation signal from the tuner. It is a frequency characteristic diagram showing. It should be noted that the tuner of this embodiment will be described as one used in a television receiver of American specifications.

This tuner is designed so that a high frequency signal received by an antenna (not shown) is subjected to high frequency amplification and then input to the mixing circuit 1 via the capacitor C1 shown in FIG. The local oscillation signal from the local oscillation circuit 5 is also input to the mixing circuit 1 via the capacitor C2. The mixing circuit 1 comprises a circuit in which the source of the dual gate field effect transistor Q1 is grounded.
The high frequency signal input to the gate G1 and the local oscillation signal are mixed, and an intermediate frequency signal (IF signal) is output from the drain terminal.
Is to be output. Here, the local oscillation circuit 5 is a circuit that always emits a local oscillation signal having a frequency higher than the video carrier frequency of the high frequency signal by the video intermediate frequency (45.75 MHz in the US) in the channel selected by the tuning device (not shown). Is. Therefore, this mixing circuit 1
Indicates a high frequency signal of the channel selected by the channel selection device at a fixed frequency (in the US specification, the video intermediate frequency is 45.
The frequency can be converted into an intermediate frequency signal of 75 MHz and an audio intermediate frequency of 41.25 MHz.

The output of the mixing circuit 1 is connected to the intermediate frequency pass filter 2. Intermediate frequency pass filter 2
Is a pie-type bandpass filter composed of one coil L1 connected in series to the signal line and two capacitors C3 and C4 connected in parallel with the other end grounded. Only the intermediate frequency signal output from is passed. The output of the intermediate frequency pass filter 2 is connected to the input of the intermediate frequency amplifying circuit 3 via the capacitor C5. Intermediate frequency amplifier circuit 3
Is an amplifying circuit for an intermediate frequency signal by the bipolar transistor Q2. The capacitor C5 is a coupling capacitor for this intermediate frequency signal, and also serves to block the DC component.

The output of the intermediate frequency amplifier circuit 3 is connected to a low pass filter 4. The low-pass filter 4 constitutes a parallel resonance circuit in which a coil L2 and a capacitor C6 are connected in parallel with each other, and is a filter including a capacitor C7 connected between the output end of the parallel resonance circuit and the ground. Therefore, the low-pass filter 4 cuts off the high band and forms a trap at the parallel resonance frequency of the coil L2 and the capacitor C6. The trap frequency here is the frequency of the local oscillation signal of two channels in the US specification 101
It is set to MHZ. The output of the low-pass filter 4 becomes the intermediate frequency signal output of the tuner through the capacitor C8 for blocking the DC component.

According to the above configuration, the intermediate frequency signal whose frequency has been converted by the mixing circuit 1 has an intermediate frequency band (41.25M).
Since it is a signal of (Hz to 45.75 MHz), it passes through the intermediate frequency pass filter 2 and the low pass filter 4, and is amplified by the intermediate frequency amplifier circuit 3 in the meantime and output from the tuner. Further, in the case of the local oscillation signal from the local oscillation circuit 5 leaking to the output of the mixing circuit 1, the high-order channel is sufficiently attenuated by the high frequency cutoff characteristics of the intermediate frequency pass filter 2 and the low pass filter 4. Then, it is not mixed in the intermediate frequency signal output of the tuner. However, since the frequency of the local oscillation signal when selecting two channels with the tuner of the US specification is 101 MHz and is the closest to the intermediate frequency band, the conventional configuration has a simple configuration as shown by the broken line B in FIG. The intermediate frequency pass filter and the low pass filter could not be sufficiently attenuated, and a relatively large leakage voltage was generated. However, according to the present embodiment, the parallel resonance circuit of the coil L2 and the capacitor C6 in the low pass filter 4 forms a trap at the frequency of 101 MHz, so that the local oscillation of 101 MHz is generated as shown by the solid line A in FIG. The signal can also be sufficiently attenuated.

In this embodiment, the capacitor C6 and the coil L2 form a parallel resonance circuit to form a trap having a frequency closest to the intermediate frequency band. However, the inductance of the low-pass filter 4 is slightly deviated from the designed value. If it is unmarried, the capacitor C6 may be specified as a variable capacitance type.

As a result, although the tuner of this embodiment uses the intermediate frequency pass filter 2 and the low pass filter 4 having a simple structure, the local oscillation signal of the two channels closest to the intermediate frequency band is used. It becomes possible to surely prevent the leakage to the output of the intermediate frequency signal.

[0018]

As is apparent from the above description, the tuner of the present invention uses a low-pass filter with a simple structure and at a low cost, but the local oscillation signal leaks to the intermediate frequency signal output. There is an effect that it can be surely prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a mixer and an intermediate frequency amplifier in a tuner according to an embodiment of the present invention.

FIG. 2 shows an embodiment of the present invention and is a frequency characteristic diagram showing a leakage voltage of a local oscillation signal from a tuner.

FIG. 3 is a circuit diagram of a mixer and an intermediate frequency amplifier in a tuner, showing a conventional example.

[Explanation of symbols]

 1 Mixing circuit 4 Low pass filter 5 Local oscillation circuit L2 coil C6 capacitor

Claims (2)

[Claims] 1. A signal obtained by mixing a high frequency signal with a local oscillation signal and performing frequency conversion to perform intermediate frequency amplification from a coil connected in series to a signal line and a capacitor connected in parallel with the other end grounded. In a tuner that removes high-frequency components and outputs by passing through a low-pass filter, a capacitor is connected in parallel to the coil of the low-pass filter, and the local resonance signal is A tuner characterized by forming a trap near the lowest frequency. 2. The tuner according to claim 1, wherein the capacitor connected in parallel with the coil of the low pass filter is set to a variable capacitance type.