JPS596010Y2 - automatic frequency control device - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an automatic frequency control device for a separate carrier type television receiver, and is intended to make it possible to always obtain high quality audio.

The separate carrier method in television receivers is to convert the tuner's medium frequency output into a video signal (58.75M).
This method processes the audio signal (54.25M) and the audio signal (54.25M) separately, and a block diagram thereof is shown in FIG.

In FIG. 1, 1 is a tuner, and a first local oscillator is present inside.

2 is a video intermediate frequency amplifier that amplifies a 58.75 MHz video signal; 3 is a video detector that detects the output of this amplifier 2; 4 is a first audio intermediate frequency amplifier that amplifies a 54.25 MHz audio signal; 5 is a first audio intermediate frequency amplifier that amplifies a 54.25 MHz audio signal; The output of this amplifier 4 is mixed with the 64.95 MHz signal of the second local oscillator 6 to generate a signal of 10.7 MHz.
7 is a second audio intermediate frequency amplifier that amplifies the output of this mixer 5; 8 is an audio FM detector that detects the output of this amplifier 8; 9 is an AFC voltage generation circuit 9. be.

This circuit uses a superheterodyne system, causes the second local oscillator 6 to stably oscillate using a crystal oscillation element, and applies AFC to the first local oscillator.

In this device, when an electronic tuning circuit is used and the tuning voltage is supplied by a potentiometer, presetting by the potentiometer becomes a problem.

It is especially difficult to synchronize the audio.

That is, if the change ratio of the local oscillation frequency to the rotational speed of the potentiometer is large, the local oscillation frequency will change greatly with a slight rotation of the potentiometer, making it difficult to set an accurate reference frequency.

Furthermore, if the ratio is made small, the number of rotations of the potentiometer becomes extremely high, especially in the UHF band, which is inconvenient.

It is the first choice in weak electric field areas, adjacent damage prevention areas, and areas where ghosts occur.
A better image may be obtained if the local oscillator frequency is shifted lower by approximately 100-300 KHz.

By the way, if the oscillation frequency of the second local oscillator 6 is set to a fixed frequency, and the frequency of the first local oscillator is shifted as described above, the audio intermediate frequency will change from 54.25 to 1.
If it deviates from 0.7MHz and no sound is produced,
There is a drawback that the sound may be distorted.

Therefore, the present invention aims to provide an automatic frequency control circuit free from such drawbacks, and embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, and the same parts as in FIG. 1 are given the same numbers and their explanation will be omitted.

The feature of this device is that a part of the output of the video intermediate frequency amplifier 2 is taken out and added to the first AFC voltage generation circuit 10, and the obtained AFC voltage is sent to the first local part of the tuner 1 via a manual stop circuit 11. Added to the oscillator.

Further, a part of the output of the second audio intermediate frequency amplifier 7 is taken out, and an AFC voltage is generated in the second AFC voltage generation circuit 12.
It is added to the second local oscillator 6.

With this precaution, when presetting the electronic tuner or shifting the frequency of the local oscillator of the tuner 1 in a weak electric field area, the stop circuit 11 is operated to turn off the IAFC circuit.

However, at this time, the second AFC circuit is operating, so the audio circuit is correct (AFC is applied and beautiful audio is obtained).

Even if the change ratio of the local frequency to the rotation speed of the potentiometer is set large, 54.25 MHz will change greatly with respect to the rotation of the potentiometer, but 10.7 MHz will change slowly due to AFC operation, so It becomes much easier to synchronize.

To explain the case of shifting the local oscillation frequency of the tuner as an example, let us assume that the above shift is now performed to a lower frequency of 150 KHz.

The first intermediate frequency is stopped at 54.1M, and the second intermediate frequency is 10.85MHz.

Here, the second AFC voltage generation circuit 12 operates, and the second AFC voltage generation circuit 12 operates.
Change the oscillation frequency of local oscillator 6 from 64.95 MHz to 6
Changes to 4.80M stop, 10.85M stop is 10.7MH
z, and audio can be received stably and without distortion.

As described above, according to the present invention, high quality audio can be received with the oscillation frequency of the first local oscillation circuit of the tuner being slightly detuned.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional automatic frequency control device, and FIG. 2 is a block diagram of an automatic frequency control device according to an embodiment of the present invention. 1... Tuner, 2... Video intermediate frequency amplifier, 10... Ith AFC voltage generation circuit,
11... Stop circuit, 4... First audio intermediate frequency amplifier, 5... Mixer, 6...
2nd local oscillator, 7...2nd audio intermediate frequency amplifier, 12...2nd AFC voltage generation circuit.

Claims (1)

[Scope of utility model registration request] The output of the tuner is applied to a video intermediate frequency amplifier and also to a first audio intermediate frequency amplifier, and the output of this amplifier is applied to a second audio intermediate frequency amplifier via a mixer to construct a separate carrier type circuit, The output of the video intermediate frequency amplifier is applied to the first local oscillation circuit of the tuner via the first AFC voltage generation circuit and a manually operated stop circuit, and the output of the second audio intermediate frequency amplifier is applied to the output of the second AFC voltage generation circuit. an automatic frequency control device in addition to a second local oscillator for said mixer via a second local oscillator for said mixer;