JPH10276111A - Aft device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To safely set an IF signal frequency constant even at the time of excess modulation and at the time of weak electric field by counting an IF signal frequency through the use of the output signal of VCO in a PLL circuit which synchronously detects a video signal and applying the count result to a local oscillator. SOLUTION: An IF signal from an IF amplifier circuit 4 is applied to a phase comparator 16 and it is phase-compared with the oscillation output signal of a VCO 15. An error signal corresponding to the phase difference of both signals is applied to LPF 17 from the phase comparator 16, is smoothed and it is applied to the VCO 15. Thus, the VCO 15 oscillates in synchronizing with an IF signal. Thus, a video signal can synchronously be detected. The IF signal frequency is counted by applying the oscillation output signal to an IF counter 9. Thus, a sufficiently large value is secured for the level of a carrier in such period. Consequently, the oscillation output signal of the VCO 15 can secure a comparatively constant level and the oscillation frequency of the local oscillator 12 is adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AF for a TV receiver.
The present invention relates to an automatic frequency tuning (T) device and an AFT device having an IF (intermediate frequency) counter circuit.

[0002]

2. Description of the Related Art In an AFT circuit for controlling an oscillation frequency of a local oscillation circuit of a TV receiver to keep an intermediate frequency constant, a 58.75 MHz (N
It is necessary to detect the IF signal frequency of the TSC system.
A method of performing the detection using a counter circuit is known. FIG. 2 shows a TV receiver using such an AFT circuit. A TV signal received by an antenna (1) is amplified by an RF amplifier circuit (2), and an IF signal is received by a mixing circuit (3). Is converted to The converted IF signal is amplified by an IF amplification circuit (4) and detected by a video detection circuit (5). The detected video signal is amplified by a video amplifier (6).

[0003] The IF counter (9) counts the IF signal frequency (58.75 MHz) from the IF amplifier circuit (4) and applies the count result to the local oscillator (12). The oscillation frequency of the local oscillator (12) is adjusted according to the result. Therefore, according to the circuit of FIG. 2, an IF signal having a constant frequency can be obtained.

[0004]

However, in the circuit shown in FIG. 2, if the IF signal level is reduced when the AM signal is overmodulated, the IF counter (9) may not operate. IF counter (9)
Performs a counting operation according to the level of the IF signal.
When the IF signal level is lowered, the device may not operate.

[0005] When a large level of noise arrives in a weak electric field, the IF counter (9) may malfunction in response to the noise. If the IF counter (9) does not operate correctly, an IF signal of a constant frequency cannot be obtained from the mixing circuit (3).

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is an AFT apparatus for automatically adjusting an IF signal frequency to a constant value, comprising: a mixing circuit for converting an RF signal into an IF signal; A PLL circuit for generating an oscillation output signal synchronized with the output IF signal of the mixing circuit, a video detection circuit for synchronously detecting the output IF signal of the mixing circuit based on the output signal of the PLL circuit, An IF counter that counts an output signal frequency, and a local oscillation circuit that controls an oscillation frequency in accordance with an output signal of the IF counter and applies the oscillation output signal to the mixing circuit.

[0007]

FIG. 1 is a block diagram showing an AFT apparatus according to the present invention, wherein (15) is a VCO oscillating at an IF signal frequency, (16) is an output IF signal of the mixing circuit (3) and the VCO. A phase comparator for comparing the phase with the oscillation output signal of the VCO (15), and (17) an LPF for smoothing the output signal of the phase comparator (16) and applying the smoothed signal to the VCO (15).

In FIG. 1, the same circuit blocks as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. VCO
(15), phase comparator (16), LPF (17) is PL
An L circuit is configured. The IF signal from the IF amplifier circuit (4) is applied to the phase comparator (16) and is compared in phase with the oscillation output signal of the VCO (15). An error signal corresponding to the phase difference between the two signals is applied from the phase comparator (16) to the LPF (17), smoothed, and applied to the VCO (15).

For this reason, the VCO (15) oscillates in synchronization with the IF signal from the IF amplifier (4). By using the VCO (15) which oscillates in synchronization with the IF signal, synchronous detection of the video signal can be performed. That is, VCO
The oscillation output signal of (15) is applied to the video detection circuit (5) to detect the video signal. The frequency of the IF signal is counted by applying the oscillation output signal of the VCO (15) to the IF counter (9). The PLL circuit including the VCO (15), the phase comparator (16), and the LPF (17) has a predetermined time constant in consideration of noise resistance and operation stability. The IF signal from the IF amplifier circuit (4) has a horizontal synchronizing signal period. In this period, the carrier level has a sufficiently large value. Generally, the time constant of the PLL circuit has a value such that the PLL circuit can operate when a stable signal arrives at a period of the horizontal synchronization signal.

Therefore, the oscillation output signal of the VCO (15) can maintain a relatively constant level even when the IF signal is overmodulated or when a large level of noise arrives in a weak electric field. Therefore, the IF counter (9) sets the VCO (1
The IF signal frequency (58.75 MHz) from 5) is counted, and the count result is applied to the local oscillator (12). The oscillation frequency of the local oscillator (12) is adjusted according to the result.

Therefore, according to the circuit shown in FIG. 1, an IF signal having a constant frequency can be obtained. The oscillation frequency of the VCO (15) in FIG. 1 is the IF signal frequency (NTSC: 58.75 MHz).
z) may be used, or an integer multiple frequency may be used on the assumption that a frequency divider is used.

[0012]

As described above, according to the present invention, the IF signal frequency is counted using the output signal of the VCO of the PLL circuit for synchronously detecting the video signal, and the count result is applied to the local oscillator. Therefore, the IF signal frequency can be stably kept constant even during overmodulation or a weak electric field.

Further, according to the present invention, since the output signal of the VCO of the existing PLL circuit is used, there is an effect that the number of elements is not increased.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an AFT apparatus according to the present invention.

FIG. 2 is a block diagram showing a conventional AFT device.

[Explanation of symbols]

 (3) Mixing circuit (9) IF counter (12) Local oscillator (15) VCO circuit (16) Phase comparator (17) LPF

Claims (2)

[Claims] 1. A method for automatically adjusting an IF signal frequency to a constant value.
An FT device, comprising: a mixing circuit that converts a frequency of an RF signal into an IF signal; a PLL circuit that generates an oscillation output signal synchronized with an output IF signal of the mixing circuit; and the mixing circuit based on an output signal of the PLL circuit. A video detection circuit that synchronously detects an output IF signal of the circuit; an IF counter that counts an output signal frequency of the PLL circuit; an oscillation frequency controlled in accordance with an output signal of the IF counter; An AFT apparatus comprising: a local oscillation circuit for applying a voltage to the AFT device. 2. A method for automatically adjusting an IF signal frequency to a constant value.
An FT device, comprising: a mixing circuit for converting an RF signal into an IF signal; a VCO that oscillates at a frequency corresponding to an IF signal frequency; and a phase comparison between an output IF signal of the mixing circuit and an oscillation output signal of the VCO. A LPF for smoothing an output signal of the phase comparator and applying the smoothed output signal to the VCO; and an output I of the mixing circuit based on the output signal of the VCO.
A video detection circuit that synchronously detects an F signal; an IF counter that counts an output signal frequency of the VCO; a local unit that controls an oscillation frequency in accordance with an output signal of the IF counter and applies the oscillation output signal to the mixing circuit An AFT apparatus comprising an oscillation circuit.