JPS5897917A - Channel selecting device - Google Patents

Abstract

PURPOSE:To perform invariably accurate AFC operation without reference to whether a tuning voltage is high or low by applying the AFC voltage of an AFC detecting circuit to an emitter follower circuit and then applying it to a synthesizing resistance network. CONSTITUTION:The AFC voltage of an AFC detecting circuit 5 is applied to the base of a transistor (TR) 7 constituting an emitter follower circuit 6 and an output is led out of the emitter and applied to a synthesizing resistance network 8; and it is synthesized with a tuning voltage at a prescribed ratio, and the resulting voltage is applied to the varactor diode 2 of an electronic tuner 1. Consequently, the AFC wave detecting circuit 5 is separated from the network 8 and the center voltage of the AFC voltage is held stably at the constant level obtained by adding the base-emitter voltage of the TR7 to the bias voltage depending upon the voltage division ratio of resistances 9 and 10.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic tuner type tuning device used in television receivers, etc., and provides a device that can perform accurate automatic frequency control regardless of the magnitude of tuning voltage. It is.

In an electronic tuner for a television receiver, a tuning voltage for channel selection and an AFC voltage for automatic frequency control (AFC) are combined using a resistor and applied to a variable capacitance diode of the tuner. However, in conventional devices of this type, when the tuning voltage increases and the current flowing into the AFC voltage generation circuit via the synthesis resistor increases, the reference level of the AFC voltage changes, causing the S curve to change. Since the characteristics are vertically asymmetrical, there is a problem that the AFC pull-in range becomes narrow.

SUMMARY OF THE INVENTION Therefore, one object of the present invention is to provide a device that can eliminate such conventional drawbacks and always achieve accurate AFC operation regardless of the magnitude of the tuning voltage.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. In the figure, 1 is the electronic tuner of the television receiver.
It has a variable capacitance diode 2 as a tuning element. 3
4 is a tuning voltage generation circuit that generates a tuning voltage to be applied to the variable capacitance diode 2 for tuning, and 4 is a tuning circuit that switches the tuning voltage depending on the channel to be tuned.

Further, 6 is an AFC detection circuit that discriminates the frequency of the video intermediate frequency (VIP) signal from the team 1 and generates an AFC voltage for AFC control.

In this device, the AFC voltage of the AFC detection circuit 5 is transferred to the transistor 7 constituting the emitter follower circuit 6.
In addition to the pace, it is taken out from its emitter and added to the 1 synthesis resistor network 8, where it is synthesized with the previous tuning voltage at a predetermined ratio and then added to the variable capacitance diode 2 of the electronic team 1. . 9, 1o are bias resistors, 11
is the emitter resistance. 12 to 16 are resistances for synthesis, 1
Reference numeral 6 denotes a field effect transistor for switching the combination ratio of AFC voltages between when selecting a VHF channel and when selecting a UHF channel. V
Only during HF tuning, the field effect transistor 16 is made conductive by a signal from the tuning circuit to increase the application ratio of the AFC voltage.

According to such a configuration, by providing the emitter follower circuit 6, the AFC detection circuit 6 is connected to the composite resistance network 8.
The center voltage of the AFC voltage can be separated from the bias voltage determined by the voltage division ratio of the resistors 9 and 1o, and the base-emitter voltage of the transistor 70 (approximately 0.
TV) can be stably maintained at a constant value.

The tuning voltage from the tuning voltage generation circuit 3 becomes high and the resistor 1
Even if the current flowing into the emitter follower circuit 6 side through the 4.degree. Thus, according to this device, the reference level of the AFC voltage can be correctly maintained at a predetermined value regardless of the magnitude of the tuning voltage applied to the variable capacitance diode 2 of the electronic tuner 1, and the S curve of the AFC voltage can be maintained vertically asymmetrically. Therefore, the AFC pull-in range can be widened and accurate AFC control operation can be ensured.

Although the emitter follower circuit was used in the above embodiment, a source follower circuit using a field effect transistor was used, and the AF from the AFC detection circuit was used at its gate.
Similar effects can be obtained by inputting the C voltage and outputting the AGC voltage from the source and adding it to the composite resistance network.

As described above, according to the present invention, the (AF
Thus, a useful channel selection device can be obtained which can maintain the reference level of the C voltage at a predetermined value and can achieve accurate operation.

[Brief explanation of the drawing]

The figure is a circuit diagram of a channel selection device in an embodiment of the present invention. 1... Electronic tuner, 2... Variable capacitance diode, 3... Tuning voltage generation circuit, 4...
...Tuning selection circuit, 6...AFC detection circuit, 6
...Emitter follower circuit, 7...Transistor, 8...Composition resistance network, 12,1
3,14.15...Resistance.

Claims (1)

[Claims:] A tuning voltage generation circuit that generates a tuning voltage for tuning to be applied to a variable capacitance diode of a tuner. an AEC detection circuit that generates an AFC voltage for automatic frequency control to be applied to the variable capacitance diode of the tuner;
and an emitter follower circuit or source follower circuit that receives the AFC voltage and outputs the AFC voltage from the emitter or source. A channel selection characterized by comprising a composite resistor that combines the tuning voltage from the tuning voltage generation circuit and the AFC voltage from the emitter follower circuit or the source follower circuit and applies the resultant to the variable capacitance diode of the tuner. Device.