JPH0611649Y2 - Chiuna - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an improvement of a tuner having a station detection circuit.

(Prior Art) FIG. 2 shows a circuit diagram of a main part of a tuner including a conventional station detection circuit. In FIG. 2, the antenna input from the antenna 1 is given to the antenna input terminal 4 of the tuner 3 via the antenna input circuit 2. The antenna input applied to the antenna input terminal 4 is applied to the gate G of the field effect transistor 5 which forms a non-tuned RF amplifier circuit. An attenuator circuit including a resistor 6 and a transistor 7 connected in series is interposed between the gate G and the ground. An AGC circuit 12 is connected to the gate G of the field effect transistor 5. Further, the antenna input amplified by the field effect transistor 5 is selected by the tuning circuit 8, frequency-converted by the mixing circuit 9, and given to the intermediate frequency amplifier circuit 10. Then, the intermediate frequency signal amplified and output from the intermediate frequency amplifier circuit 10 is given to a detection circuit (not shown) in the subsequent stage, and a part of the intermediate frequency signal is given to the station detection circuit 11. The station detection circuit 11 detects the presence of a station from the level change of the intermediate frequency signal which is equal to or higher than the station detection level of a constant threshold with respect to the sweep of the reception frequency selected by the tuning circuit 8. In addition, AGC
The circuit 12 controls the intensity of the input signal supplied to the field effect transistor 5 by applying a part of the intermediate frequency signal to a predetermined range, and is well known.

Then, Dx selects a weak radio wave from a broadcasting station at a long distance.
At the time of reception, even if a search start operation for starting the sweep of the reception frequency is performed, a control signal is not output from a control circuit such as a CPU (not shown), the transistor 7 is non-conductive and the AGC circuit 12 is non-operationally controlled. To be done. Therefore, the antenna input applied to the field effect transistor 5 is not attenuated, the tuner 3 has high reception sensitivity, and the intermediate frequency signal is applied to the station detection circuit 11 while its level is high. When receiving this Dx, as indicated by the solid line in FIG.
The circuit is set so that the receiving sensitivity is constant. Also,
At the time of Lo reception that selects a strong radio wave from a broadcasting station in a short distance, when a search start operation is started to start sweeping the reception frequency, a control signal such as a CPU (not shown) outputs a control signal, and the control signal is a transistor. 7 and AGC circuit
12 and the transistor 7 is conduction-controlled to attenuate the antenna input and the AGC circuit 12 is controlled to operate, and the tuner 3 has a low receiving sensitivity and the intermediate frequency signal level is lowered to be given to the station detection circuit 11. To be

Therefore, at the time of Lo reception, only the intermediate frequency signal corresponding to the strong antenna input exceeds the station detection level of the station detection circuit 11 and the presence of the station is detected, but the station with the weak antenna input is not detected. Further, at the time of receiving Dx, the presence of a station having a weak antenna input exceeds the station detection level of the station detection circuit 11 and is detected.

(Problems to be Solved by the Invention) By the way, the RF amplifier circuit including the transistor 7 that is non-conducting controlled and the AGC circuit 12 that is non-operating controlled during Dx reception and the antenna circuit 2 are as shown by the solid line in FIG. It is set so that it does not have a resonance frequency within the reception band, but during Lo reception, the transistor 7 is conduction controlled and AG
By the operation control of the C circuit 12, the inductance component and the capacitance component are largely changed and the transistor 7 whose conduction is controlled and the RF amplification circuit including the AGC circuit 12 whose operation is controlled and the antenna circuit 2 are within the reception band. It is easy to form a resonance circuit having a resonance frequency. Then, as shown in FIG. 3, the impedance of the antenna 1 viewed from the tuner 3 becomes lower than the other frequencies, for example, at 1350 KHz in the AM band of 550 to 1600 KHz. Therefore, the receiving sensitivity of the tuner 3 at the time of Lo reception becomes low at 1350 KHz where the impedance in FIG. 3 is low, as indicated by the broken line in FIG.

As described above, since the reception sensitivity during Lo reception does not have a flat characteristic with respect to the frequency within the band, a station existing in the vicinity of the frequency with low reception sensitivity of the tuner 3 is a station with a frequency with high reception sensitivity. Station detection circuit 11 if not stronger than the radio wave
Cannot be detected by. Therefore, there is a problem that the strength of the antenna input that can be detected by the station detection circuit 11 varies.

As described above, the fact that the reception sensitivity of the tuner 3 during Lo reception caused by providing the transistor 7 and the AGC circuit 12 for attenuating the antenna input to the RF amplification circuit is not flat is due to the AGC circuit 12 itself or the RF amplification circuit. It is difficult to correct by the circuit configuration of. Also,
Correcting this defect in the latter stage of the RF amplifier circuit is a practical measure because the S / N ratio is likely to deteriorate with respect to a low level signal with low reception sensitivity such as when receiving Dx because the correction width is large. Not.

The purpose of the present invention is to solve the above-mentioned problems of the conventional tuner, and does not provide a tuner in which the strength of the antenna input that can be detected by the station detection circuit during Lo reception is constant. To do so.

(Means for Solving the Problems) In order to achieve the above object, the tuner of the present invention has an RF amplifier circuit connected to an antenna circuit, and the RF amplifier circuit is provided between the antenna circuit and the RF amplifier circuit. Is connected to an AGC circuit for controlling the intensity of the input signal input to the device, and the output of the RF amplification circuit is applied to a frequency conversion circuit for frequency conversion, and this signal is applied to an intermediate frequency amplification circuit to provide an intermediate frequency signal. Is amplified and output, a part of the amplified intermediate frequency signal is supplied to the station detection circuit, the intermediate frequency amplification circuit is provided with a gain switching means, and the gain switching means is operated and controlled at the time of Lo reception to perform the intermediate frequency amplification. It is configured so that the amplification gain of the circuit is reduced, and the gain switching means is not operated when Dx is received so that the amplification gain is not reduced.

(Function) The intermediate frequency amplification circuit is provided with a gain switching means, the gain switching means is controlled in operation during Lo reception to lower the amplification gain of the intermediate frequency amplification circuit, and the gain switching means is deactivated during Dx reception. Since the level is prevented from decreasing, the level of the intermediate frequency signal given to the station detection circuit can be switched between Lo reception and Dx reception, and the RF amplification circuit can be switched between Lo reception and Dx reception as in the conventional case. A circuit for switching the strength of the input signal that is input is not required. Here, when Lo is received, the strength of the input signal of the RF amplifier circuit is controlled within a predetermined range by the AGC circuit.
Although the inductance component and the capacitance component change due to the operation control of the C circuit, a large change does not occur unlike the conventional switching circuit. Therefore, there is no fear that the RF amplifier circuit including the AGC circuit whose operation is controlled during Lo reception and the antenna circuit form a resonance circuit having a resonance frequency within the reception band. Therefore, the reception sensitivity becomes relatively flat even when receiving Lo. Further, even if the circuit is set to correct the reception sensitivity to the reception frequency in the subsequent stage of the RF amplifier circuit, the correction width is small, so that the S / N ratio is less deteriorated and the station input circuit can detect the antenna input during Lo reception. It is easy to make the strength of the constant.

(Description of Embodiments) Hereinafter, embodiments of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram of an essential part of an embodiment of a tuner provided with the station detecting circuit of the present invention. In FIG. 1, the same circuits as those in FIG. 2 are designated by the same reference numerals, and duplicate description will be omitted.

1 is different from FIG. 2 in that a gain switching means composed of a series connection of a resistor 16 and a transistor 17 is interposed between the intermediate frequency amplifier circuit 10 and the ground, and the resistor 6 in FIG. The transistor 7 is omitted. The transistor 17 shown in FIG. 1 is conductively controlled by receiving a control signal from a control circuit such as a CPU (not shown) when a search start operation is performed during Lo reception. Further, when Dx is received, the control signal is not given and the non-conduction control is performed.

When a control signal is applied during Lo reception to control the conduction of the transistor 17, the gain of the intermediate frequency amplifier circuit 10 decreases, and during Dx reception, the control signal is not applied and the transistor 17 is non-conducting. Controlled, intermediate frequency amplifier circuit 10
The gain of does not decrease.

Therefore, the intensity of the intermediate frequency signal output from the intermediate frequency amplifier circuit 10 is switched to low and is given to the station detection circuit 11 during Lo reception, and the intensity of the intermediate frequency signal is not switched to low during Dx reception.

By the way, in FIG. 1, the AGC circuit is used at the time of Lo reception.
By controlling the operation of 12, the inductance component and the capacitance component change, but the change is slight, and a large change does not occur unlike the conventional switching circuit shown in FIG. Therefore, when receiving Lo, the RF amplifier circuit including the AGC circuit 12 whose operation is controlled and the antenna circuit 2 are
A resonance circuit having a resonance frequency in the reception band is not constructed. Therefore, the reception sensitivity becomes relatively flat even when receiving Lo. Then, in response to the fact that the reception sensitivity of the RF amplification circuit slightly changes due to the operation of the AGC circuit 12 during Lo reception, a circuit setting for correcting the reception sensitivity of the tuner 3 with respect to the reception frequency is set in the RF amplification circuit. Even if it is performed in the latter stage, the reception sensitivity range to be corrected is small, so that the deterioration of the S / N ratio is small. Therefore, it is easy to flatten the reception sensitivity during Lo reception, and as a result, the strength of the antenna input that can be detected by the station detection circuit 11 can be made constant with little variation.

Note that by setting the dynamic range of the AGC circuit 12 appropriately, distortion will not occur due to a strong antenna input.

(Effect of the Invention) As described above, according to the tuner of the present invention, the Lo
An excellent effect that the strength of the antenna input that can be detected by the station detection circuit at the time of reception can be made constant with little variation, and the difference in reception sensitivity during Lo reception and Dx reception can be easily set to a predetermined value. Play.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an essential part of an embodiment of a tuner having a station detecting circuit according to the present invention, and FIG. 2 is a circuit diagram of an essential part of a tuner having a conventional station detecting circuit. Figure 3 shows the second
FIG. 4 shows impedance characteristics of the antenna as seen from the tuner of FIG. 4, and FIG. 4 shows reception sensitivity characteristics of FIG. 2: Antenna circuit, 3: Tuner, 5: Field effect transistor, 8: Tuning circuit, 9: Mixed circuit, 10: Intermediate frequency amplification circuit, 11: Station detection circuit, 12: AGC circuit, 16: Resistor, 17: Transistor .

Claims (1)

[Scope of utility model registration request] 1. An RF amplifier circuit is connected to an antenna circuit, and an AGC circuit for controlling the intensity of an input signal input to the RF amplifier circuit within a predetermined range is connected between the antenna circuit and the RF amplifier circuit. The output of the RF amplification circuit is applied to a frequency conversion circuit for frequency conversion, this signal is applied to an intermediate frequency amplification circuit to amplify and output the intermediate frequency signal, and a part of the amplified intermediate frequency signal is detected by the station detection circuit. The gain switching means is provided in the intermediate frequency amplifying circuit, the gain switching means is controlled in operation during Lo reception to reduce the amplification gain of the intermediate frequency amplifying circuit, and the gain switching means is deactivated in Dx reception. A tuner configured so that the amplification gain does not decrease.