JPS6133725Y2 - - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to a tuning circuit for a receiver.

In the receiver, sensitivity switching is required to improve the interference characteristics of strong inputs or to receive only strong input stations at night. Conventionally, such sensitivity switching has been considered, for example, by installing an attenuator in the antenna, and one proposed method is to lower the Q of the antenna tuning circuit.
This method is particularly effective when the Q is high, such as a ferrite bar antenna in the medium wave band. FIG. 1 is an example of a conventional sensitivity switching circuit. In the figure, reference numeral 1 denotes a field effect transistor for amplifying high frequency signals, and together with a resonant circuit 2 connected to the input stage of the field effect transistor 1, a tuning circuit is formed. This tuning circuit includes a primary coil 4 and a secondary coil 5 of a bar antenna 3, and a semi-fixed capacitor 6 and a variable capacitor 7 as tuning elements connected to the primary coil 4. One end of the next coil 5 is connected to the gate of the field effect transistor 1, and the other end is connected to the source of the field effect transistor 1. A resistor Rp is connected in parallel to the secondary coil 5 via a sensitivity selector switch 8, and in the DX reception mode, the switch 8 is turned off (shown by the solid line) to maintain high sensitivity, and in the local reception mode, the switch 8 is turned on (as shown by the dotted line), the resistance Rp
is connected in parallel to the tuning circuit, thereby lowering the Q of the tuning circuit and reducing sensitivity.

However, in a sensitivity switching circuit with such a configuration, even if switch 8 is turned off in the DX reception mode, stray capacitance occurs between the hot side wiring leading to switch 8 and the ground, and this stray capacitance causes resistance Rp to is inserted in parallel to the tuned circuit, and
As a result, in the DX reception mode, the Q of the tuning circuit is
There was a drawback that the sensitivity decreased and high sensitivity could not be ensured.

The present invention is designed to effectively avoid a drop in Q when the sensitivity is high (in DX reception mode) in a receiver tuning circuit that controls reception sensitivity by controlling the Q of the tuning circuit. This is what I did.

Hereinafter, an example of the receiver sensitivity switching circuit according to the present invention will be explained using FIG.

In FIG. 2, 1 is a field effect transistor for amplifying a high frequency signal, and 2 is a resonant circuit connected to the input stage of this field effect transistor 1. This example shows a resonant circuit constituted by a primary coil 4 and a secondary coil 5 of a bar antenna 3, and a semi-fixed capacitor 6 and a variable capacitor 7 connected in parallel to the primary coil 4.

In the present invention, one end of the secondary coil 5 of the bar antenna 3 is connected to the gate of the field effect transistor 1, and the other end is connected to the field effect transistor 1 through a high frequency signal bias capacitor 9.
Connect to the source (or drain) of On the other hand, the common contact 10a, the first and second contacts 10b and 10
A sensitivity changeover switch 11 is provided as a bypass control means with A bias that connects the connection midpoint of the source (or drain) of the field effect transistor 1 to the first contact 10b of the switch 11, and supplies a predetermined bias current to the field effect transistor 1 through the resistor 12 to the second contact 10c. Connect the current supply source (+B).

In such a configuration, when in the so-called DX reception mode with a weak electric field, the sensitivity selector switch 11 is set to the first contact 1.
Switch to the 0b side (shown by solid line). By switching this switch, the gate of field effect transistor 1 becomes 0 bias, so it shows the original high impedance of the field effect transistor, and the Q of the tuning circuit at this time
This is the Q _O when _L has almost infinite impedance, and a high Q can be obtained. In this case, since the wiring leading to the changeover switch 11 is on the ground side of the secondary coil, the Q does not decrease due to the routing of the wiring in the DX reception mode, and high Q and high sensitivity can be ensured. Next, when in the so-called local reception mode, which is a strong electric field, the changeover switch 11 is switched to the second contact 10c side. By switching this switch, a bias current flows from the bias current supply source (+B) to the ground through the resistor 12 and the diode between the gate and source (or drain) of the field effect transistor 1.
The impedance decreases in accordance with the bias current, and the Q of the tuned circuit decreases, resulting in a decrease in sensitivity. Note that even when a bias signal is supplied to the field effect transistor 1, the other end of the secondary coil 5 is AC grounded via the bypass capacitor 9, so that adverse effects caused by controlling the bias signal are prevented. be done.

In the above example, a sensitivity switching circuit is added to the amplification circuit using a field effect transistor, but in addition, for example, a field effect transistor for amplification, the first and second coils electromagnetically coupled to each other, and the A sensitivity switching circuit constructed as shown in FIG. 2 can also be added to a mixer circuit comprising a resonant circuit constructed of a capacitor connected to one coil.

Further, as the sensitivity switching means, for example, a variable resistor or automatic gain control (AGC) may be used in place of the changeover switch 11, and the former can be used as manual gain control (MGC), and the latter can be used as high frequency automatic gain control. .

As mentioned above, according to the present invention, the Q of the resonant circuit is controlled by utilizing the decrease in impedance of the field effect transistor for amplifying high frequency signals, and the receiving sensitivity is thereby controlled. A decrease in Q due to wiring during an electric field is avoided, and high sensitivity can be ensured.
Further, the circuit configuration is simple, and sensitivity reduction can be freely selected by controlling the bias current flowing through the gate source (or drain). Furthermore, since there is no need to use a diode switch or the like, it can be provided at low cost.

[Brief explanation of the drawing]

FIG. 1 is a tuning circuit diagram of a conventional receiver, and FIG. 2 is a tuning circuit diagram of a receiver according to the present invention. 1 is a field effect transistor, 2 is a resonant circuit, 9
1 is a bypass capacitor, and 11 is a sensitivity selector switch.

Claims (1)

[Claims for Utility Model Registration] A tuning element is connected to the primary coil.
In a tuned circuit of a receiver, a field effect transistor for amplifying a signal induced in the secondary coil is connected to one end of the secondary coil that is electromagnetically coupled to the secondary coil. A tuning circuit for a receiver, characterized in that the other end is grounded via a capacitor and a bias control means is connected to the other end to control the bias of the field effect transistor to control the Q of the tuning circuit.