JPS6230343Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an improvement of a receiving band switching circuit in an AM receiver.

Multiple reception bands (e.g. MW: 520~
In AM receivers that can receive 1680KHz, LW: 150 to 370KHz), it is extremely difficult to support the above multiple reception bands with a single oscillation circuit in the local oscillation circuit, so it is usually necessary to use multiple oscillations. A circuit is provided and configured to be switched at appropriate times.

To explain this in more detail with reference to Figure 1,
In the figure, VC is a variable capacitor, and by selectively switching between the variable capacitor and the oscillation coils L1 and L2 with a switch SW1, one of the resonant circuits is formed.

For example, variable capacitor VC and oscillation coil L
1, the oscillation frequency is 975~2135KHz, and variable capacitor VC and oscillation coil L2 allow the oscillation frequency to be 605~2135KHz.
An oscillation frequency of 825KHz can be obtained.

The oscillation output taken out from the intermediate tap of each oscillation coil L1, L2 is selectively switched by a switch SW2, and is input to the emitter of a mixed transistor TR1.

On the other hand, the received high frequency signal is applied to the base of the mixed transistor TR1, and an intermediate frequency signal (455KHz) corresponding to the difference between the two signals is output from the collector and selectively input to the intermediate frequency transformer L3 by the switch SW3. Ru.

Note that the switches SW1, SW2, and SW3 are configured to move, respectively.

For example, if switches SW1, SW2, and SW3 are connected to terminal a, the variable capacitor
The oscillation output (975~2135KHz) from the oscillation circuit of VC and oscillation coil L1 is input to mixed transistor TR1, and the difference between it and MW (520~1680KHz) is
The 455KHz intermediate frequency signal is transferred to intermediate frequency transformer L3.
is input into the MW band reception state.

On the other hand, when switches SW1, SW2, and SW3 are each connected to terminal b, the LW band reception state is established in the same manner.

In a configuration in which a plurality of oscillation circuits are switched in response to a plurality of reception bands to obtain a corresponding local oscillation output, a problem arises in that noise occurs when the switches are switched.

In particular, since the switches SW2 and SW3 are interposed in the signal path, the influence is large, impairing the hearing sense and causing a decrease in product value.

Therefore, this invention is a switching circuit which suppresses the generation of noise at the time of switching with an extremely simple configuration, and further simplifies combination with accessory circuits such as a reception band display circuit and a muting circuit.
Below, FIGS. 2 and 3 will be explained in more detail. (In Figures 2 and 3, the same numbers as in Figure 1 indicate the same parts.) In Figure 2, one end of each of the oscillation coils L1 and L2 is selectively connected to a variable capacitor VC via a switch SW1. The other ends of each are grounded through forward-connected diodes D1 and D2, and selectively connected to the B power source through a resistor R1 and a switch SW4, respectively.

Furthermore, the oscillation outputs to be taken out from the respective intermediate taps of the oscillation coils L1 and L2 are respectively input to the emitter of the mixed transistor TR1 via the DC blocking capacitor C1, and the intermediate frequency which is the difference from the high frequency signal applied to the base is input to the emitter of the mixed transistor TR1. The signal is taken out from the collector and transferred to intermediate frequency transformer L.
3 is input.

Note that SW1 and SW4 are switched in conjunction with each other.

In the above configuration, switches SW1 and SW4
When connected to terminal a, diode D1 is forward biased by power supply B and turns on, activating an oscillation circuit consisting of oscillation coil L1 and variable capacitor VC, and its output is taken out from the intermediate tap and connected to the mixed transistor. The intermediate frequency signal is applied to the emitter, and as described above, the intermediate frequency signal is input to the intermediate frequency transformer L3.

Further, when the switches SW1 and SW4 are connected to terminal b, the diode D2 is turned on, and an oscillation output is obtained from the variable capacitor VC and the oscillation coil L2, and the same operation as described above is performed.

FIG. 3 shows the band switching circuit of this invention with a band switching display and muting circuit added.

Since the switching circuit is the same as that shown in FIG. 2, the explanation will be omitted.Light emitting diodes D3 and D4 are connected to terminals a and b of the switch SW4 through resistors, respectively, to form a display circuit, and the terminal a , b through capacitor C3,
NPN transistors with their bases connected
TR2 is connected to the base of PNP transistor TR3, and the base is grounded via resistor R3. Furthermore, the emitter of transistor TR2 and the collector of TR3 are grounded, respectively, and the collector of TR2 and the emitter of TR3 are connected to each other after the detection output. It is connected to the audio signal path 2 to constitute a muting circuit.

Next, to explain the operation of this circuit, the switch
When SW4 is connected to terminal a, the oscillation coil L1 oscillates and the light emitting diode D3
emits light to indicate the reception band.

Next, when switching switch SW4 to terminal b, the switch piece separates from terminal a and at the same time terminal a
falls to the O potential and the light emitting diode D3 goes out, and the falling part of this voltage waveform is connected to the capacitor C3.
It is converted into a negative pulse by a differentiating circuit consisting of a resistor R3, and this negative pulse makes TR3 conductive and short-circuits the audio signal path.

Also, at the same time that the switch piece contacts terminal b, voltage B is generated at terminal b and the light emitting diode D4 emits light, and the rising part of this voltage waveform is converted into a positive pulse by the differentiating circuit of capacitor C3 and resistor R3, and this positive pulse is The pulse causes TR2 to conduct and short the audio signal path.

With this, it is possible to mute the noise when the switch SW4 breaks and makes.

In this way, this invention connects diodes in the forward direction between multiple oscillation coils and the ground, and selectively applies a bias current to the diodes to make them conductive, so that only one corresponding oscillation circuit is connected. Since it is configured to operate and not to switch the signal path, the noise during switching is extremely small, and it is also extremely easy to connect a muting circuit as shown in Figure 3, so it is used in combination. This has practical value as it can completely prevent noise during switching, enable good reception, and significantly improve product value.

[Brief explanation of the drawing]

FIG. 1 is a conventional band switching circuit diagram, FIG. 2 is a band switching circuit diagram of this invention, and FIG. 3 is a circuit diagram of an example of the same use. L1 and L2 are local oscillation coils, D1 and D2 are diodes, R1 is a resistor, SW4 is a switch, VC is a variable capacitor, and SW1 is a switch.

Claims (1)

[Scope of utility model registration request] In an AM receiver having a plurality of local oscillation coils L1 and L2 for receiving a plurality of reception bands, diodes D1 and D2 are connected in the forward direction between the respective oscillation coils L1 and L2 and the ground, and the diodes D1 and D2 are connected to each other. a resistor R1 connected in series with the diode to allow a forward bias current to flow;
A switch SW4 for selectively connecting the B power source to the resistor R1, and a switch SW1 corresponding to the switch SW4 for connecting one of the oscillation coils L1 and L2 to the variable capacitor VC. A receiving band switching circuit for an AM receiver characterized by the following.