JPS5830336Y2 - local oscillation circuit - Google Patents

Description

[Detailed description of the invention] This invention relates to a local oscillation circuit for a multi-band radio receiver, and its purpose is to control the bias voltage of the oscillation transistor of the frequency band that is not being received. It is an object of the present invention to provide a local oscillation circuit which can stop the oscillation of a local oscillator and short-circuit the oscillation coil by the above-mentioned transistor, and which requires fewer circuits of band changeover switches.

Figure 1 is a circuit diagram of a local oscillator circuit for a conventional multi-band radio receiver, where MW indicates a local oscillator for medium waves, and Tro
is an oscillation transistor, to which a base voltage is applied by resistors R1 and R2, and a collector voltage is supplied through a resistor R3 and the primary side of the oscillation coil L.

Further, the emitter of the transistor Tr1 is the resistor R4.
It is connected to the tap on the primary side of the oscillation coil L through the capacitor C1 to perform oscillation, and the oscillation frequency is determined by the oscillation coil L and the variable capacitor VC.

Although FIG. 1 only shows such a local oscillator for medium-wave MW, a multi-band radio receiver is equipped with a plurality of other local oscillators, and these are used for short-wave MW, for example. SWl and shortwave2. If SW2 is used, a band changeover switch is required to switch these according to the reception frequency band.

Sl, S2. S3. S4 is a band changeover switch for this purpose, and a four-circuit, three-contact switch is used to switch between the MW and the local oscillators of SW1 and SW2 (not shown).

The switch S1 is used to switch the oscillation coil L to the variable capacitor VC, and the switch S2 is a switch on the secondary side of the oscillation coil L, and switches the oscillation output to the emitter of the frequency conversion transistor Tr2 through the capacitor C3. Inject into.

The switch S3 is for short-circuiting the oscillation coil L of the frequency that is not being received, and the oscillation coil of the lower frequency of the adjacent band is grounded through the converter C2.
In the illustrated MW reception state, none of the oscillation coils is grounded,
When receiving the SW1 band, the oscillation coil for the MW band is grounded, and when receiving the SW2 band, the oscillation coil for the SW1 band is grounded.

Furthermore, the switch S4 is for switching the power supply of the oscillation transistor 1r1.

Note that the transistor Tr2 injects oscillation output into its emitter, and also applies a received signal from the antenna circuit to its base, performs a frequency conversion function, and extracts an intermediate frequency output signal from its collector, but this part is the gist of this invention. Since it is not directly related to this, detailed explanation will be omitted.

As described above, in the conventional local oscillator circuit for a multi-band radio receiver, when switching the oscillation frequency band, four circuit changeover switches are required, which complicates the circuit configuration and increases costs. Another disadvantage is that harmful coupling is likely to occur between the oscillation coils of different frequency bands or between the antenna coil and the oscillation coil due to the vibration of the switching circuit.

This invention is intended to eliminate the above drawbacks, and is the second
The figure shows a circuit diagram of a local oscillation circuit according to one embodiment.

In this embodiment as well, in order to receive multiple frequency bands, a local oscillator for MW and a local oscillator for SW1 and SW2 (
A local oscillator (not shown) is provided, and each local oscillator is provided with an oscillation coil L and an oscillation resistor 1 to transistor Tri, but the oscillation function is almost the same as that of the conventional example, so a detailed explanation will be omitted.

The switch S1 is a three-contact switch for switching the oscillation coil L to the variable capacitor VC, and the oscillation transistor Tri is given a base voltage by resistors R1 and R2, but the resistor R2 is directly grounded as in the conventional example. It is connected to one end of the secondary side of the oscillation coil L, and the other end of the secondary side of the oscillation coil L is connected to the emitters of frequency converter 1 to transistor Tr2 through a capacitor C3. .

The switch S2 is used to ground the connection point between the secondary side of the oscillation coil L of the local oscillator in the receiving state and the resistor R2. In FIG. 2, the local oscillator MW is grounded, and the oscillation transistor Tri is connected to the resistor. It is in an oscillating state with a regular base bias voltage applied by R□ and R2, and the oscillation output on the secondary side of the oscillation coil L is generated by the capacitor C.
3 and is injected into the frequency conversion I transistor Tr□.

However, in the local oscillators SW1 and 5W2 (not shown) which are not grounded by the switch S2, the resistor R2 is not grounded, and the oscillation transistor Trl
cannot perform oscillation because the collector current is saturated due to overbiasing.

Further, in this case, the collector-emitter resistance of the transistor Tr1 becomes low, and the oscillation coil L becomes in a state equivalent to being short-circuited due to the low resistance.

In this way, the switch S2 maintains the local oscillator of the frequency band in the reception state in the oscillation state and supplies its oscillation output to the frequency conversion circuit, and also stops the oscillation of the local oscillator that is not in the reception state, Further, the oscillation coil L is short-circuited by the transistor Tr1.

As described above, in the circuit of this embodiment, the local oscillator can be switched using only two band switching switches by allowing the switch S2 to perform multi-functional actions.

As described above, when switching the local oscillator, this device can significantly reduce the number of changeover switch circuits compared to the conventional method, reduce the cost of the local oscillator circuit by 1 or more, and improve the reception condition. The oscillation coils of frequency bands not included in the frequency band are all short-circuited, not just the adjacent bands, and the short-circuiting effect is achieved by direct current control of transistors, which reduces the number of wires in the high-frequency circuit. Harmful coupling between the oscillation coils or between the oscillation coil and the antenna coil can be prevented, and wiring can be simplified by reducing the number of switching circuits, contributing to equipment downsizing and assembly work efficiency. It is effective though.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional local oscillation circuit for a multi-band radio receiver, and FIG. 2 is a circuit diagram of an embodiment of the local oscillation circuit for a multi-band radio receiver according to the present invention. MW...Local oscillator for medium waves, SWl...
...Local oscillator for short waves, SW2...Local oscillator for short waves, Tr+...Transistor for oscillation,
Tr2... Frequency conversion transistor, L...
...Oscillation coil, Sl, S2. S3゜S4...
・・Band selection switch, C1, C2, C3・・・・
・Capacitor, R,, R2, R3, R4...Resistance.

Claims (1)

[Scope of utility model registration request] In a multi-band radio receiver equipped with a plurality of local oscillators made up of oscillation transistors and oscillation coils, and a band changeover switch that switches the local oscillators according to the reception frequency band, there is a A second resistor is connected between the base and one end of the secondary side of the oscillation coil, and the other end of the secondary side of the oscillation coil is connected to a frequency conversion circuit. , by configuring the connection point between the secondary side of the oscillation coil and the second resistor to be selectively grounded by a changeover switch,
The oscillation transistor whose connection point between the second resistor and the secondary side of the oscillation coil is grounded by the changeover switch is given a normal base bias voltage by the first and second resistors and enters an oscillating state. At the same time, the oscillation output is supplied to the frequency conversion circuit, but the oscillation transistor whose connection point is not grounded by the changeover switch stops its oscillation action due to the base voltage becoming overbiased, and this transistor causes the oscillation coil to A local oscillation circuit characterized in that the circuit is short-circuited.