JPH01259631A - Local oscillating circuit - Google Patents

Abstract

PURPOSE:To make the title circuit excellent in characteristics and stability by switching first and second oscillating circuits whose oscillating frequency areas are different so that one may be on and the other may be off and supplying the outputs of the oscillating circuits to the differential inputs of one differential buffer circuit. CONSTITUTION:When a switch 33 is turned off, an FM oscillator 31 is off and when the switch 33 is turned on, it becomes on. The oscillation output of the FM oscillator 31 is supplied to the base of a transistor TR11 which is one component of a differential buffer circuit 50 through a coupling capacitor 34. On the other hand, a switch 43 is turned on, a TV oscillator 41 is on and when the switch 43 is turned off, it becomes off. The oscillation output of the TV oscillator 41 is supplied to the base of a transistor TR12 which is the other component of the differential buffer circuit 50 through a coupling capacitor 44. Thus, the title local oscillating circuit excellent in characteristics and stability is obtained without generating the large increase of cost.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a local oscillation circuit for a receiver that receives audio signals in, for example, AM, FM bands, and television bands.

(Prior Art) Recent cheese circuits have been developed to be able to receive not only AM and FM band audio signals but also television band audio signals. In an electronic tuner circuit that uses a Paris cap diode as an element to vary the rooting frequency,
It has two local oscillation circuits, one for FM and one for television, and is configured by sharing only the mixing circuit.

FIG. 4 shows a front end circuit of a two-band receiver capable of receiving audio signals in the FM band and the television band.

11Fi is a mixing circuit, and when receiving an FM signal, the RF multiplied signal from the FM high frequency amplifier 12 and the local oscillation output from the FM oscillator 13 are input. Furthermore, when receiving a television audio signal, the local oscillation output from the RF multiplied signal TV oscillator 15 from the TV high frequency amplifier 14 is input.

Switches SWI and SW2 are band changeover switches, and in the FM reception mode, the FM high frequency amplifier 12
A power source is connected to the FM oscillator 13, and a power source is connected to the TV high frequency amplifier B14 and the TV oscillator 15 in the television reception mode.

The mixing circuit 1 consists of a mixing transistor Tr and an intermediate frequency resonant circuit 11h, and the output is derived from the emitter or flipter of the transistor Tr and is supplied to an amplifier via an intermediate frequency filter.

The circuit of FIG. 4 above is based on the transistor T of the mixing circuit 11.
This is a type in which local oscillation output is directly injected into the r pace. For this reason, if you try to fully stabilize the oscillation by raising the oscillation level of the oscillator to a sufficient level and supply it to the pace of the transistor Tr, you will have to rely on the impedance of the transistor Tr, and in the end it will be impossible to obtain strong input characteristics. This may result in unstable oscillation. Furthermore, since the radiation is directly injected into the transistor Tr, a large proportion of unnecessary radiation is included in the output. Furthermore, the oscillators 13 and 15 tend to have mutual resonant circuits that tend to affect each other, and the oscillation tends to become unstable.

FIG. 5 shows a circuit devised to improve the problems of the circuit shown in FIG.

The FM oscillator 21 and the buffer circuit 22 consisting of a differential pair of transistors form a first set, and the TV oscillator 25 and a buffer circuit 26 consisting of a differential pair of transistors form a second group. has been done. The FM oscillator 21 and Gunofer circuit 22 are turned on or off simultaneously by switches S11 and S12, and the TV oscillator 25 and buffer circuit 26 are turned off or turned on simultaneously by switches 821 and 822. Ru. In other words, switch S11
and 812. S21 and 822 are interlocked, and the first
One of the second set can be turned on completely and the other set can be turned off. The outputs of the buffer circuits 22 , 26 are fed to a common mixing circuit 28 .

Although the circuit shown in FIG. 5 is suitable for IC implementation, there is a problem in that the chip size becomes large due to the large number of prime numbers. Along with this, the price is also good.

(Problem to be solved by the invention) As mentioned above, the conventional two-band local oscillation circuit is
The problem is that if the device shown in FIG. 4 is used, a certain amount of deterioration in characteristics and stability must be sacrificed, and if the device is provided by the device shown in FIG. 5, the price increases significantly.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a local oscillation circuit with excellent characteristics and stability without causing a significant increase in price.

[A1Q Construction of the Invention] (Means for Solving the Problem) This invention switches first and second oscillation circuits having different oscillation frequency regions so that one is turned on and the other is turned off by a switching circuit. The configuration is such that the output of each oscillation circuit is supplied to the differential input of one differential buffer circuit.

(Function) By the above means, it is possible to obtain a thread for taking out the oscillation output from the output terminal of the differential buffer circuit and supplying it to the mixing circuit, so that the oscillation circuit is adjusted to stabilize the oscillation. The buffer circuit has a simple configuration, and the cost does not increase. Since the oscillator on the non-operating side has a low output impedance, it functions as a pinocess circuit and does not adversely affect the other oscillator.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. Reference numeral 31 denotes an FM oscillator, which is grounded via a current source 32 and a switch 33. In FIG. When the switch 33 is turned off, the FM transmitter 31 is turned off, and when the switch 33 is turned on, it is turned on. The oscillation output of the FM oscillator 31 is supplied via the coupling capacitor 34 to one of the transistors Tr, 1 forming the differential buffer circuit 50.

On the other hand, 41 is a TV oscillator, which is grounded through a current source 42 and a switch 43'II. When the switch 43 is turned on, the TVV oscillator 41 is turned on, and when the switch 43 is turned off, the TVV oscillator 41 is turned off.

The oscillation output of the total TV oscillator 4 is supplied via a coupling capacitor 44 to the other transistor Tr12 constituting the differential amplifier circuit 5θ.

The transistors Tr11*Tr12 form a differential pair, with common emitters grounded via a constant current source 5 and collectors connected to a power source via resistors 52 and 53, respectively. Each collector is also connected to a common mixing circuit. Furthermore, resistors 54 and 55 are connected between the power supply and the paces of each transistor "11 + Tr12 for pace bias.

In the above circuit, when obtaining an oscillation output for FM band reception, the switch 33 is turned on, the switch 43 is turned off, the FM oscillator 3 is turned on, and all the TV oscillators 41 are turned off. As a result, the oscillation output for FM band reception is
It is injected into the mixing circuit via the pace collector of transistor Tr11. Conversely, when obtaining oscillation output for television broadcast band reception, switch 43 is turned on and switch 3 is turned on.
3 is turned off. As a result, the oscillation output of the entire TV oscillator 41 is supplied to the mixing circuit via the pace collector of the transistor Tr12.

By the band switching circuit using switches 33 and 43,
An oscillator that is turned off has a very small output impedance. This is because the capacitors forming the oscillation circuit function as pi-thousand capacitors. Therefore, one input terminal of the differential buffer circuit 50 (the side on which the oscillator is turned off) is equivalent to being grounded, and does not adversely affect the other oscillator. In addition, the output of each oscillator 31 and 4 is supplied to the mixing circuit via the oscillator circuit, which is convenient for stabilizing one oscillator.In other words, even if the oscillators are adjusted, the mixing circuit The effect of that adjustment is small! The degree of freedom of the bottom is at/-x
6, and the JM output from iL injected into the mixing circuit is also stabilized by the buffer circuit. Moreover, the configuration of the buffer circuit is simple and does not cause an increase in n-mountains.

The oscillator used in this embodiment may be either a Colpitts type oscillator shown in FIG. 2 or a Clapp type oscillator shown in FIG. 3.

In Fig. 2, Tr2o is a dust-generating transistor, capacitor C11s C12' coil Li1 is a resonance element, C
13 is a feedback capacitor. In Figure 3,
Tr50 is an oscillation transistor, capacitor C21*0
22. Coil L21 is a resonant element and C23 is a feedback capacitor.

[Effects of the Invention] As explained above, according to the present invention, a local oscillation circuit with excellent characteristics and stability can be obtained without causing a significant increase in price.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the invention, FIGS. 2 and 3 are diagrams showing an example of the oscillator shown in FIG. 1, and FIGS.
Each figure is a circuit diagram showing a conventional oscillation circuit. 31...FM oscillator, 32.42...Current source, 3
3.43...Switch, J4, 44...Coupling capacitor, 50...Differential buffer circuit, 52
~55...Resistance. Applicant's agent Patent attorney Takeshi Suzue 2 Figure 1 Figure 2 Figure g3

Claims (1)

[Claims] first and second oscillation circuits having different oscillation frequency regions; a switching circuit that switches the first and second oscillation circuits so that one is on and the other is off; and the first and second oscillation circuits. and a differential buffer circuit whose output end is connected to the differential input end via a coupling capacitor, respectively.
A local oscillation circuit characterized in that differential output ends of the differential buffer circuits are commonly connected to a mixing circuit.