JP2565979B2 - Local oscillator circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a local oscillator circuit of a receiver for receiving audio signals in, for example, AM, FM bands and television bands.

(Prior Art) Recent tuner circuits have been developed so that they can receive not only AM and FM band signals but also television band audio signals. In an electronic tuner circuit using a varicap diode as an element for changing the oscillation frequency, FM
It has two local oscillating circuits, one for audio and one for television, and is configured to share only the mixing circuit.

FIG. 4 shows a front end circuit of a 2-band receiver capable of receiving audio signals in the FM band and the television band.

Reference numeral 11 denotes a mixing circuit, to which the RF signal from the FM high frequency amplifier 12 and the local oscillation output from the FM oscillator 13 are input when the FM signal is received. Further, at the time of receiving the television audio signal, the RF signal from the TV high frequency amplifier 14 and the local oscillation output from the TV oscillator 15 are input.

Switches SW1 and SW2 are band changeover switches
FM high-frequency amplifier 12 and FM oscillator 1 in M reception mode
Connect the power supply to 3 and enter T in the TV reception mode.
A power supply is connected to the V high-frequency amplifier 14 and the TV oscillator 15. The mixing circuit 11 includes a mixing transistor Tr and an intermediate frequency resonance circuit.
11a, the output of which is derived from the emitter or collector of the transistor Tr and is supplied to the amplifier via the intermediate frequency filter.

The circuit shown in FIG. 4 is the transistor Tr of the mixing circuit 11.
This is a type in which the local oscillation output is directly injected into the base of. For this reason, if the oscillation level of the oscillator is set to a sufficient level to stabilize the oscillation and supply it to the base of the transistor Tr, it is unavoidable to rely on the impedance of the transistor Tr and eventually a strong input characteristic can be obtained. I can't
The oscillation may become unstable. In addition, transistor Tr
Since it is injected directly into the base of, the output contains a large proportion of unwanted radiation. Further, in the oscillators 13 and 15, the mutual resonance circuits are likely to electrically affect each other, and the oscillation is likely to be unstable.

FIG. 5 is a circuit devised to improve the problems of the circuit of FIG.

The FM oscillator 21 and the buffer circuit 22 including the differential pair transistor are configured as a first group, and the TV oscillator 25 and the buffer circuit 26 including the differential pair transistor are configured as the second group. The FM oscillator 21 and the buffer circuit 22 are simultaneously turned on or off by the switches S11 and S12, and the TV oscillator 25 and the buffer circuit 26 are switched to the switch S21.
And S22 simultaneously turn off or simultaneously turn on. That is,
The switches S11 and S12, S21 and S22 are interlocked with each other, and can switch on and off one of the first and second sets. The outputs of the buffer circuits 22 and 26 are supplied to the common mixing circuit 28.

The circuit of FIG. 5 described above is suitable for being integrated into an IC, but there is a problem that the chip size becomes large due to the large number of prime numbers. In addition, the price is also high.

(Problems to be Solved by the Invention) As described above, in the conventional local oscillator circuit for two bands, if it is the one shown in FIG. The one shown in FIG. 5 has a problem that the price is unavoidably increased.

Therefore, an object of the present invention is to provide a local oscillation circuit having excellent characteristics and stability without causing a large increase in price.

[Configuration of the Invention] (Means for Solving the Problem) According to the present invention, a switching circuit switches a first oscillation circuit and a second oscillation circuit having different oscillation frequency regions so that one of them is turned on and the other is turned off. The output of each oscillation circuit is supplied to the differential input of one differential buffer circuit.

(Operation) By the above means, it is possible to obtain a system path for taking the oscillation output from the output end of the differential buffer circuit and supplying it to the mixing circuit. Therefore, it is easy to adjust the oscillation circuit for stabilizing the oscillation, and The circuit has a simple structure and the price does not increase. Since the oscillator on the non-operating side has a low output impedance, it functions as a bypass circuit and does not adversely affect the other oscillator.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, 31 is an FM oscillator, which is grounded via a current source 32 and a switch 33. When the switch 33 is turned off, the FM oscillator 31 is 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 the base of one transistor Tr ₁₁ forming the differential buffer circuit 50.

On the other hand, 41 is a TV oscillator, which includes a current source 42 and a switch 43.
Grounded through. When the switch 43 is turned on, the TV oscillator 41 is on, and when the switch 43 is turned off, it is turned off. The oscillation output of the TV oscillator 41 is supplied to the base of the other transistor Tr ₁₂ forming the differential buffer circuit 50 via the coupling capacitor 44.

The transistors Tr ₁₁ and Tr _{12 form} a differential pair, a common emitter is grounded via a constant current source 51, and respective collectors are connected to a power source via resistors 52 and 53, respectively. Also, each collector is connected to a common mixing circuit. Furthermore, resistors 54 between the base of the power supply and the transistor Tr _11, Tr _12,
55 is connected for base bias.

In the above circuit, when obtaining the oscillation output for FM band reception, the switch 33 is turned on and the switch 43 is turned off,
The FM oscillator 31 is turned on and the TV oscillator 41 is turned off. Accordingly, the oscillation output for FM band reception, the transistor Tr ₁₁
Injected into the mixing circuit via the base collector of the. Conversely, if you want to obtain the oscillation output for television band reception,
The switch 43 is turned on and the switch 33 is turned off. As a result, the oscillation output of the TV oscillator 41 is supplied to the mixing circuit via the base / collector of the transistor Tr ₁₂ .

The output impedance of the oscillator that is turned off by the band switching circuit formed by the switches 33 and 43 becomes extremely small. This is because the capacitor that constitutes the oscillation circuit functions as a bypass capacitor. Therefore,
One input terminal of the differential buffer circuit 50 (the side where the oscillator is off) is equivalent to being grounded, and does not adversely affect the other oscillator. In addition, each oscillator 31,4
Since the output of 1 is supplied to the mixing circuit via the buffer circuit, it is convenient to obtain the stabilization of the oscillator. High degree of freedom. Moreover, the oscillation output injected into the mixing circuit is also stabilized by the buffer circuit. Moreover, the structure of the buffer circuit is simple and does not increase the price.

The oscillator used in this embodiment may be either the Colpitts type shown in FIG. 2 or the clapp type shown in FIG. In FIG. 2, Tr ₂₀ is an oscillation transistor, capacitors C ₁₁ , C ₁₂ and coil L ₁₁ are resonance elements, and C ₁₃ is a feedback capacitor. In FIG. 3, Tr ₃₀ is an oscillation transistor, capacitors C ₂₁ , C ₂₂ and coil L ₂₁ are resonance elements, and C ₂₃ is a feedback capacitor.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a local oscillation circuit having excellent characteristics and stability without causing a large increase in price.

[Brief description of drawings]

1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 and FIG. 3 are diagrams showing an example of the oscillator shown in FIG. 1, FIG. 4 and FIG.
Each of the figures is a circuit diagram showing a conventional oscillator circuit. 31 …… FM oscillator, 32,42 …… current source, 33,43 …… switch, 34,44 …… coupling capacitor, 50 …… differential buffer circuit, 52 to 55 …… resistor.

Claims (1)

(57) [Claims] 1. A first and a second oscillation circuit having different oscillation frequency regions, a switching circuit for switching the first and the second oscillation circuit so that one is on and the other is off, and the first and second oscillation circuits are provided. A differential buffer circuit in which an output terminal of the second oscillator circuit is connected to a differential input terminal via a coupling capacitor, respectively, and a differential output terminal of the differential buffer circuit is commonly connected to a mixing circuit. A local oscillator circuit characterized in that