JP2003309430A - Local oscillation circuit for receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a local oscillation circuit capable of stably being oscillated over a wide range so as to extend the oscillating frequency range. <P>SOLUTION: The local oscillation circuit comprises: an oscillation transistor 1, a feedback circuit 2 connected between the emitter and base of the transistor; and a tuning circuit 3 connected to the base of the transistor and the resonance frequency of which is controlled by a tuning voltage, and a varactor D3 for adjusting a feedback amount is connected to the feedback circuit 2 and the varactor D3 can adjust the oscillation gain. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a local oscillator circuit of a receiver, and more particularly to a technique for increasing oscillation stability and expanding an oscillation frequency range.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional local oscillator circuit of a receiver. In FIG. 3, reference numeral 1 is an IC (integrated circuit) that constitutes the mixer section, and includes a transistor Q1 and its emitter resistance R1.
Others are omitted.

Reference numeral 2'is a feedback circuit for determining the oscillation gain,
It comprises a feedback capacitor C1 connected between the base and emitter of the transistor Q1, and a feedback capacitor C2 connected between the emitter of the transistor Q1 and ground.

3 is a tuning voltage VT applied to the tuning terminal 4.
Is a tuning circuit that determines the resonance frequency by: variable capacitance diodes D1, D2, capacitances C3, C4, C5, and a resistor R
2, R3, R4 and inductor L1. Variable capacitance diodes D1 and D2, capacitances C3 and C4 are for tuning, and capacitance C
5 is for coupling for connecting the tuning circuit 3 to the base of the transistor Q1, inductor L1 is for tuning, resistor R2 is for supplying tuning voltage VT to the variable capacitance diode D1, and R3 and R4 are for variable capacitance diode D2. It is for supplying the tuning voltage VT.

C5 is a bypass capacitor for bypassing the oscillation signal and is connected to the power supply terminal 5.

In this local oscillator circuit, the transistor Q1
A signal is positively fed back from the emitter of the transistor Q1 to the base by the feedback circuit 2'and the feedback circuit 2'and the tuning circuit 3
Oscillation is performed at a frequency determined by.

[0007]

However, in the conventional local oscillation circuit, since the feedback amount of the feedback circuit 2'is fixed,
The variable range of the oscillation frequency depends on the tuning circuit 3, that is, is determined by the variable capacitance range of the variable capacitance diodes D1 and D2.

Therefore, even if the variable range of the tuning circuit 3 is expanded in order to expand the variable range of the oscillation frequency, the oscillation gain decreases and the oscillation stability decreases at the band edge on the low frequency side. The variable range could not be expanded.

The present invention has been made in view of the above circumstances, and an object thereof is to make it possible to adjust a feedback amount of a feedback circuit, that is, an oscillation gain, so that stable oscillation is performed in a wide range, and an oscillation frequency range is set. It is to provide a local oscillation circuit that can be expanded.

[0010]

According to a first aspect of the present invention, an oscillation transistor, a feedback circuit connected between the emitter and the base of the transistor, and a resonance frequency which is connected to the base of the transistor and has a tuning voltage are used. In a local oscillation circuit of a receiver having a controlled tuning circuit, a variable capacitance element for adjusting a feedback amount is connected to the feedback circuit, and an oscillation gain can be adjusted by the variable capacitance element. The local oscillator circuit of the receiver is used.

According to a second aspect of the invention, there is provided a local oscillation circuit of a receiving device according to the first aspect of the invention, wherein the variable capacitance element is controlled by the tuning voltage.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION [First Embodiment] FIG. 1 is a diagram showing a local oscillator circuit according to a first embodiment of the present invention.
Reference numeral 1 denotes an IC (integrated circuit) that constitutes the mixer section, and the transistor Q1 and its emitter resistance R1 are omitted.

Reference numeral 2 is a feedback circuit for determining the oscillation gain. A feedback capacitance C1 has one end connected to the base of the transistor Q1, a capacitance 2 connected between the other end of the feedback capacitance C1 and the ground, a variable capacitance diode D3, and a variable capacitance diode. D3 and capacity C
1. A coupling capacitor C7 connected between the common connection point of C1 and C2 and the emitter of the transistor Q1, and a tuning connected between the common connection point of the variable capacitance diode D3 and the capacitors C1 and C2 and the tuning terminal 4. Resistor R for supplying voltage VT
It consists of 5.

3 is a tuning voltage VT applied to the tuning terminal 4.
Is a tuning circuit that determines the resonance frequency by: variable capacitance diodes D1, D2, capacitances C3, C4, C5, and a resistor R
2, R3, R4 and inductor L1. Variable capacitance diodes D1 and D2, capacitances C3 and C4 are for tuning, and capacitance C
Reference numeral 5 is for coupling for connecting the tuning circuit 3 to the base of the transistor Q1, inductor L1 is for tuning, resistor R2 is for supplying tuning voltage VT to the variable capacitance diode D1, and resistors R3 and R4 are for variable capacitance diode D2. For supplying the tuning voltage VT. The tuning circuit 3 is the same as that described in FIG.

C6 is a bypass capacitor for bypassing the oscillation signal, and is connected to the power supply terminal 5.

In this local oscillator circuit, the transistor Q1
A signal is positively fed back from the emitter of the transistor Q1 to the base by the feedback circuit 2, and oscillation is performed at a frequency determined by the feedback circuit 2 and the tuning circuit 3.

The variable capacitance diodes D1, D2, D3 have the same or similar voltage / capacitance characteristics, and the resistance R
2, tuning voltage VT applied via R3, R4 and R5
Thus, the capacitance value is controlled and the desired oscillation frequency is determined.

In order to obtain a low oscillation frequency, the tuning voltage VT is lowered so that the variable capacitance diode D1,
The capacitance value of D2 increases and the resonance frequency of the tuning circuit 3 decreases. At this time, since the capacitance value of the variable capacitance diode D3 also increases, the total capacitance with the capacitance C2 increases, its impedance decreases, the amount of feedback decreases, and the oscillation gain increases.

In order to obtain a high oscillation frequency, the tuning diode VT is increased to increase the variable capacitance diode D1,
The capacitance value of D2 decreases and the resonance frequency of the tuning circuit increases. At this time, the capacitance value of the variable capacitance diode D3 also decreases, but since there is the capacitance C2 connected in parallel, an increase in impedance at that portion is suppressed, and an increase in feedback amount and a decrease in oscillation gain are also suppressed.

As described above, the feedback amount of the feedback circuit 2 changes in association with the variation of the resonance frequency of the tuning circuit 3, the oscillation gain increases more than before, especially at a low frequency, and the oscillation gain decreases at a high frequency. Since it is suppressed, its oscillation gain is optimized and oscillation is stabilized. Further, by stabilizing the oscillation, the variable capacitance diodes D1 and D2 of the tuning circuit 3 are
The capacitance can be changed over a wider range, and the oscillation frequency can be changed over a wider range.

[Second Embodiment] FIG. 2 is a diagram showing a local oscillation circuit according to a second embodiment of the present invention. The tuning circuit 3 of the local oscillation circuit shown in FIG. 1 is tuned including a band switching unit. It is replaced with the circuit 3A. This is applied to a VHF tuner including a CATV channel, replacing the inductor L1 with a high band (HB) inductor L2,
In addition, a low band (LB) inductor L3, a high band capacitor C8, a low band capacitor C9, a switch diode D4, a high band selection terminal 6, a low band selection terminal 7, voltage supply resistors R6 and R7, and a coupling capacitor are newly added. C10 is provided.

In this local oscillator circuit, when the VHF high band is selected, the high band selection terminal 6 is set to a high voltage (“H”) and the low band selection terminal 7 is set to a low voltage (“L”). As a result, the diode D4 is turned on, so that the capacitance C3, the variable capacitance diodes D1 and D2,
The inductor L2 and the capacitor C8 are dominant (the impedance of the inductor L3 and the capacitor C9 is larger than the impedance of the capacitor C8), and the frequency range is VH.
Selected for F high band.

When selecting the VHF low band, the high band selection terminal 6 is set to a low voltage ("L") and the low band selection terminal 7 is set to a high voltage ("H"). As a result, the diode D4 is turned off, so that the capacitance C8 is removed from the circuit and the frequency range is selected for the VHF low band by the capacitances C3 and C9, the variable capacitance diodes D1 and D2, and the inductors L2 and L3.

[0024]

As described above, according to the present invention, stable oscillation can be obtained and the oscillation frequency range can be expanded.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a local oscillator circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a local oscillator circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a local oscillator circuit of a conventional embodiment.

[Explanation of symbols]

1: IC (integrated circuit) 2, 2 ': Feedback circuit 3,3A: Tuning circuit 4: Tuning terminal 5: Power supply terminal 6: High band selection terminal 7: Low band selection terminal

Claims (2)

[Claims] 1. A local part of a receiver including an oscillating transistor, a feedback circuit connected between an emitter and a base of the transistor, and a tuning circuit connected to the base of the transistor and having a resonance frequency controlled by a tuning voltage. In the oscillation circuit, a local oscillating circuit of a receiving device, wherein a variable capacitance element for adjusting a feedback amount is connected to the feedback circuit, and the oscillation gain can be adjusted by the variable capacitance element. 2. The local oscillator circuit of a receiving device according to claim 1, wherein the variable capacitance element is controlled by the tuning voltage.