JPH0815244B2 - High frequency oscillator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-frequency oscillator circuit used for a local oscillator circuit of a CATV (cable television) tuner.

[Outline of Invention]

The present invention relates to a high-frequency oscillator circuit in which the tuning frequency of the tuning circuit is prevented from being affected by stray capacitance by blocking the tuning circuit for oscillation from ground at high frequencies. By connecting another capacitor between the point and ground, parasitic oscillation can be suppressed, and safe oscillation operation is possible over the entire band of the VHF band CATV channel, for example.

[Conventional technology]

Recent television tuners are strongly demanded to be downsized and to have a low voltage (power saving), and are also required to have performances satisfying legal regulations such as interference wave removing ability and unnecessary radiation. In order to satisfy these requirements, a mixer using a balanced mixer and an IC configuration of a local oscillation circuit are being introduced, and the effect is enhanced in the CATV channel of the VHF band.

On the other hand, full-bandization of CATV channels in the VHF band of television tuners (up to 470 MHz reception) has progressed considerably in the United States, and is scheduled to start soon in Europe. The above mixer and local oscillator circuit IC
It was difficult to operate stably up to the maximum frequency of the F-band CATV channel. Therefore, in the past, the Hyper (Hype) which is the CATV band on the high frequency side in the VHF band
Part of the r) band (300MHz to 470MHz) extended the operating frequency of the UHF band local oscillation circuit and mixer to the lower side, or provided a dedicated local oscillation circuit. Therefore, there are problems that the cost increases due to the increase in the number of parts, the miniaturization is hindered, the interference wave removing capability and the unnecessary radiation are deteriorated when the hyperband is received.

In order to solve this problem, the applicant of the present invention filed a patent application No.
It proposes a high-frequency oscillator circuit as described in the specification of 282326. This high frequency oscillator circuit
It can operate stably in the band up to 470MHz and the local oscillation frequency 510MHz. By using this high frequency oscillation circuit, band switching can be performed only by switching the inductance, so that a single local oscillation circuit can receive up to the VHF hyperband. Therefore, cost reduction and miniaturization of the tuner are achieved.

The previously proposed high-frequency oscillation circuit suffers from the adverse effects of the diode for protecting the transistor inside the IC and stray capacitance that occur in the conventional configuration in which the oscillation feedback loop is connected to the ground pattern via the tuning circuit. It solves the problem that high-frequency oscillation becomes difficult, and has a configuration in which a tuning circuit is connected between the collector and base of the transistor inside the IC without a ground pattern.

FIG. 2 shows a previously proposed high-frequency oscillator circuit used as a local oscillator circuit part of an IC-mixed mixer and a local oscillator circuit. In FIG. 2, Q1 is a transistor for oscillation and Q2 is a transistor for feedback and amplification.

The collector of the transistor Q2 is connected to the power supply terminal to which the power supply voltage + B is supplied, the emitter of the transistor Q1 and the emitter of the transistor Q2 are connected, and the transistor Q1
Is connected to the base of the transistor Q2 via the feedback capacitor C1. In this case, a feedback capacitor C2 may be inserted between the base of the transistor Q1 and the base of the transistor Q2, as shown by the broken line in FIG.

The collector of the transistor Q1 is connected to the power supply terminal via the choke coil L2 and the resistor R. A capacitor C4 for grounding the power supply at high frequency is inserted between the power supply terminal and the ground. The collector of the transistor Q1 is a tuning coil
It is connected to the base of the transistor Q1 via L1 and the variable capacitor C3. The variable capacitance C3 is actually composed of a variable capacitance diode. The high-frequency oscillator circuit described above is a type of Colpitts oscillator circuit.

[Problems to be solved by the invention]

The high-frequency oscillation circuit shown in FIG. 2 may have an unstable operation due to parasitic oscillation in a high frequency region such as the VHF band hyperband. This point will be described with reference to FIGS. 3 and 4.

FIG. 3 is a configuration in which the circuit configuration of FIG. 2 is rewritten. In FIG. 3, as indicated by the broken line, the transistor
If the stray capacitance C5 of Q1 and Q2 exists between the emitter of the transistor Q2 and the power supply terminal + B, parasitic oscillation may occur due to the feedback and amplification transistor Q2 and the choke coil L1.

As shown in FIG. 4, the base of the oscillating transistor Q1 and the power supply line are easily coupled via the stray capacitance C6 of the ground pattern, and as a result, parasitic oscillation may occur in the circuit including the choke coil L2. It was

Regular oscillation stops due to the above parasitic oscillation,
An unstable operation such as generation of two frequency components as the IF output occurs.

Therefore, an object of the present invention is to improve a high-frequency oscillation circuit in which a tuning circuit for oscillation is cut off from a ground pattern, and the possibility of parasitic oscillation is eliminated, and a high-frequency oscillation circuit capable of performing stable oscillation operation. To provide a circuit.

[Means for solving problems]

According to the present invention, the collector of the first transistor Q1 is connected to a power supply terminal via a choke coil, and the first and second
The collectors of the transistors Q1 and Q2 are connected in high frequency, the emitters of the first and second transistors Q1 and Q2 are connected to each other, and the base and collector of the first transistor Q1 are connected via a tuning circuit. The tuning circuit is
A high frequency oscillation circuit characterized in that it is connected to the base of a second transistor Q2 via a feedback capacitor, and the base of the second transistor Q2 is grounded via a capacitor.

[Action]

Since the tuning circuit for oscillation is cut off from ground at high frequencies, the capacitor C7 has no effect on this tuning circuit. However, the tuning circuit in the case of parasitic oscillation is
Since the capacitor C7 is not cut off from the ground in high frequency, the frequency of parasitic oscillation is changed to a frequency that does not cause a problem, and the amount of feedback is reduced by the capacitor C7, and parasitic oscillation is less likely to occur.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. This embodiment is used for full bands including the hyper band of CATV channels in the VHF band.

In FIG. 1, Q1 is a transistor for oscillation and Q2 is a transistor for feedback and amplification. The collector of the transistor Q2 is connected to the power supply terminal to which the power supply voltage + B is supplied, the emitter of the transistor Q1 and the emitter of the transistor Q2 are connected, and the collector of the transistor Q1 and the base of the transistor Q2 form a feedback capacitor C1. Connected through. In this case, a feedback capacitor C2 may be inserted between the base of the transistor Q1 and the base of the transistor Q2, as indicated by the broken line in FIG.

The collector of the transistor Q1 is connected to the power supply terminal via the choke coil L2 and the resistor R. A capacitor C4 for grounding the power supply at high frequency is inserted between the power supply terminal and the ground. The collector of the transistor Q1 is a tuning coil
It is connected to the base of the transistor Q1 via L1 and the variable capacitor C3. The variable capacitance C3 is actually composed of a variable capacitance diode. The high-frequency oscillator circuit according to this embodiment is a Colpitts type.

In this embodiment, capacitor C7 is inserted between the base of transistor Q2 and ground. As mentioned above, coil L1
Since the tuning circuit composed of the variable capacitor C3 and the variable capacitor C3 does not include the ground pattern, the feedback amount does not change when the capacitor C7 is connected, and the capacitor C7 does not affect the normal oscillation operation.

When parasitic oscillation occurs due to the transistor Q2 and the choke coil L1, the capacitor C7 changes the frequency of parasitic oscillation to a frequency that does not cause a problem, and the amount of feedback is reduced by the capacitor C7, making parasitic oscillation less likely to occur. Become.

As shown in FIG. 5, when the capacitor C7 is connected when the stray capacitance C5 of the transistors Q1 and Q2 exists,
Capacitor C7 and capacitor C4 for grounding the power supply are connected in parallel to choke coil L2, and even if parasitic oscillation occurs, the oscillation frequency shifts to the lower side, and parasitic oscillation at the required frequency can be prevented.

Further, FIG. 6 shows a configuration in which the capacitor C7 is connected when the stray capacitance C6 exists between the base of the transistor Q1 and the ground. The capacitor C4 for grounding the power supply is larger than the capacitor C6 and is ignored in the high frequency equivalent circuit of FIG. As is clear from FIG. 6, since the parasitic oscillation tuning circuit is not cut off from the ground, the feedback amount is divided and reduced by the capacitors C1 and C7. In this way, the oscillation conditions are worsened, and parasitic oscillation is less likely to occur.

〔The invention's effect〕

According to the present invention, since the tuning circuit for oscillation is cut off from the ground in a high frequency, VH up to 470MHz is received.
It is possible to realize a local oscillation circuit for receiving all bands of the F-band CATV channel with a single configuration and switching the inductance. In particular, in the present invention, since the feedback capacitor and the semiconductor element are grounded via the capacitor, parasitic oscillation can be suppressed and the oscillation operation becomes stable.

[Brief description of drawings]

FIG. 1 is a connection diagram of an embodiment of the present invention, FIG. 2 is a connection diagram of a previously proposed high-frequency oscillator circuit, FIGS. 3 and 4
The drawings are connection diagrams used for explaining the previously proposed high-frequency oscillation circuit, and FIGS. 5 and 6 are connection diagrams used for explaining the effects of the present invention. Description of main symbols in the drawings Q1: Transistor for oscillation, Q2: Transistor for feedback and amplification, C1: Capacitor for feedback, C3: Variable capacitance, C4: Capacitor for grounding power supply, C7: Capacitor, L1: Tuning Coil, L2: choke coil.

Claims (1)

[Claims] 1. A collector of a first transistor is connected to a power supply terminal via a choke coil, and the first and second collectors are connected.
The collectors of the transistors are connected to each other at high frequencies, the emitters of the first and second transistors are connected to each other, the base and collector of the first transistor are connected via a tuning circuit, and the tuning circuit is Is connected to the base of the second transistor via a feedback capacitor, and the base of the second transistor is grounded via the capacitor.