JPH0254704B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oscillation circuit used in a tuner of a television receiver or the like.

In recent years, broadcast reception functions in the UHF band have become indispensable for television receiver tuners and the like. Therefore, the oscillation circuit used in such a tuner must be configured to have sufficient characteristics in the UHF band, but in the UHF band, stray inductance and This has a large effect on the characteristics of stray capacitance, so in most cases, as a practical configuration to obtain the required characteristics, circuit components are arranged spatially and wired three-dimensionally (hereinafter referred to as three-dimensional structure). structure) is adopted.

However, such a three-dimensional structure has poor workability and is problematic in terms of cost, so a planar structure using a printed circuit board has been desired.

However, with a planar structure, the arrangement of circuit components is limited to a plane, so the area occupied is larger than that of a three-dimensional structure, and there are also constraints on printed pattern formation on the board, resulting in longer wiring sections. At the same time, the wiring patterns become unnecessarily close to each other, which increases stray inductance and stray capacitance compared to a three-dimensional structure, which has a large negative impact on the characteristics of the oscillation circuit, making it impossible to obtain the required characteristics. .

In particular, the effect is large when stray inductance increases and the ground terminal of each component is no longer kept at a sufficiently low potential with respect to the common potential point (hereinafter referred to as ground).For example, in an oscillation circuit, the ground terminal of the oscillation transistor If there is a stray inductance greater than a certain value between the capacitor and ground, normal oscillation will not occur.

This will be explained with respect to the circuit shown in FIG.

In FIG. 1, 1 is an oscillation transistor, 2
3 is a feedback capacitor, 3 is a bypass capacitor, 4 is a coupling capacitor, 5 is a tracking capacitor for tuning, 6 is a tuning coil, and 7 is a variable capacitance diode for tuning. 8 is a resonant circuit that determines the oscillation frequency of the oscillation circuit.

Inductances 9 and 10 are inevitably formed due to the circuit configuration, and 9 is named base lead inductance, and 10 is named variable capacitance diode lead inductance.

This oscillation circuit is composed of a base-grounded transistor 1, which oscillates due to the feedback effect generated from the collector to the emitter by a capacitor 2, and a resonant circuit 8.
The oscillation operation is performed at a frequency determined by .
Then, by changing the capacitance of variable capacitance diode 7,
It is now tuned to the frequency required for UHF band tuners. This is a so-called electronic tuning type.

Now, in such an oscillation circuit, it has a planar structure using a printed circuit board, for example, and as a result, the base lead inductance 9 and the variable capacitance diode lead inductance 10 increase, and if the inductance value exceeds a certain limit, the oscillation operation will be affected. It will appear.

Among the effects when the lead inductances 9 and 10 increase, a particular problem is the hysteresis phenomenon when the oscillation frequency is changed.

In other words, as shown in Figure 2, if the horizontal axis represents the capacitance of the tuning diode, and the resulting change in the oscillation frequency is represented on the vertical axis, part of the change is due to the hysteresis phenomenon, as shown in the curve in the figure. If the capacitance of the tuning diode is changed, the oscillation frequency will change dramatically within a certain capacitance range and will suddenly shift from a low frequency to a high frequency or vice versa, making it difficult to obtain sufficient tuning. You won't be able to take it.

This phenomenon does not occur when the lead inductances 9 and 10 are suppressed to sufficiently small values. However, when using a planar structure as described above, the lead inductances 9 and 10 cannot be made as small as when using a three-dimensional structure, so hysteresis inevitably occurs, making it impossible to obtain a product that can withstand practical use. .

An object of the present invention is to provide an oscillation circuit which eliminates the above-mentioned drawbacks of the prior art and whose characteristics do not deteriorate even in a planar structure, which is excellent in terms of production.

In order to achieve this object, in the present invention, the tracking capacitor is constituted by two capacitors, one of which is grounded to the ground of the oscillation transistor, and the other is connected to the ground of the resonant circuit.

Hereinafter, embodiments of the present invention will be described in detail based on the drawings. FIG. 3 shows an oscillation circuit according to an embodiment of the present invention. Parts equivalent to or identical to those of the circuit in FIG. 1 are given the same numbers, and detailed explanation thereof is omitted. This embodiment is different from the circuit shown in FIG. 1 in that the tuning tracking capacitor 5 is composed of two capacitors (capacitances), and the tracking capacitor C _Tj formed on the ground side of the tuning diode and the oscillation A tracking capacitor C _Tb is formed on the base side of the transistor 1.

Next, its operation will be explained.

As already explained, the base lead inductance 9 and the variable capacitance diode lead inductance 1 are reduced by making the oscillation circuit a planar structure.
When 0 increases, a hysteresis phenomenon occurs in the oscillation operation.

Therefore, if the capacitance of the tracking capacitor is reduced, the hysteresis phenomenon can be suppressed, but since the tracking becomes significantly different, the noise figure, gain, etc., which are the basic characteristics of the tuner, deteriorate and become unusable.

This state will be described with reference to FIG.

The characteristic curve in Figure 4 shows the tracking capacitance C _Tb on the base/earth side on the horizontal axis, and the tracking capacitance C _Tj on the diode/earth side on the vertical axis, with the tracking state as a parameter. Curve A shows a state where a tracking abnormality occurs where the local speed becomes faster, curve B shows a normal tracking state, and curve C shows a state where a tracking abnormality occurs where the local speed becomes slower. In each curve, the x mark indicates a portion where a hysteresis phenomenon occurs, and the 〇 mark indicates a portion where a hysteresis phenomenon is suppressed.

Here, the tracking capacity of the tuning diode is
The case where C _Tj is 7PF and the tracking capacitance C _Tb on the base/earth side of the transistor is 0PF corresponds to the conventional case.

As is clear from FIG. 4, the hysteresis phenomenon can be suppressed by reducing the tracking capacitance C _Tj . However, normal tracking cannot be obtained, which affects the NF characteristics, power gain, etc.

However, in the present invention, since the reduced tracking capacitance is compensated for by the tracking capacitance C _Tb on the base side of the transistor, the problem of tracking abnormality is solved. Furthermore, an oscillation circuit with sufficient characteristics is obtained even if the inductance due to the base lead and the lead inductance of the tuning diode become considerably large. As a result, the oscillation circuit can have a planar structure with excellent workability.

Further, in the above embodiments, the oscillation transistor is used as a common base circuit, but the present invention is not limited to this, and the same effect can be achieved even if the transistor is configured as a common collector circuit. Furthermore, as is clear from the above, the present invention is particularly effective when the oscillation circuit has a planar structure, but even when a three-dimensional structure is adopted, the lead inductance becomes large due to the structure. It works the same way.

As explained above, according to the present invention, even when a large stray inductance occurs due to the configuration of the oscillation circuit, abnormal oscillation does not occur in the UHF band, and it is possible to obtain an oscillation circuit with excellent characteristics with a simple configuration. can. As a result, even when the oscillation circuit in the UHF band has a planar structure with good workability,
An oscillation circuit that does not cause abnormal oscillation can be provided.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional oscillation circuit used in a UHF tuner, etc., FIG. 2 is a characteristic curve diagram thereof, and FIG. 3 is a circuit diagram of an oscillation circuit according to an embodiment of the present invention. FIG. 4 is a diagram of its characteristic curve. 1...transistor, 6...tuning coil, 7...
...variable capacitance diode, 8...resonant circuit, C _Tb ,
C _Tj ...Tracking capacitor for tuning.

Claims (1)

[Claims] 1 A resonant circuit that uses a variable capacitance diode at one end of a tuning coil and a tuning capacitor at the other end, and an oscillation transistor that is coupled at high frequency to a connection point between the tuning coil and the tuning capacitor via a capacitor. In a local oscillation circuit having a tuning coil, the tuning capacitor is divided into a first and a second capacitor, and one end of each of the first and second capacitors is connected to a variable capacitance diode of the tuning coil. The other connecting end of the first capacitor is connected to the ground terminal of the oscillation transistor, and the other connecting end of the second capacitor is connected to the connecting end on the opposite side. A local oscillation circuit characterized in that a capacitive diode is connected to a connecting end on the opposite side of the connecting end to a tuning coil.