JPH0116056B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic tuning tuner, and particularly to a vehicle-mounted electronic tuning tuner in which an antenna tuning circuit is formed by a variable capacitance diode.

Currently, electronic tuning tuners using variable capacitance diodes in antenna tuning circuits are widely used. As is well known, a variable capacitance diode is a variable capacitor element that utilizes the junction capacitance of the diode, and by varying the reverse bias voltage applied to it, its capacitance value can be changed and tuned to the signal of the desired frequency. fulfills the function of Note that in a frequency synthesizer type electronic tuning tuner, the reverse bias voltage, so-called tuning voltage, is supplied from a PLL (Phase Locked-Loop) circuit.

By the way, this variable capacitance diode is for example
They are broadly divided into low-voltage types that operate up to about 9V, and high-voltage types that operate up to about 25V. The low-voltage type has the advantage of being directly driven by the vehicle battery voltage (12V), but has the disadvantage of having non-linearity in the characteristic of tuning voltage versus capacitance value (described later). On the other hand, the high-voltage type has the advantage of having a linear characteristic of tuning voltage versus capacitance value (described later), but on the other hand, it cannot be directly driven by the vehicle's battery voltage, so a DC-DC converter etc. must be used in conjunction with it. There is a disadvantage.
In the electronic tuning tuner referred to in the present invention,
A low-voltage variable capacitance diode is used, taking advantage of the fact that it can be driven directly by the vehicle's battery voltage. However, in this case, the aforementioned disadvantages of low-voltage variable capacitance diodes, specifically when the antenna input becomes very large,
The problem of nonlinear distortion in the output audio must be solved.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an electronically tuned tuner capable of solving the above-mentioned problems.

In accordance with the above object, the present invention provides an electronic tuning tuner having an antenna tuning circuit constituted by a variable capacitance diode.
This feature is characterized by a lower quality factor.

The present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing the general configuration of an antenna tuning circuit that forms the main part of an electronic tuning tuner. In this figure, 10 is an antenna tuning circuit, its input is connected to the antenna ANT, and its output is an RF (radio frequency) amplifier RF・AMP.
connected to. The antenna tuning circuit 10 includes a variable capacitance diode 11 and a variable capacitance diode 11.
A DC blocking capacitor 12 connected in series with the antenna ANT, a tuning coil 13 forming a resonant circuit together with these capacitance components, a DC blocking capacitor 14 blocking the DC component entering from the antenna ANT, and a coupling coil for extracting the resonance output. It consists of 15.
Of these, the present invention mainly focuses on the variable capacitance diode 1.
This refers to 1. Various tuning voltages TV, which are reverse bias voltages, are applied to the variable capacitance diode 11 to select a desired receiving frequency. Note that, as described above, this tuning voltage TV is supplied from, for example, a PLL circuit (not shown).

By the way, although it was decided to use a low voltage type variable capacitance diode 11 in the present invention,
The characteristic of the tuning voltage TV versus the capacitance value C is as shown in FIG. FIG. 2 is a graph for explaining the relationship between TV and C, with the horizontal axis showing the TV scale and the vertical axis showing the logarithmic C scale. Solid curve in this graph
VC indicates the characteristics of the low-voltage variable capacitance diode 11, and the dotted curve VC indicates the characteristics of the high-voltage variable capacitance diode. The approximate operating ranges for each are 9V and 25V as shown in the diagram. Comparing these curves, it is clear that the high pressure type (VC)
has linearity, while the low pressure type (vc) has nonlinearity. This is caused by factors related to semiconductor manufacturing technology.

As shown in FIG. 2, the low voltage type variable capacitance diode 11 has a non-linear curve, so the antenna
The greater the antenna input received by the ANT, the more distortion will be included in the audio output. The graph in Figure 3 provides a simple illustration of this situation. The definitions of the horizontal and vertical axes in this graph are the same as those in FIG. Now, hypothetically, the tuning voltage T that should be tuned to a signal with a certain desired frequency.
Suppose that V. is located at one inflection point R1 of the curve vc. At this time, if the antenna input is a small amplitude signal like s in the figure, the capacitance value that changes in response includes almost no distortion, like Cs. However, if the antenna input is a large amplitude signal like S in the figure, the capacitance value that changes accordingly will include distortion like CS. As a result, the audio output will include distortion. This also applies to the other inflection point R2 of the curve vc.

In order to prevent this kind of distortion, a conventional method has been adopted, for example, by inserting an attenuator into the input stage of the antenna tuning circuit and increasing its attenuation ratio when the antenna input becomes large. However, when the electric field strength changes frequently, such as in a car radio, it is necessary to automatically change the attenuation ratio, which has the disadvantage of complicating the circuit. In addition, when such an attenuator is inserted, the resonance point is shifted due to the capacitance component specific to the attenuator.

Therefore, the present invention proposes a method that can limit antenna input without using such an attenuator. The gist of this inventive approach is to lower the Q of the antenna tuning circuit when the antenna input becomes excessive. If the Q decreases, the antenna input applied to the variable capacitance diode 11 will inevitably decrease, and the above-mentioned audio output distortion will not occur. However, here, various effects caused by changing Q must be taken into consideration. First, setting Q high generally has the effect of improving the S/N ratio. Therefore, in the case of the present invention, lowering the Q results in deterioration of the S/N, which is not preferable. However, it should be understood that this does not actually lead to deterioration of the S/N ratio. This is because according to the present invention, the Q is reduced only when the antenna input is large, and in this case the S component of the S/N is naturally large, so in reality, the S/N is reduced. This does not result in deterioration of N. Of course, when the antenna input is small,
Since Q is a normal high value, the S/N is good. Furthermore, setting Q high is effective in improving not only S/N but also cross modulation. Therefore, in the case of the present invention, lowering the Q makes the signal more susceptible to interference waves existing near the signal of the desired frequency. However, it should be understood that such cross-modulation does not actually occur. This is because Q is reduced based on the present invention only when the level of the signal of the desired frequency received at the antenna input is high, and even if a small amount of interference waves enter here, the high level The signal at the desired frequency can sufficiently cancel out the interference waves. Even if the level of the interference wave is high and the level of the signal of the desired frequency is low,
At this time, Q is high and the interference waves can be sufficiently eliminated. Thus, from the above considerations, it becomes clear that the method of lowering the Q according to the present invention does not have any adverse effect on the original radio reception function.

FIG. 4 is a block diagram showing an example of the configuration of a radio receiver employing an electronically tuned tuner according to the present invention. In this figure, the same reference numbers or symbols as in FIG. 1 represent the same components.
In this figure, RF・AMP is an RF amplifier connected to the coupling coil 15, MIX is a mixer,
OSC is an oscillator that constitutes a local oscillator.
IF (intermediate frequency)/AMP is an IF amplifier, detector DET, audio frequency amplifier AF/AMP, and SP (speaker) are audio output circuits. In addition, the above configuration and the antenna tuning circuit 10
is a common component. Also, the output of IF/AMP is used as input, and AGC is applied to RF/AMP.
A level detector 41 that adds automatic gain control is also a conventional and common component. However, the present level detector 41 is also an important part in implementing the present invention. That is, the level detector 41
controls the switch circuit 42. switch circuit 4
2 is connected to a Q reduction circuit 43, and the Q reduction circuit 43 acts on the antenna tuning circuit 10. This Q reduction circuit 43 becomes active on the antenna tuning circuit 10, that is, operates to lower the Q of the circuit 10, when the antenna input exceeds a predetermined value, and this A state exceeding the value can be detected by the level detector 41. When this detection is made, detector 41 turns on switch circuit 42 and makes Q reduction circuit 43 active. In other words, the antenna tuning circuit 10 is operated to lower its Q. In the end, portions 42 and 43 in this figure are added according to the present invention.

FIG. 5 is a diagram showing an example of a configuration that more specifically represents the configuration of FIG. 4. However, those having the same reference numbers or symbols as in FIG. 4 are the same components. In this figure, diode 51 corresponds to level detector 41 in FIG. 4.
Further, resistors 52, 53 and 54 are a resistor group forming a general AGC circuit, and 55 is a capacitor forming a part of the AGC circuit. 56 is an amplification transistor that forms the main part of the RF/AMP. Now, the part related to the present invention is the switching transistor 5.
7, a capacitor 58, and a resistor 59, the switching transistor 57 corresponds to the switch circuit 42 in FIG.
The pair corresponds to the Q reduction circuit 43 in FIG. The operation of the main parts of the circuit shown in FIG. 5 will be explained with reference to FIG. 6. FIG. 6 is a graph showing the antenna input level ANT _io on the horizontal _axis and the DC voltage V _dc on the vertical axis.
Voltage applied to the base of transistor 56 in the figure
V _b and the AGC voltage V _agc appearing across resistor 53 and capacitor 55 in FIG. Fifth
6 and 6, the DC voltage
V _b and V _agc are voltage-divided at a constant voltage by resistors 52 and 53, and both change in parallel as shown by curves V _b and V _agc in FIG. First, when the antenna input ANT _io is almost zero, that is, near the vertical axis of the graph in Figure 6, the detection output from the diode 51 is also almost zero, so V _b
and V _agc hardly change (the antenna input ANT _io in FIG. 6 is in the region). However, when the antenna input ANT _io gradually increases, a detection output in the negative direction is generated in the diode 51, lowering V _agc and V _b (the antenna input level ANT _io in FIG. 6 is in the region). When the antenna input level ANT _io increases further, V _b is further reduced to around 0.6V, and the transistor 56
As a result, V _b
changes to converge to 0.6V, and along with this, V _agc changes to converge to approximately zero V (the antenna input level ANT _io in FIG. 6 is in the region). And further antenna input level
ANT _io exceeds the predetermined value and enters the area. Antenna input level ANT _io that belongs to this region
is exactly the excessive input that causes the audio output distortion described above. In the case of such an excessive antenna input level, the antenna input itself directly turns on the transistor 56.
In other words, the antenna input signal itself is the transistor 56
bias. In such a state, the AGC function is no longer achieved, and the detection output of the diode 51 is further lowered in the negative direction. and
When V _agc eventually falls below −0.6V,
Transistor 57, which is a pnp transistor, is turned on, and capacitor 58 and resistor 59, which form the Q reduction circuit (43 in FIG. 4), are connected in parallel to antenna tuning circuit 10 with one end grounded. Here, the Q of the antenna tuning circuit 10 is
This decreases due to the insertion of . In addition, the normal Q
If the Q is set to 60 and the reduced Q is set to about 20, the input level applied to the variable capacitance diode 11 will be reduced by about 10 dB, and the objective will almost be achieved.

As explained above, according to the present invention, audio output distortion that occurs when the antenna input is excessive can be eliminated by simply adding a few extremely simple elements.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the general configuration of the antenna tuning circuit that forms the main part of an electronic tuning tuner.
The figure is a graph showing the relationship between tuning voltage and capacitance value, Figure 3 is a graph to illustrate that audio output distortion is caused when the antenna input becomes large, and Figure 4 is a graph showing the invention of the present invention. 5 is a block diagram showing an example of the configuration of a radio receiver that employs an electronically tuned tuner based on FIG. This is a graph used to explain the operation of the circuit. In the figure, 10 is an antenna tuning circuit, 11 is a variable capacitance diode, 13 is a tuning coil, 41 is a level detector, 42 is a switch circuit, and 43 is a Q reduction circuit.

Claims (1)

[Scope of Claims] 1. An antenna, an antenna tuning circuit consisting of a pair of a variable capacitance diode and a tuning coil connected to the antenna, and a coupling coil provided after the antenna tuning circuit and coupled to the tuning coil through a transformer. An amplification stage including an RF amplifier, a local oscillator, a mixer, an IF amplifier, and an audio output circuit at the input, and a level detector that detects a change in the output level of the IF amplifier and applies AGC to the RF amplifier. In an electronic tuning tuner used in a radio receiver to select a desired reception frequency by applying a predetermined tuning voltage to the variable capacitance diode in the antenna tuning circuit, the Q of the antenna tuning circuit is reduced. and a switch circuit for switching the Q reduction circuit to an active state or an inactive state with respect to the antenna tuning circuit, the switch circuit comprising:
When the input signal level to the antenna becomes excessive beyond a predetermined value, switching the Q reduction circuit from the inactive state to the active state with respect to the antenna tuning circuit via the level detector; An electronic tuning tuner characterized by reducing Q. 2 The switch circuit consists of a pnp transistor,
2. An electronically tuned tuner as claimed in claim 1, wherein the base of the pnp transistor is connected to the output of a level detector. 3. An electronically tuned tuner according to claim 2, wherein the Q reduction circuit comprises a series connected capacitor and resistor pair, which pair is connected in parallel to the antenna tuning circuit when the pnp transistor is turned on.