JPH0210673Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an electronic tuning circuit having a variable capacitance diode, and especially when an excessive signal is input, the variable capacitance connected to a tuning circuit such as an antenna input stage or an RF amplification stage is used. Even if the diode becomes temporarily forward biased, current does not flow through the diode, so that the oscillation frequency of the local oscillation circuit in the subsequent stage does not change, and the resonance circuit including the variable capacitance diode is designed to prevent current from flowing through the diode. This suppresses a decrease in the Q value.

FIG. 1 shows the front end of a conventional electronic tuning circuit, which includes an antenna input stage 1, an RF amplification stage 2 with an RF amplification transistor Q ₁ , a mixing stage 3 with a mixing transistor Q ₂ , and an oscillation It consists of a local oscillation stage 4 having a transistor _Q3 . Antenna input stage 1, RF amplification stage 2,
The resonant circuits provided in each local oscillation stage 4 include variable capacitance diodes VC ₁ , VC ₂ and VC ₃ , respectively. These variable capacitance diodes VC ₁ , VC ₂ ,
Tuning voltage Tu is applied to VC ₃ in a reverse bias state via resistors R ₁ , R ₂ , and R ₃ , and decoupling capacitors C ₁ ,
C ₂ and C ₃ are also provided. By the way, FIG. 3 shows the voltage-capacitance characteristics of a variable capacitance diode, and normally a voltage of 3 to 25 V is applied to the variable capacitance diode as a tuning voltage.

Now, using the antenna input stage 1 as an example, we will consider the case where an excessive signal is input. The peak level of the excessive signal is the tuning voltage to the variable capacitance diode VC ₁
If Tu is exceeded, the variable capacitance diode VC ₁ connected to the FET Q ₁ side becomes conductive, current flows into the decoupling capacitor C ₁ via the variable capacitance diode VC ₁ and the resistor R ₁ , and the capacitor
C ₁ will be charged. That is, the tuning voltage Tu
As a result, the oscillation output (local oscillation frequency) of the local oscillation stage 4 fluctuates, causing a tuning shift in the reception frequency. This tuning deviation of the receiving frequency directly leads to the inability to receive signals, and is therefore a major hindrance. Furthermore, when the variable capacitance diodes VC ₁ and VC ₂ become conductive, the Q value of the resonant circuit decreases, and _the resonant circuit no longer performs its function, resulting in a significant decrease in selectivity _. , _VC3 becomes conductive, causing the diode to operate as a diode mixer, resulting in a problem that the selection desired signal and other signals are mixed.

In order to solve this problem, the present invention connects a reverse current blocking diode _D1 in the forward direction between the decoupling capacitor _C1 and the variable capacitance diode _VC1 . Although FIG. 2 shows only the resonant circuit portion of the antenna input stage 1, reverse current blocking diodes are similarly connected to other portions, such as the resonant circuit of the RF amplification stage 2, as required. With this configuration, even if an excessive input signal voltage occurs in the direction of making the variable capacitance diode _VC1 conductive,
Since this signal voltage becomes a reverse voltage with respect to the diode _D1 , the diode _D1 will not become conductive. Therefore, no current flows through the variable capacitance diode VC ₁ , and this variable capacitance diode
VC ₁ never conducts. Therefore, a decrease in the Q value of the resonant circuit including the variable capacitance diode _VC1 is suppressed. Furthermore, considering the voltage drop of this diode _D1 caused by inserting the diode _D1 , the current flowing through this diode _D1 is:
Since only the reverse leakage current flows through the variable capacitance diode _VC1 , it is almost negligible and poses no problem.

Next, the operation at the time of an excessive input signal will be explained in more detail. In other words, at the positive peak of an excessive input signal, the FET
Variable capacitance diode on Q ₁ and transistor Q ₂ side
Although VC ₁ and VC ₂ are forward biased, no current flows due to the action of diode D ₁ . Furthermore, no current flows through VC ₁ or VC ₂ even at the negative peak of an excessive input signal. Therefore, even if the variable capacitance diodes VC 1 and VC 2 on the side of FET Q _{1 and transistor Q 2 temporarily become forward biased during the positive peak of an excessive input signal, VC 1 and VC 2} are normally in the forward bias state during the negative _peak . In the end, if we consider both the positive and negative cycles of the AC input signal, VC ₁ and VC ₂ are normally biased, so there is no problem.

Also, even if the positive peak of the input signal is excessive,
Even if the Q value of the resonant circuit of the antenna input stage 1 and the resonant circuit of the RF amplification stage 2 decreases due to the influence of VC ₁ and VC ₂ , this has almost no effect on the reception sensitivity because the input signal is large. To further explain this, VC ₁
Just before VC ₂ or VC2 becomes conductive, the capacitance value becomes quite large, but only up to a certain limit, and once it begins to conduct, it loses its function as a capacitor and becomes a resistor of several ohms or less. Therefore, the Q value of the tuning circuit is lowered, and the frequency selection characteristics of the tuning circuit are extremely impaired. In such a state, since the input signal is large, an input signal whose difference from the local oscillation frequency _L corresponds exactly to the intermediate frequency (10.7 MHz in the case of FM) will be received. If the Q of the tuning circuit is extremely low, the antenna input stage 1
The tuned load circuit of the RF amplifier stage 2 becomes like a simple resistive load, and the RF amplifier stage 2 operates like a broadband amplifier with low gain.

In addition, virtually no current flows through the reverse current blocking diode D ₁ of the present invention during normal operation, and the variable capacitance diode VC ₁ flows through the diode D ₁ .
It only acts to apply a tuning voltage to the cathode side of the . To explain this more precisely, during normal operation, a voltage is applied to the variable capacitance diode _VC1 in the opposite direction, and a very small leakage current flows. Since the current only flows through this diode, it may be considered that no current substantially flows through the diode _D1 .

Now, since there is a circuit that is visually similar to the reverse current blocking diode _D1 of the present invention, the differences from the circuit of the present invention will be explained next. That is, in the past, for example, as shown in Figure 2 of Utility Model Publication No. 58-28367 (H03J3/04), a temperature compensation diode was used to compensate for the temperature characteristics of a variable capacitance diode connected in parallel to a tuning circuit. The technology of connecting is known. In this conventional technology, in order to compensate for variations in capacitance due to temperature in the variable capacitance diode, a current is always passed through the aforementioned temperature compensation diode, and the voltage drop across this diode changes depending on the ambient temperature. This fact is utilized to compensate for temperature changes in the variable capacitance diode. In addition, in order to compensate for the temperature characteristics of variable capacitance diodes, a technology for connecting temperature compensation diodes was developed.
No. 20162 (H03J 3/04) is known, but in Figure 1, one resistor is used to simplify the drawing.
Although the resistor connected between the connection point between 3 and the temperature compensating diode and the ground is omitted from illustration, this resistor is essential in principle since it performs the temperature compensating function.

According to the present invention described above, since the tuning voltage is applied to the variable capacitance diode constituting the resonant circuit through the forward direction diode, when an excessive signal is input, the variable capacitance diode is temporarily turned off. No current flows through the variable capacitance diode even if the variable capacitance diode becomes forward biased. Therefore, there is no shift in the tuning voltage due to charging of the decoupling capacitor, so the oscillation frequency of the local oscillation circuit does not change, and therefore no shift in tuning of the receiving frequency occurs. Furthermore, there is no problem such as deterioration in the selection characteristics of the resonant circuit based on the conduction of the variable capacitance diode. Furthermore, even if the voltage applied to the variable capacitance diode (tuning voltage) is lowered, the conduction of the variable capacitance diode is always blocked by diode _D1 , so the tuning voltage can be lower than the conventional lower limit (3V), for example, up to around 1V. The tuning frequency range can be widened accordingly.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a front end having a conventional electronic tuning circuit, FIG. 2 is a diagram showing an antenna input stage having an electronic tuning circuit of the present invention, and FIG. 3 is a characteristic diagram of a variable capacitance diode. 1 is an antenna input stage, 2 is an RF amplification stage, 3 is a mixing stage, 4 is a local oscillation stage, VC ₁ , VC ₂ , VC ₃ are variable capacitance diodes, C ₁ , C ₂ , C ₃ are decoupling capacitors, D ₁ is a backflow blocking diode.

Claims (1)

[Scope of utility model registration request] In an electronic tuning circuit configured to apply a tuning voltage to a variable capacitance diode via a decoupling capacitor, there is substantially no current between the variable capacitance diode and the decoupling capacitor during steady operation. By connecting a reverse reverse blocking diode in the forward direction in which no current flows, a tuning voltage is supplied to the variable capacitance diode through the reverse current blocking diode, and the reverse current flowing from the variable capacitance diode to the decoupling capacitor at the time of an excessive input signal is An electronic tuning circuit characterized in that the current is blocked by the reverse current blocking diode.