JPH0260212A - Modulation circuit - Google Patents

Abstract

PURPOSE:To cut the supply of external noise and the like to attain stable modulation by providing a switching diode in a forward direction with respect to an input signal for the prestage of a signal supply means, and supplying the input signal to the signal supply means through the switching diode. CONSTITUTION:An inverse direction voltage is impressed on a varactor diode 2. when the pulse wave of 5V, for example, is inputted from an input terminal Ti in such a state, the static capacity of the varactor diode 2 is changed. Thus, a system is made to a state that modulation by an SAW resonator 6 provided in a subsequent level, furthermore, a transistor 1 is attained. When the external noise of about 1.2V is inputted, there is hardly any output from the switching diode 4. Consequently, noise is hardly supplied to the varactor diode 2. Thus, the static capacity of the varactor diode 2 can stably be controlled, and modulation by the SAW resonator 6 and the transistor 1 provided to the next stage can stably be executed.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a frequency modulation circuit, and particularly to a modulation circuit that can perform stable modulation. [Prior Art] FIG. 5 is a circuit diagram of a modulation circuit used in transceivers, wireless data transmission equipment, and the like. The modulation circuit IO is an NPNI-transistor 1 as an amplifier.
, a varactor diode (variable capacitance diode) 2 whose capacitance is changed based on a change in a human power signal VT input from an input terminal T1 is provided. Further, the modulation circuit 10 has a S A W (5u
An rface acoustic wave resonator 6 is provided. Modulation by this modulation circuit 10 is performed by first applying a bias voltage VB between the base B and emitter E of the transistor 1 via a bias resistor R1, and also applying a signal wave v1 of a predetermined level via a resistor R2. Change the capacitance of 2. When the capacitance of varactor diode 2 changes, the oscillation frequency changes,
FM waves can be obtained. This FM wave is extracted by a circuit (not shown) connected to the cap C or emitter E of the transistor 1, and is oscillated. [Problems to be Solved by the Invention] For example, consider a case where a 5V pulse signal is supplied to the varactor diode 2 from the input terminal Ti. in this case,
A 5V pulse signal is supplied via resistor R2. Therefore, when external noise or minute noise is input from the input terminal Ti for some reason, these noises are also supplied to the varactor diode 2 in proportion to their magnitude. Therefore, unnecessary fluctuations in the capacitance of the varactor diode 2 occur randomly, making FM modulation unstable. Moreover, even when the pulse signal is Ov, randomly generated noise such as external noise enters from the input terminal Ti, causing F
This has the disadvantage that M modulation becomes unstable. The present invention has been made in view of the above-mentioned conventional problems, and it is an object of the present invention to provide a modulation circuit that can perform stable FM modulation. [Means for Solving the Problems] The present invention provides an oscillation transistor and an S connected to the oscillation transistor.
An AW resonator and a signal supply means for supplying different signals to the SAW resonator according to the level of an input signal are provided, and a switching diode is provided at a stage before the signal supply means in a forward direction with respect to the input signal. is provided, and modulation is performed by supplying an input signal to the signal supply means via this switching diode. Further, the present invention includes an oscillation transistor, a SAW resonator connected to the oscillation transistor, and a variable capacitance diode that supplies different signals to the SAW resonator depending on the level of an input signal. It is characterized in that a fixed capacitor is connected in parallel with the variable capacitance diode. [Function] According to the above-described means, a switching diode is provided in the forward direction with respect to the input signal at the front stage of the signal supply means, and the input signal is supplied to the signal supply means via this switching diode. Since the switching diode hardly outputs a signal below a predetermined level (for example, 1.2V), most external noises having a very small level (1.2V or below) are almost completely cut out by the switching diode. Therefore the signal provider °
The supply of external noise etc. to the stage (varactor diode) is cut off. As a result, random and unnecessary fluctuations in the capacitance of the signal supply means (varactor diode) are eliminated.
The signal supply means (varactor diode) becomes well involved in modulation, and stable modulation is achieved. In addition, by connecting a fixed capacitor in parallel with the variable capacitance diode, the capacitance ratio of the variable capacitance diode can be lowered, and capacitance changes affected by external noise to the input section can be relatively lowered, and the modulation frequency The impact of this can be reduced. [Example] Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a modulation circuit according to a first embodiment of the present invention. This modulation circuit 20 and the conventional modulation circuit 10 are the modulation circuit 2
The difference is that a switching diode 4 is provided with respect to 0. Incidentally, in FIG. 1, the same reference numerals are given to the same parts as in the prior art. Reference numeral 1 indicates an NPN transistor as an oscillator, and its collector C is connected to a power supply VB via a coil L. Further, the emitter E of the transistor 1 is grounded via a bias resistor R2. As a result, a collector-emitter voltage VB is applied between the collector C and emitter E of the transistor 1. Further, the base B of the transistor 1 is connected to the power supply VB via the bias resistor R1, so that the operating point of the pace current IB of the transistor 1 can be set. A bias voltage VBE based on the bias resistor R1 is applied between the base B and emitter E of the transistor 1.
is applied. Reference numeral 2 indicates a varactor diode (variable capacitance diode) as a signal supply means, and as shown in the figure, the anode side of the varactor diode 2 is grounded, and the cathode side is connected to the power supply VB via a capacitor 5 etc.
A DC voltage (reverse voltage) VB is applied. Further, the cathode side of the varactor diode 2 is connected to the input terminal Ti via a resistor R3. and,
A signal at a predetermined level, for example, a 5V pulse signal, is supplied to the varactor diode 2 from this input terminal Ti. When the input signal VT is supplied to the varactor diode 2, the voltage applied to the varactor diode 2 changes, and the capacitance of the varactor diode 2 changes. Reference numeral 4 indicates a switching diode, and the resistor R
3 in parallel. The current/voltage characteristics of this switching diode 4 are as shown in FIG. 2, for example. That is, when the input voltage is 1.2 V or less, there is almost no output. Therefore, very little noise (usually 1.2 V or less) such as external noise is output. Fusuma No. 6 is S A W (5 surface Acoust
ic Wave ) resonator, which is connected between the collector C and emitter E of the transistor 1 as a feedback capacitor. The SAW resonator 6 is for stabilizing modulation. Modulation by the modulation circuit 20 is performed as follows. Bias voltage VB is applied to transistor 1 to enable transistor 1 to operate. Further, a reverse voltage is applied to the varactor diode 2. In this state, a pulse wave of, for example, 5V is input from the input terminal Ti. This makes the varactor diode 2
The capacitance of is changed. Then, the S equipped in the next stage
Modulation by the AW resonator 6 and further by the transistor l is enabled. Here, even if an external noise of about 1.2 V or less is input, there is almost no output from the switching diode 4 as shown in FIG. Therefore, almost no noise is supplied to the varactor diode 2. Therefore, the supply of unnecessary randomly generated noise to the varactor diode 2 is cut off, and the capacitance of the varactor diode 2 can be stably controlled. As a result, modulation by the SAW resonator 6 and transistor 1 provided at the next stage of the varactor diode 2 can be stably performed. FIG. 3 is a circuit diagram of a modulation circuit according to a second embodiment of the present invention. This embodiment differs from the first embodiment in that a capacitor 7 as a fixed capacitance is provided in parallel with the varactor diode 2, as shown in the figure, and that a switching diode 4 is not provided. ing. Capacitor 7 acts to reduce the capacitance of varactor diode 2 when the voltage is kept constant. That is, the capacitance of varactor diode 2 and capacitor 7
The charge is distributed based on the ratio of the capacitances, and the charge is stored in the varactor diode 2 and the capacitor 7. Therefore, even if external noise is supplied to the varactor diode 2, the rate of variation in the capacitance of the varactor diode 2 due to this is reduced, so that stable modulation can be performed. FIG. 4 is a circuit diagram of a modulation circuit according to a third embodiment of the present invention. In this embodiment, a switching diode is provided in parallel to the resistor R3. Furthermore, in this embodiment in which a capacitor 7 is provided in parallel to the varactor diode 2, external noise input via the input terminal Ti is blocked by the switching diode 4. Further, the capacitance of the varactor diode 2 is reduced by the capacitor 7, and as a result, fluctuations in capacitance due to noise in the varactor diode 2 are reduced. Therefore, this embodiment, which can stabilize modulation, can suppress fluctuations in the capacitance of the varactor diode 2 itself, and is therefore effective for minute frequency changes in automatic frequency control (AFC) or adjustment of oscillation frequency. be. Although the invention made by the present inventor has been specifically explained based on the embodiments above, it is not limited thereto. That is, in the embodiments described above, an input signal is supplied from the input terminal Ti via the resistor R3. However, the resistance R
3 may be supplied without passing through. Furthermore, although a capacitor is provided in parallel with the varactor diode 2 in the second and third embodiments, a plurality of capacitors may be provided in parallel.

【Effect of the invention】

As described above, according to the present invention, at the front stage of the signal supply means,
A switching diode is provided in the forward direction with respect to the input signal, and the input signal is supplied to the signal supply means via this switching diode. Therefore, most of the external noise, which has a lot of minute levels, is cut out by the switching diode. Therefore, the supply of external noise to the signal supply means (varactor diode) is cut off, and stable modulation can be performed. Furthermore, according to the present invention, a fixed capacitance is provided in parallel with the signal supply means (varactor diode) to reduce fluctuations due to the capacitance of the signal supply means, making it less susceptible to the influence of external noise intrusion, resulting in less modulation. This has the effect that it can be performed stably.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a modulation circuit according to a first embodiment of the present invention,
FIG. 2 is a diagram showing the current/voltage characteristics of a switching diode, FIG. 3 is a circuit diagram of a modulation circuit according to a second embodiment of the present invention, and FIG. 4 is a diagram of a modulation circuit according to a third embodiment of the present invention. Circuit Diagram FIG. 5 is a circuit diagram of a conventional modulation circuit. l...Transistor, 2...Varactor diode,
4... Switching diode, 5.7... Capacitor, 6... SAW resonator. Figure 3

Claims (1)

[Scope of Claims] 1. An oscillation transistor, a SAW resonator connected thereto, and signal supply means for supplying different signals to the SAW resonator according to the level of an input signal, and the A modulation circuit characterized in that a switching diode is provided in a forward direction with respect to the input signal at a stage before the signal supply means, and modulation is performed by supplying the input signal to the signal supply means via the switching diode. 2. It is equipped with an oscillation transistor, a SAW resonator connected to it, and a variable capacitance diode that supplies different signals to the SAW resonator depending on the level of the input signal, and the variable capacitance diode and A modulation circuit characterized by a fixed capacitance connected in parallel.