JPS6318888B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a simple and stable frequency modulation circuit. More specifically, it modulates audio signals and control signals as a wireless microphone using a relatively high frequency (250 to 330MHz).
It is also an object of the present invention to obtain a frequency modulation circuit that uses a battery and has low voltage and low power consumption.

Conventionally, wireless microphones have adopted a method of performing frequency modulation using a modulation transformer between the base and emitter of a self-excited oscillation circuit, a method using a variable capacitance element, etc. However, this method is limited to relatively low oscillation frequencies and uses a modulation transformer or a variable capacitance element for the capacitance of the oscillator tank circuit, but this method has a temperature characteristic of -30°C to It was difficult to satisfy +60°C.

In addition, as a circuit that modulates only a control signal using a relatively high frequency (250 to 330MHz),
As shown in Figure 1, the RF of the RF self-excited oscillation circuit 1
There is a method of modulating the amplitude of a low frequency rectangular wave by adding a low frequency rectangular wave signal from a control signal oscillation circuit 2 mainly composed of an inverter to the (high frequency) signal using a modulation transistor 3. This satisfies the temperature characteristics from -30℃ to +60℃, but it can only modulate control signals and it is difficult to modulate audio signals.Moreover, only AM signals can be modulated, and FM signals cannot be modulated. It was hot.

The present invention has been made in view of these points. In conventional oscillation circuits, changes in the output capacitance of the transistor affect the oscillation frequency, so the circuit was designed to avoid this as much as possible. , attempts to actively utilize this change in the output capacitance of the transistor.

That is, regarding the output capacitance of the transistor changing depending on the ambient conditions, the collector capacitance (Cob) of the transistor is approximately inversely proportional to the square root of the collector voltage (Vc) as shown in equation (1) below.

Therefore, a frequency modulated wave can be obtained by changing the collector voltage of the self-excited oscillation circuit.

Therefore, the present invention connects a modulation transistor to a totem pole on the collector-tuned circuit side of a collector-tuned RF self-excited oscillator circuit, and changes the power supply voltage of the oscillation circuit according to the amplitude level of the modulation signal. The collector output capacitance of the transistor constituting the oscillation circuit is varied to obtain a frequency modulated wave.

Embodiments of the present invention will be described below based on the drawings.

FIG. 2 shows a frequency modulation circuit, which is composed of a collector-tuned RF self-excited oscillation circuit 10 and a modulation circuit 26. As a configuration of this collector-tuned RF self-excited oscillation circuit 10, an oscillation transistor 11
A tuned circuit consisting of a capacitor 14 and a coil 13 is connected to the collector of , and between the other end of this tuned circuit and the base of the oscillation transistor 11,
A feedback capacitor 15 for exciting oscillation is connected to the other end, and an RF choke coil 12 is connected to the emitter of a transistor 20 of a modulation circuit 26, which will be described later, and is connected to the base of the oscillation transistor 11 and modulation. transistor 20
A bias supply resistor 16 for the oscillation circuit is connected between the emitters of the oscillation circuit. Further, an RF bypass capacitor 19 and a resistor 18 for applying DC feedback to stabilize the oscillation frequency are connected in parallel to the emitter of the oscillation transistor 11.

Next, the configuration of the modulation circuit 26 will be described. This modulation circuit 26 is connected to the oscillation circuit 10 in a totem pole manner. That is, the transistor 20 of this modulation circuit 26 is coupled to the oscillation circuit 10 with a common collector, the collector is connected to the power supply terminal 21, the base is connected to the voltage dividing resistors 22 and 23, and the capacitor 24 is connected to the transistor 20. It is connected to the modulation input terminal 25. Furthermore, an RF bypass capacitor 17 is inserted between the modulation circuit 26 and the oscillation circuit 10.

Next, the circuit operation will be explained based on the above configuration.

The tuning circuit of this frequency modulation circuit is the coil 1
3. Since it is composed of the capacitor 14 and the collector output capacitance (Cob) of the oscillation transistor 11, the power supply voltage (Vcc) is set to 9V, and the base applied voltage of the modulation transistor 20 is set to 3~
When varied within the range of 8V, the collector potential (Vc) of the transistor 11 in the oscillation circuit 10
As the emitter potential (Ve) changes as shown in the curves (Vc) and (Ve) shown in FIG. 3, the collector output capacitance (Cob) of the oscillation transistor 11 changes, resulting in the tuning of the tuning circuit. The frequency changes depending on the collector output capacitance (Cob) of the oscillation transistor 11. Therefore, different frequencies can be obtained with the same oscillation circuit. This relationship is shown in curve A of FIG.

In addition, when such an oscillation circuit generates a relatively high frequency (250 to 330 MHz), for example around 280 MHz, even if the capacitance value of the collector tuning circuit changes by only 0.01 pF, the oscillation circuit will not oscillate. The frequency changes around 1MHz.
Therefore, in order to keep the oscillation frequency stable, it is better to use a transistor with a small collector output capacitance (Cob) and a small collector cut-off current (I _CBO ).

Next, the modulation operation will be explained based on the characteristics of the oscillation circuit.

The power supply potential (Vc) of the oscillation circuit 10 is determined by the modulation circuit 26 which is totem pole connected to the oscillation circuit 10, and therefore a low frequency modulation signal is supplied to the input terminal 25 of the modulation circuit 26. By applying , the power supply potential (Vc) of the oscillation circuit 10 changes according to the amplitude of the modulation signal. Here, since the oscillation output voltage level of the RF self-excited oscillation circuit 10 is determined by the Q of the collector tuning circuit, a frequency modulation operation is performed.
Incidentally, in order to improve the modulation characteristics, the modulation circuit 26
The emitter feedback resistor 18 of the oscillation circuit 10 is also used as a load resistor of the modulation circuit 26.

In general, the collector voltage (Vc) of a transistor
The change in the collector output capacitance (Cob) when changing linearly does not change linearly as shown in equation (1) above. For example, transistor
The amount of change in the collector output capacitance (Cob) of 2SC2026 is shown as a curve in FIG.

Therefore, in the present invention, the collector voltage of the oscillation transistor 11 is actually improved to (Vc-Ve) by the resistor 18 inserted into the emitter of the oscillation transistor 11, and good linearity is maintained. At the same time, the resistor 18 applies feedback to the transistor 11 of the oscillation circuit 10,
It has the effect of keeping the oscillation frequency stable.
Further, the capacitor 19 bypasses the high frequency, and as described above, maintains the oscillation frequency stably, and at the same time acts so as not to affect the low frequency modulation signal. Furthermore, the output capacitance change of the modulation circuit 26 is
In order to prevent the oscillation frequency of the oscillation circuit 10 from being adversely affected, a capacitor 17 is used between the modulation circuit 26 and the oscillation circuit 10 to bypass high frequencies.

In the above embodiment, by using the modulation transistor 20 with the collector grounded, the linearity of the modulation circuit 26 is not impaired, and the modulation circuit 26 has a substantially linear frequency characteristic in the low frequency region. has. Here, the upper limit of the modulation frequency characteristic is
The output end of the modulation circuit 26, that is, the oscillation circuit 10
It is regulated by the RF bypass capacitor 17 and is approximately 23KHz. Therefore, approximately DC−10K
Hz is used for the audio modulation band, and 12-23KHz is used for the control signal modulation band consisting of a combination of low frequency tone signals, allowing simultaneous modulation of audio and control signals, and the entire band is used for modulating the audio signal. May be used.

Moreover, the modulation circuit 26 and the oscillation circuit 10
Since the modulation circuit 26 can be operated at a low working voltage, it is easy to connect the modulation circuit 26 to the oscillation circuit 10 in a totem pole.
By connecting the oscillation circuit 10 and the oscillation circuit 10 to the totem pole, the current used by both circuits can be borrowed, resulting in less power consumption. Furthermore, the modulation transistor 20 may be used with its emitter grounded or its base grounded, in addition to the usage according to the embodiments described above.

Since the present invention is configured as described above, it has the following effects.

(1) By connecting the oscillation circuit and modulation circuit to a totem pole to form a frequency modulation circuit, the current used can be shared, resulting in less power consumption and operation at low voltage.

(2) Good modulation can be performed with a small modulation voltage, and at the same time, the linearity of the modulation is also excellent. In particular, by inserting a DC feedback resistor, which is shared as a load resistor in the modulation circuit, into the emitter of the oscillation transistor, the collector voltage of the oscillation transistor is improved to (Vc - Ve), and good linearity is maintained. At the same time, this resistor applies feedback to the transistor in the oscillation circuit, which has the effect of keeping the oscillation frequency stable.

(3) The oscillation output voltage level of the oscillation circuit is determined by the Q of the collector-tuned circuit, so it is extremely stable.
FM waves can be obtained and the temperature characteristics are -30℃ to +60℃.
Satisfied up to ℃.

(4) Relatively high frequencies (250~330MHz,
Not only control signals but also high quality audio signals can be modulated.

Incidentally, the upper limit of the modulation frequency characteristic is defined by the output end of the modulation circuit, that is, the oscillation circuit and the RF bypass capacitor, and is approximately 23KHz. Therefore, approximately DC-10KHz is used for the audio modulation band, and 12 to 23KHz is used for the control signal modulation band consisting of a combination of low frequency tone signals, allowing simultaneous modulation of audio and control signals, and the entire band can also be used to modulate audio signals.

(5) Since a capacitor is inserted between the modulation circuit and the oscillation circuit, it bypasses high frequencies and keeps the oscillation frequency even more stable, while at the same time acting on low frequency modulation signals without affecting them. Furthermore, changes in the output capacitance of the modulation circuit are prevented from adversely affecting the oscillation frequency of the oscillation circuit.

[Brief explanation of the drawing]

Fig. 1 is an electric circuit diagram showing a conventional modulation circuit, Fig. 2 is an electric circuit diagram showing an embodiment of the frequency modulation circuit according to the present invention, and Fig. 3 is a characteristic showing the relationship between the voltage of each part and the oscillation frequency. FIG. 4 is a characteristic curve diagram showing the relationship between the collector-base voltage and the collector output capacitance of a transistor. 10...RF self-excited oscillation circuit, 11...Oscillation transistor, 13...Tuning circuit coil, 14...
... Tuning circuit capacitor, 15... Feedback capacitor, 16... Bias resistor, 17, 19...
RF bias capacitor, 18... DC feedback resistor, 20... Modulation transistor, 21... Power supply terminal, 23, 24... Bias resistor, 25...
Modulation input terminal, 26...Modulation circuit.

Claims (1)

[Claims] 1. A transistor of a modulation circuit is connected to a totem pole between the collector-tuned circuit side of the collector-tuned RF self-excited oscillation circuit and a power supply terminal, and a transistor of the collector-tuned RF self-excited oscillator circuit is connected to a totem pole. A DC feedback resistor, which is also used as a load resistor of the modulation circuit, is connected to the emitter of the collector-tuned RF.
A high-frequency signal bypass capacitor is inserted between the self-excited oscillation circuit and the transistor of the modulation circuit connected to the totem pole, and the power supply potential of the oscillation circuit is adjusted according to the amplitude change of the modulated signal applied to the modulation circuit. 1. A frequency modulation circuit characterized in that the oscillation frequency is changed by directly changing the output capacitance of a transistor constituting the oscillation circuit. 2. The frequency modulation circuit according to claim 1, wherein the modulation circuit coupled to the collector-tuned RF self-excited oscillation circuit operates in an analog manner.