JPH0936665A - Fm modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FM modulator which has high linearity of the potential- frequency displacing characteristic and also has a small distortion factor. SOLUTION: An FM modulator includes of an operational amplifier 12, a time constant circuit consisting of a capacitor 17 which is connected between the inverted input terminal of the amplifier 12 and the ground and a 1st resistor 16 which is connected between the output terminal and the inverted input terminal of the amplifier 12, and an astable multivibrator 2 containing a feedback 2nd resistor 15 which connected between the output terminal and the non- inverted input terminal of the amplifier 12. Furthermore, the bias voltage generation means 4 and 5 are added to apply the bias voltage to the non-inverted input terminal of the amplifier 12. Then the modulated signals are superimposed on the bias voltage of fixed level generated by the means 4 and 5.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an FM modulator.

[0002]

2. Description of the Related Art A conventional FM modulator (frequency modulator) will be described below with reference to FIG. This FM modulator constitutes a PLL circuit as a whole. The phase comparator 32 compares the oscillation signal from the reference oscillator 31 with the oscillation signal from the unstable multivibrator 36 as a potential control type oscillator, and supplies the comparison output to the low pass filter 33. Then, the adder 35 including a DC amplifier adds the output of the low-pass filter 33 and the modulation signal from the modulation signal source 34, supplies the added output (superimposed output) to the oscillator 36, and oscillates the same. Control the frequency. The output side of the oscillator 36, that is, the output terminal 37,
The modulated signal is output.

The non-stable multivibrator 36 used here has a pair of transistors whose emitters are grounded, and the collector of each one of the transistors is connected through a capacitor to the base of the other transistor. Is connected to a common input terminal through a resistor, and the collector of each transistor is connected to a power supply through a load resistor.

Such an FM modulator is used as a transmitter for testing a receiving circuit for receiving a US second audio program broadcast. In this case, the added signal of the right audio signal and the FM-modulated left audio signal is supplied to the above DC amplifier as a modulation signal.

[0005]

In the conventional FM modulator shown in FIG. 4, the modulation distortion is about 1% because the linearity of the potential-frequency shift characteristic of the astable multivibrator itself is not so good. Fidelity is not very high.

In view of the above point, the present invention intends to propose an FM modulator having a good potential-frequency shift characteristic linearity and a low distortion rate.

[0007]

According to the present invention, there is provided an operational amplifier, a capacitor connected between an inverting input terminal of the operational amplifier and ground, and a first capacitor connected between an output terminal and an inverting input terminal of the operational amplifier. To a non-inverting input terminal of the operational amplifier, and a non-stable multivibrator including a time constant circuit composed of a resistor and a second resistor for feedback connected between the output terminal and the non-inverting input terminal of the operational amplifier. An FM modulator having a bias voltage generating means for applying a constant bias voltage, wherein a modulation signal is superimposed on the constant bias voltage from the bias voltage generating means.

According to the present invention, the one in which the modulation signal is superimposed on the constant bias voltage from the bias voltage generating means is applied to the non-inverting input terminal of the operational amplifier which constitutes the astable multivibrator.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. First, the configuration of the unstable multivibrator 2 will be described. The feedback resistor 15 is connected between the output terminal and the non-inverting input terminal of the operational amplifier 12.
The inverting input terminal of the operational amplifier 12 is grounded through the capacitor 17, and the resistor 16 is connected between the output terminal and the inverting input terminal of the operational amplifier 12. The capacitor 17 and the resistor 16 form a time constant circuit. Operational amplifier 1
The two non-inverting input terminals are grounded through a resistor. The DC voltage from the power source + B is applied to the operational amplifier 12. Output terminal of operational amplifier 12

Here, an example of the value of each element connected to the operational amplifier 12 is shown. Resistor 14: 6.8 kΩ, resistor 15: 8.2 kΩ, resistor 16: 2 kΩ, capacitor 1
7: 220 pF.

Next, the amplifier 1 will be described. Power +
Between B and ground, resistor 4 and Zener diode 5
Is connected to form a constant voltage circuit.
The anode side of the diode 5 is grounded. The midpoint of connection between the resistor 4 and the Zener diode 5 is connected to the non-inverting input terminal of the operational amplifier 3 through the resistor 6.
A capacitor 7 is connected to the non-inverting input terminal 3 of the operational amplifier 3.
The signal source 8 of the modulation signal is connected through the. Although not shown, the other end of the signal source 8 is grounded. Operational amplifier 3
The inverting input terminal of is grounded through a series circuit of a resistor 9 and a capacitor 10. Incidentally, one end of the capacitor 10 is grounded. A resistor 11 is connected between the output terminal and the inverting input terminal of the operational amplifier 3. A DC voltage from the power source + B is applied to the operational amplifier 3.

It should be noted that by changing the ratio of the resistance values of the resistors 13, 14 and 15, the FM target obtained at the output terminal 19 can be obtained.
The duty of the modulation signal can be changed.

The output terminal of the operational amplifier 3 of the amplifier 1 is connected to the astable multivibrator 2 through the resistor 13.
Connected to the non-inverting input terminal of the operational amplifier 12. An example of the value of the resistor 13 is 6.8 kΩ.

In this way, the pulse signal whose frequency changes according to the level of the modulation signal from the signal source 8 is output terminal 1.
9 is obtained. Further, the FM-modulated signal from the output terminal 19 is FM demodulated by being supplied to the low-pass filter 20 and integrated, and the demodulated potential is obtained by the resistor 1 of the amplifier 1.
Connect to the connection midpoint of 1 and 9. The low pass filter 20 may be omitted. However, the low-pass filter 20
The distortion factor is further reduced and the linearity of the voltage-frequency shift characteristic is further improved.

The modulation signal from the signal source 8 can be an audio signal, a video signal or the like. When the present invention is applied to a transmitter for testing a receiving circuit for receiving a second audio program broadcast in the United States as described at the beginning,
A superimposed signal of the right audio signal and the FM-modulated left audio signal may be supplied as a modulation signal to the non-inverting input terminal of the operational amplifier 3 of the amplifier 1 through the capacitor 7.

FIG. 2 shows waveforms of a point b on the non-inverting input terminal side, a point c on the inverting input terminal side, and a point d on the output terminal side of the operational amplifier 12 of the astable multivibrator 2.
The left side and the right side of FIG. 2 respectively show the waveforms at points b, c, and d when the potential at the point a on the output terminal side of the operational amplifier 1 is raised and lowered, respectively. In this case, the AC amplitudes of the waveforms at the points b, c, and d and the ON duration of the point d are substantially constant regardless of the potential at the point a. The lower the potential at point a, the longer it becomes. From this, it is understood that the output terminal of the operational amplifier 12 of the astable multivibrator 2 can obtain the FM modulated signal whose period (frequency) changes according to the potential of the output terminal of the operational amplifier 3 of the amplifier 1. .

As is well known, in the operational amplifier, when the potential of the inverting and non-inverting input terminals exceeds the offset voltage of the input terminals on the opposite sides (voltage unique to the operational amplifier), the potential of the output terminal becomes the saturation potential. Change to the point. Therefore, when a positive potential is applied to the non-inverting input terminal of the operational amplifier 12 of the unstable multivibrator 2, the potential of the output terminal instantaneously saturates at the non-inverting input terminal due to the positive feedback of the resistor 15. It rises to the potential point. Thus, the voltage at this output terminal causes the capacitor 17 to move to the resistor 16
Is charged through and rises along the waveform of point C in FIG.
When the potential exceeds the potential at the point b by the offset, the potential at the point d instantly drops to the saturation potential point of the inverting input terminal. In this way, the potential of the non-inverting input terminal drops relatively quickly due to the time constant due to the stray capacitance of the non-inverting input terminal system. The potentials of the inverting input terminal and the non-inverting input terminal start decreasing at the same time, and the capacitor 1 has a time constant determined by the resistance value of the resistor 16 and the capacitance value of the capacitor 17.
When the electric charge of 7 is discharged and the potential thereof falls below the offset potential, the potential of the point d instantly reaches the saturation potential point of the non-inverting input terminal again, and the above-mentioned operation is repeated.

Next, if the potential at the point a is lowered, the potential at the point b is also lowered. Therefore, it takes a considerable time until the potential at the point c is lowered to a potential lower than the potential at the point b. The inversion cycle of the potential of d becomes long.

On the other hand, the point d continues to be on until the charging of the capacitor 17 is completed, and the charging of the capacitor 17 begins at the same time when the point d shifts toward the saturation point of the non-inverting input terminal of the operational amplifier 12, but the saturation potential. The point is reached instantly,
It can be considered that the substantial start of charging is the time when the voltage at the point d reaches the saturation potential point of the non-inverting input terminal of the operational amplifier 12. Then, the saturation potential point of the inverting input terminal of the operational amplifier 12 is
Since it has nothing to do with the potential of the saturation potential point of the non-inverting input terminal of the operational amplifier 12, the charging completion time becomes constant regardless of the saturation potential point of the non-inverting input terminal of the operational amplifier 12.

Therefore, by superimposing the modulation signal on the point a, a frequency-modulated signal whose pulse width is constant and only the pulse repetition period changes is obtained at the point d.

After the audio signal of a compact disk player, a mini disk player, a helical scan type digital audio player, or the like is FM-modulated, it is transmitted to the space by an infrared light emitting device, and the transmitted FM-modulated audio signal is transmitted to the infrared of the wireless headphones. The FM modulator in the wireless headphone system that receives the signal by the light receiver and then demodulates it to obtain the audio signal
It is suitable to apply a modulator. In this case, the carrier frequency is practically 30 kHz to 70 kHz.

Of course, it is preferable to apply the above FM modulator to a transmitter for testing a receiving circuit for receiving a second audio program broadcast in the United States as described at the beginning.

[0023]

According to the first invention described above, the operational amplifier, the capacitor connected between the inverting input terminal of the operational amplifier and the ground, and the output terminal and the inverting input terminal of the operational amplifier are connected. And a non-inverting multi-vibrator including a time constant circuit including a first resistor and a feedback second resistor connected between the output terminal and the non-inverting input terminal of the operational amplifier, and the non-inverting of the operational amplifier. Since the input terminal has a bias voltage generating means for applying a constant bias voltage, and the modulation signal is superimposed on the constant bias voltage from the bias voltage generating means, the linearity of the potential-frequency shift characteristic is good and the distortion factor is high. A low FM modulator can be obtained.

According to the second aspect of the present invention described above, in the first aspect of the present invention, the bias voltage generating means is provided with the amplifier for amplifying the modulation signal. Therefore, in addition to the effect of the first aspect of the present invention, the amplification is performed. The modulated signal thus generated can be supplied to the astable multivibrator.

[Brief description of drawings]

FIG. 1 is a block diagram showing an FM modulator according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing a partial signal waveform of the FM modulator.

FIG. 3 is a modulation input vs. distortion factor / frequency shift characteristic curve diagram of the FM modulator.

FIG. 4 is a block diagram showing a conventional FM modulator.

[Explanation of symbols] 1 amplifier 2 astable multivibrator 3 operational amplifier 8 signal source of modulation signal 12 resistor 13 resistor 14 resistor 15 resistor 16 resistor 17 capacitor 20 low pass filter

Claims (2)

[Claims] 1. An operational amplifier, a capacitor connected between the inverting input terminal and the ground of the operational amplifier, and a first connected between the output terminal and the inverting input terminal of the operational amplifier.
A non-stable multivibrator including a time constant circuit including a resistor and a second resistor for feedback connected between the output terminal and the non-inverting input terminal of the operational amplifier, and the non-inverting input of the operational amplifier. An FM modulator comprising: a bias voltage generating means for applying a constant bias voltage to a terminal, wherein a modulation signal is superimposed on the constant bias voltage from the bias voltage generating means. 2. The FM modulator according to claim 1, wherein the bias voltage generating means is provided with an amplifier for amplifying the modulation signal.