JPS6126728B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplitude modulation circuit, and attempts to provide a modulation circuit that performs amplitude modulation with a relatively simple configuration and is effective even when the carrier frequency and modulation frequency are close to each other. It is something to do.

FIG. 1 shows a block diagram of a conventional amplitude modulation circuit. In FIG. 1, numeral 1 denotes an adder circuit that adds a carrier wave and a modulation signal. 2 is a circuit that passes signals above a certain level, and 3 is a band pass filter that passes only the spectrum related to amplitude modulation in the output of the circuit 2. The operation of each part at this time will be explained with reference to FIG. The signals added by the adder circuit 1 are converted into signals as shown by the solid line in FIG. 2A by the circuit 2 which passes signals above a certain level. At this time, if the frequencies of the carrier wave and modulation signal are c and s, the spectrum will be as shown in FIG.
This spectrum includes c-s, c, c+s, which is a spectrum caused by amplitude modulation.
In addition, the spectrum of the modulated signal, higher harmonics including the second harmonic of the carrier wave, and sideband components thereof are included. Therefore, when these unnecessary spectra are removed by the bandpass filter 3, an amplitude modulated signal as shown in C in the figure is obtained. The spectrum is shown in D.

An actual example of a circuit that performs amplitude modulation in this manner is shown in FIG. Here, the carrier wave is applied through the transformer _T1 to the base of the transistor TR1. The modulation signal is connected to the capacitor
It passes through _C1 , passes through the secondary winding of the transformer _T1 , is applied to the base of the transistor TR1, and is added to the carrier wave. TR1 is biased so that it becomes conductive above a certain level of the input signal and non-conductive below a certain level, and only the signal above a certain level of the superimposed carrier wave and modulation signal is passed through. At the same time, the signal that has passed is amplified. This signal has band-pass characteristics due to the resonant circuit made up of the primary inductance of transformer T ₂ connected to the collector of TR1 and capacitor C ₂ , and high-pass characteristics due to the low-frequency transmission loss of transformer T ₂ . , only the spectrum of amplitude modulation, i.e. c, c+
Extract only s, c-s and remove other unnecessary,
By removing harmonic components and modulation signals, a signal as shown in FIG. 2C is obtained, which is extracted as a modulated wave output from the secondary side of the transformer _T2 .

By the way, in such an amplitude modulation circuit,
Disadvantages arise as the frequencies of the carrier wave and modulating signal become closer. This situation is shown in FIG. In FIG. 4, the frequency s of the modulation signal is greater than 1/2 of the carrier frequency c, for example, c=3MHz and s=2MHz. At this time, the spectrum due to amplitude modulation is
c=3MHz and c-s=1MHz and c+s=
Occurs at 5MHz. However, a modulation signal spectrum occurs at 2MHz. Furthermore, to 4MHz,
2c-s caused by the second harmonic of the carrier wave
spectrum is occurring. Therefore, the spectrum c-
It is impossible to extract only s, c+s. In this way, s and 2c-s are sideband components due to amplitude modulation, c-s and c+
The condition that s falls inside is s≧c/2, and in such a case, it is not possible to perform amplitude modulation with an amplitude modulation circuit as shown in FIG.

It is an object of the present invention to eliminate such conventional drawbacks and to realize an amplitude modulation circuit with a simple circuit configuration without using a transformer as shown in FIG. FIG. 5 shows a block diagram of the basic configuration of the present invention. A modulation signal is input to the terminal d, and is input to the variable amplitude limiting circuit 5. Further, the polarity of a part of the input signal to the terminal d is inverted by the inverting amplifier circuit 4, and the polarity is inverted by the variable amplitude limiting circuit 5.
is input to. Furthermore, a carrier wave is input to the terminal e. The variable amplitude limiting circuit 5 operates at a certain voltage V ₀
A carrier wave is passed through the center, and the carrier wave is
When the level exceeds V ₁ which is a constant voltage away from V ₀ in the positive direction, the amplitude is limited so as not to allow the carrier wave to pass. On the other hand, when the input carrier wave reaches a level that exceeds the voltage V ₂ in the negative direction, which is a constant voltage away from V ₀ in the negative direction, the amplitude is limited so as not to pass the carrier wave. The upper and lower amplitudes of the carrier waves at voltages V ₁ and V ₂ are simultaneously limited by one variable amplitude limiting circuit 5 . This V ₁ and V ₂ are determined by the modulation signal,
Each changes while maintaining an antiphase relationship. From the output of the variable amplitude limiting circuit 5, only a carrier wave component and sideband components are extracted by a bandpass circuit 6 to obtain an amplitude modulated signal.

The operation of each part at this time is shown in FIG. Same figure A
shows the modulated signal input waveform. B in the figure shows a carrier wave input signal. C in the figure shows the variable amplitude limiting circuit 5.
Indicates the operating voltage. Here, V ₀ causes a carrier wave shown in B to pass around this voltage. V ₁
is a voltage higher than _V0 , this voltage changes depending on the modulation signal A, and the variable amplitude limiting circuit 5 allows the carrier wave to pass when the voltage of the carrier wave is lower than this voltage, and when the voltage of the carrier wave is higher than this voltage. At this time, the carrier wave is not allowed to pass through. V ₂ is a voltage lower than V ₀ , and this voltage changes depending on the output voltage of the inverting amplifier circuit 4, that is, the signal obtained by inverting the modulation signal A, and the variable amplitude limiting circuit 5 is lower than this voltage. When the voltage of the carrier wave is high, the carrier wave is passed; when the voltage is lower than this voltage, the carrier wave is not passed. In this case,
The amplitude of the carrier wave is chosen to be larger than the maximum value of V ₁ −V ₂ . Variable amplitude limiting circuit 5 that operates as described above
The output waveform of is shown by the solid line in FIG. 6D. 6th
Diagram D is vertically symmetrical with respect to its center of exchange,
Furthermore, since the modulation signal is not included, the second harmonic component of the carrier wave and the modulation signal component are not included in the spectrum, so the spectrum is as shown in Figure 6E. Become. When unnecessary components in this spectrum are removed by the band pass filter 6, an amplitude modulated signal as shown in FIG. 6F is extracted.

What should be noted here is the spectrum s and 2c-
s, 2c, 2c+s do not occur in FIG. 6E. Therefore, compared to the conventional example, the present invention not only facilitates the design of the bandpass filter 6 of FIG. 5, but also eliminates the conventional drawback shown in FIG. This is the seventh
Illustrated in the diagram. In Fig. 4, the case where s≧1/2c, for example, s = 2MHz, c = 3MHz, is shown, but in the conventional example, as shown in Fig. 4, frequency modulation in such a case is was impossible. However, in the case of the present invention, the spectral distribution under similar conditions is as shown in FIG. 7, and the spectrum c-s, c, c,
It is possible to extract only c+s, and the problem shown in FIG. 4 does not occur. Therefore,
In principle, up to the frequency immediately before c=s where c+s and 3c-s intersect, that is,
Amplitude modulation is possible within the condition of s<c.

Referring to FIG. 8, a specific embodiment of the present invention will be explained. The modulated signal is input to terminal g. The transistor TR3 constitutes an inverting amplifier circuit (corresponding to 4 in FIG. 5) with a voltage gain of 1. Transistors TR2, TR4, TR5 and diodes D ₁ , D ₂
constitutes a variable amplitude limiting circuit (corresponding to 5 in FIG. 5). Here, transistors TR2 and
TR4 is an emitter follower circuit, and the emitter voltages of transistors TR2 and TR4 correspond to the voltages _V1 and _V2 in FIG. 6C, respectively, and the output impedance thereof is very low. The transistor TR5 is an amplifier circuit that amplifies the carrier wave input to the terminal h to an amplitude larger than the voltage _V1 - _V2 , and its output impedance is mainly determined by the collector resistor _R1 , and it has a relatively high impedance. Become. The carrier wave amplified by TR5 and output from its collector is output from the emitter follower circuit of transistor TR6 through terminal i when diodes _D1 and _D2 are non-conducting, but when diode _D1 or D When ₂ is conductive, TR5 collector → C ₃ → D ₁ → TR
2 emitter or TR5 collector → C ₃ → D ₂
→ Current flows to the emitter of TR4 and the carrier wave is not output to terminal i. At this time, the average value of the current flowing through D ₁ or D ₂ becomes the same due to the function of the capacitor C ₃ to block direct current. Therefore, the average voltage at the connection point j between D ₁ and D ₂ is equal to the intermediate voltage between V ₁ and V ₂ . That is, it becomes equal to V ₀ in FIG. 6C.
Therefore, when the voltage of the positive half wave of the carrier wave output from the collector of TR5 becomes larger than V ₁ −V ₀ , D ₁ becomes conductive and the absolute value of the voltage of the negative half wave becomes V ₀
When it becomes larger than −V ₂ , D ₂ becomes conductive and D ₁ or D ₂
However, when conductive, the carrier wave is not added to the base of TR6. Therefore, the signal output to terminal i is
The waveform becomes as shown in FIG. 6D, and the variable amplitude limiting circuit is configured. A bandpass filter that passes sideband components and carrier wave components can be further connected to the terminal i to obtain an amplitude-modulated signal, but the bandpass filter is omitted in this figure.

FIG. 8B is a diagram showing another embodiment of the present invention. The same parts as A are given the same symbols. Transistor TR7 is a common emitter circuit with a voltage gain of 1, and its collector and emitter output voltages have the same amplitude and opposite polarity. Transistors TR8 and TR9 obtain a direct bias from the collector and emitter of the transistor TR7, and at the same time perform impedance conversion, and apply the voltages V ₁ and V ₂ of FIG. 6C to their respective emitters.
This is an emitter follower circuit that obtains the following. The following operations are the same as those in FIG. 8A. put it here,
By obtaining the bias for TR8 and TR9 from the collector and emitter of TR7, the bias circuit can be simplified compared to FIG. 8A. Furthermore, TR7
It is also possible to use a PNP transistor.

In this way, the present invention provides an amplitude modulation circuit that includes:
Create second and third voltages that are separated up and down by the same voltage from the center voltage (first voltage) and change to opposite polarities depending on the input modulation signal, and It is characterized by a configuration in which the amplitude of the carrier wave is modulated by simultaneously controlling the amplitude of both the upper and lower carrier waves using voltage, and one variable amplitude limiting circuit is used to control the amplitude of the upper and lower carrier waves according to the modulation signal. Therefore, it is possible to perform amplitude modulation that does not generate second harmonic components, thereby making it easier to control the carrier wave by the filter in the subsequent stage. By limiting the upper and lower amplitudes at the same time, it is possible to have the simplest circuit configuration using only one amplitude limiting circuit, and since the amplitude is limited by one circuit, an imbalance between the upper and lower waveforms may occur. This eliminates the generation of even-order harmonics and makes it easy to separate the carrier wave and modulated output signal using a filter.

As described above, according to the present invention, in principle, the frequency of the modulation signal can be increased to just before the frequency of the carrier wave, and an amplitude modulation circuit can be realized with a simple configuration. Furthermore, as shown in Figure 8, there is no need to use a relatively expensive transformer.
Since it can be constructed using only resistors, capacitors, transistors, and diodes, it is inexpensive and
It has the characteristic that it can be easily integrated into an IC.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a conventional amplitude modulation circuit, Fig. 2 is a waveform diagram showing the operation of each part in Fig. 1, Fig. 3 is a circuit diagram showing a conventional amplitude modulation circuit, and Fig. 4 5 is a block diagram showing the basic configuration of the present invention. FIG. 6 is a waveform diagram showing the operation of the present invention.
8 is a diagram showing the spectral distribution of the present invention, and FIG. 8 is a circuit diagram showing an embodiment of the present invention. 4...Inverting amplifier circuit, 5...Variable amplitude limiting circuit, 6...Band pass filter.

Claims (1)

[Scope of Claims] 1. Means for creating second and third voltages that are separated by the same voltage above and below a constant first voltage and whose voltage values change in opposite polarities in accordance with an input modulation signal. , the second and third voltages, and a carrier wave with a constant amplitude centered around the first voltage are applied, and a carrier wave exists between the second voltage and the third voltage among the carrier waves. and one variable amplitude limiting circuit that simultaneously limits the amplitude of a portion of the amplitude of the carrier wave that exceeds the second and third voltages using the second and third voltages so that only the portion that exceeds the second and third voltages passes through. An amplitude modulation circuit characterized by: 2. A second voltage and a third voltage are each obtained from a low impedance circuit, a first diode whose cathode is connected to the low impedance circuit that obtains the second voltage, and a cathode connected to the anode of the first diode. and a second diode having an anode connected to a low impedance circuit that obtains the third voltage, and supplying a carrier wave to the connection point of the first and second diodes through the high impedance circuit. An amplitude modulation circuit according to claim 1, characterized in that: 3. The amplitude modulation circuit according to claim 2, wherein the low impedance circuit is constituted by an emitter follower circuit. 4. The amplitude modulation circuit according to claim 1, wherein the second and third voltages are obtained from a low impedance circuit connected to the collector and emitter of a grounded emitter circuit with a voltage gain of 1.