Amplitude modulation with a suppressed carrier
In this article, I will explain Amplitude modulation with a suppressed carrier in detail. In this I have discussed all the 3 types of AM with suppressed carrier generator along with the required circuit diagram and wave forms.
In the conventional amplitude modulation with respect to the information to be transmitted a bad money. Most of the transmitter power has to be applied for the carrier, which contains no information as auxiliary signal. Suitable measures may be suppressed in the modulation of the carrier. The Modulation product has then only the two sidebands. The process is called amplitude modulation with suppressed carrier or double-sideband modulation ZM, English as a DSBM, double sideband modulation. The currently best known applications of the ZM are stereophonic sound transmissions and analog color television. It will be shown several methods for AM with suppressed carrier.The mathematical background
Modulation or the information signal and the high-frequency carrier are multiplied directly with each other. A preceding signal superposition or addition does not take place. Notes on the multiplication of the cosine functions can be found in Section mathematics - trigonometric functions . The result indicates the presence of the two sidebands. The clean carrier signal is not present.
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Own created using CAD softwareAmplitude modulation multiplier IC
In the simulation, the signals are multiplied by software mathematically. In practice, using a four-quadrant multiplier as a modulator IC. The module requires a symmetrical DC voltage supply. To the one input signal and the NF on the other hand, the RF signal is applied. The output signal is an amplitude modulated signal with suppressed carrier.
The signal timing diagram on the oscilloscope differs significantly from normal AM signal. In the frequency spectrum, only the two side lines are visible in the single-tone modulation. Both input signals do not appear in the output signal. Higher modulation products are generated. The audio input signal (blue) as a comparison signal in the output of AM-represented.
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In the frequency diagram of a voice or music frequency band is disposed as the upper sideband and lower sideband USB OSB symmetrically on both sides of the carrier frequency location. The volume corresponds to the signal amplitude. The spectrum is like the normal AM twice the highest frequency to be transmitted NF. NF b = 2 f max ·Amplitude modulation with diode ring modulator
The AM with suppressed carrier was used long before the development of integrated multiplier. Is modulated at the curved diode characteristic. The circuit uses a quartet of selected diodes diodes. To form a conductive ring in a direction in a series circuit. The carrier serves as the switching voltage for the diodes. Its amplitude must be able to turn on the diode only in the conducting or the blocked state, and has no effect on the amplitude of the output signal. Without NF-information signal is to be measured at the output winding at U ZM no signal. The diode ring modulator has the disadvantage that two equal-sized small RF transformers are needed with center. The circuit configuration is shown in the following figure.
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The carrier signal is present at the center taps of the transformers that are connected via the diode ring. Alternating the polarity of the half-wave HF times to the left or right transformer coil positive signal. As a result, either the external or internal diodes are conducting. Without AF signal is at the ends of each coil transformers no potential difference and the output winding no signal is induced.
Located on the left transformer, the modulation signal, so it transfers the AC voltage on the inner winding. From there, the NF passes through the straight-conducting diode pair to the output transformer and is transformed as change size on the output coil. The output signal of the shifting operation can be clearly seen. The zero crossing of the two signal voltages, a phase jump occurs. The polarity change of the phase shifts is alternately in even-numbered frequency ratio. The following circuit diagram illustrates the emergence of the phase jumps.
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The frequency spectrum of the output signal has a number of high-frequency modulation products. They are located at the odd integer multiple of the carrier frequency. The carrier has a function of switching voltage for the diodes, and thus could have been replaced by a symmetric square-wave signal of the same frequency. The Fourier analysis of this square wave would spectral lines at 10, 30, 50 kHz, etc. show.
In the AM with suppressed carrier come to this support only symmetric positions on the sidebands. The higher modulation products must be removed by a good low-pass filter. The first ZM-harmonic of the signal may be very well be filtered out with a resonant circuit which is tuned to the fundamental frequency of the carrier.AM with product or coincidence modulator
Transformers and coils are quite large. Their properties are not identical in pairs and also not long-term stability. Where it is, therefore, one tries to replace these components. Instead of a diode ring modulator transistor circuit is used for double-sideband modulation. They are available as an integrated circuit, and coincidence of the product modulator. Coincidence, from the Latin, means here that only the simultaneous occurrence of two signals generated at the inputs of an output signal. In the circuit operate three differential amplifier stages V 1.2, V 3.4 and V 5.6. The two collector resistances are equal. Arises as a bridge between them, the modulation voltage U ZM product.
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The circuit is dimensioned such that the RF carrier with its positive half cycle voltage of the transistor V is guided 5th In the negative half cycle V 6 is conductive because its base is then relative to that of 5 V positive. Without low-frequency signal, the upper differential amplifier are non-conductive. The result is no bridge voltage. The positive half-wave of the LF signal will be guided V 1 and V 4th The negative half-wave control, and V 2 V 3 conductive. It follows a reciprocal change in potential at the collector resistors. The final product, the polarity of the modulation is determined by the lower switching transistors.