What Is Amplitude Modulation?

A modulation method in which the amplitude of a carrier wave is changed in accordance with a change rule of a required transmission signal, but the frequency remains unchanged. AM is widely used in wired or radio communications and broadcasting. ^[1] Amplitude modulation Modulation (AM) in which the amplitude of a high-frequency carrier changes with the signal. Among them, the amplitude of the carrier signal changes with the transformation of certain characteristics of the modulation signal. For example, 0 or 1 corresponds to no carrier or carrier output respectively, and the image signal of TV uses amplitude modulation. FM has strong anti-interference ability, small distortion, but small service radius.

Chinese name: AM
Foreign name: Amplitude Modulation

Range: At 530 --- 1600KHz
Explanation: Medium wave

AM Introduction

Amplitude Modulation (AM). Amplitude modulation is also commonly called medium wave, with a range of 530 --- 1600KHz. Amplitude modulation is an electric signal that changes the amplitude by the level of sound. The transmission distance is long, but it is greatly affected by weather factors, which is suitable for inter-provincial radio broadcasting. Most of the early mobile communication stations in the VHF band used amplitude modulation. Due to the channel fading, the analog amplitude modulation will generate additional amplitude modulation, which will cause distortion. It is also easy to be eavesdropped during the transmission process, and it has been rarely used. At present, it is also used in simple communication equipment. For example, the AM band in the radio is an AM wave, and the sound quality is relatively poor compared to the FM wave.

AM classification

Amplitude modulation can be divided into several different ways: ordinary amplitude modulation (AM), double-sideband amplitude modulation (DSB-AM), single-sideband amplitude modulation (SSB-AM) and residual sideband amplitude modulation (VSB-AM). ^[2]

AM and AM

The double-sideband AM signal contains only two side frequencies and no carrier component, and its frequency band width is still twice the frequency of the modulation signal.

AM Single Sideband AM

The single-sideband AM signal contains only one side frequency.

AM sideband AM

Residual sideband amplitude modulation refers to the method of amplitude modulation of a transmitted signal including a complete sideband, a carrier, and a small portion of another sideband.

Waveforms and expressions of ordinary AM signals

Let the expression of the carrier u _c ( t ) and the expression of the modulation signal u ( t ) be:

According to the definition of amplitude modulation, when the amplitude of the carrier wave changes linearly with the size of the modulation signal, it is an amplitude modulation signal. The waveform of the modulated wave is shown in Figure (c) above, and Figures (a) and (b) are Modulated signal and carrier waveform. It can be seen from the figure that the envelope shape of the amplitude change of the AM wave is the same as that of the modulation signal, and the high frequency oscillation frequency in the envelope is still the same as the carrier frequency, indicating that the AM wave is actually a high frequency signal. It can be seen that the amplitude modulation process only changes the amplitude of the carrier, so that the carrier amplitude and the modulation signal have a linear relationship, even if U _cm becomes U _cm + K _a U _m cos t , according to this, the expression of the AM wave can be written as

M _{a is} called the amplitude modulation coefficient, U _max represents the maximum value of the amplitude modulation wave envelope, and U _min represents the minimum value of the amplitude modulation wave envelope. M _a indicates the degree to which the carrier amplitude is controlled by the modulation. Generally, 0 M _a 1 is required so that the envelope of the amplitude modulated wave can correctly show the change of the modulation signal. The case where M _a > 1 is called overmodulation, and the following figure shows the modulated waveform when different M _a .

In order to analyze the frequency components contained in the AM signal, equation (4-3) can be expanded according to the trigonometric function formula to obtain

It can be seen that there are three frequency components in the modulated wave: _c , _c + and _c - . _c + is called the upper side frequency, and _c - is called the lower side frequency. The resulting spectrum of the AM wave is shown below.

From the spectrum of the AM wave, the bandwidth of the AM wave is BW = 2 F , where F is the modulation frequency.

(1) If the modulation signal is a complex multi-frequency signal, its spectrum is shown in the figure below.

For example, the frequency range of a voice signal is 300 ~ 3400Hz, then the amplitude bandwidth of the voice signal is 2 × 3400 = 6800Hz. Observing the frequency spectrum of the AM wave, it is found that whether it is a single audio modulation signal or a complex modulation signal, the modulation process is a linear transfer process of the frequency spectrum, that is, the frequency spectrum of the modulation signal is moved to both sides of the carrier frequency without distortion. Therefore, amplitude modulation is called linear modulation. The AM circuit is a linear shift circuit of the frequency spectrum.

(2) If the modulation signal is a single-frequency cosine signal and the load resistance is R _L , the power of the modulated wave mainly has the following types.

Carrier power

2. Upper and lower sideband power

3. Total average power

4.Maximum instantaneous power

Method for generating and demodulating ordinary AM signal

Generation of AM AM Signal

The generation of ordinary AM signals can be achieved by adding the modulation signal to DC and multiplying it with the carrier signal to achieve ordinary AM. Low level AM method and high level AM method can be used.

AM demodulation method

(1) Envelope detection

The envelope of a normal AM signal is used to reflect the characteristics of the modulation signal waveform change. If the envelope can be extracted, the original modulation signal can be restored.

(2) Synchronous detection

Synchronous detection must use a signal with the same frequency and phase as the carrier at the transmitting end. This signal is called a synchronization signal.

Note: Double-sideband AM, single-sideband AM, and residual sideband AM can only use synchronous detection.

AM and AM circuit

The principle of AM circuits is mainly divided into two categories: high-level AM circuits and low-level AM circuits, as follows:

AM high level AM

High-level AM requires the output power of the circuit to be large enough. The circuit performs power amplification at the same time as amplitude modulation. The modulation process is usually performed in a Class C amplifier stage. According to the electrodes controlled by the modulation signal, the modulation methods mainly include collector amplitude modulation, base amplitude modulation, and emitter amplitude modulation.

1. Collector AM

(1) The characteristics of the collector AM circuit are:

The low-frequency modulation signal is applied to the collector circuit. B ₁ and B ₂ are high-frequency transformers; B ₃ is a low-frequency transformer. The low-frequency modulation signal u (t) is connected in series with the DC power supply of the Class C amplifier, so the effective collector supply voltage V _cc (t) of the amplifier is equal to the sum of the two voltages, which changes with the modulation signal. The capacitors C _b and C` in the figure are high-frequency bypass capacitors. The role of C` is to prevent high-frequency current from passing through the secondary coil of the modulation transformer B ₃ and the DC power supply. The signal frequency should be equivalent to an open circuit.

For class C high-frequency power amplifiers, when the base bias Vbb, the high-frequency excitation signal voltage amplitude Ubm, and the collector loop impedance Rp are unchanged, and only the effective supply voltage of the collector is changed, the collector current pulse is in the undervoltage region. Can be considered unchanged. In the overvoltage region, the amplitude of the collector current pulse will vary with the effective supply voltage of the collector. Therefore, the collector AM must work in the overvoltage region.

(2) Collector AM can only produce ordinary AM waves.

The advantage is that the AM linearity is better than the base AM. In addition, because the collector amplitude modulation always works in the critical and weak overvoltage regions, the efficiency is relatively high.

The disadvantage is that the power supplied by the modulation signal in the collector loop is relatively large.

2.Base amplitude modulation

The characteristic of the base AM circuit is that the modulation signal is added to the base loop. In the figure, C ₁ and C ₃ are high-frequency bypass capacitors; C ₂ is a low-frequency bypass capacitor; B ₁ is a high-frequency transformer; B ₂ is a low-frequency transformer; and the LC loop is a band-pass filter. Make sure that the loop is tuned to _C and the passband is 2.

The principle of base-amplitude modulation is realized by using a class-C power amplifier under the condition that the power supply voltage Vcc, the input signal amplitude Ubm, and the resonance resistance Rp are unchanged, and Vbb is changed in the undervoltage region. AM.

The advantage of base-amplitude modulation is that because the modulation signal is connected to the base loop, only a small amount of power is needed for the modulation signal.

The disadvantages are: low efficiency, and the modulation linearity is not as good as the collector amplitude modulation.

AM low level AM circuit

(1) Analog multiplier AM circuit

Role: Multiplying two analog signals

symbol:

Circuit diagram:

(2) Diode modulation circuit

The diode modulation circuit includes a single diode modulation circuit, a diode balance circuit, and a diode double balance modulation circuit.

1) Single diode circuit

The single diode circuit is shown below.

When the voltage UD across the diode is greater than the on-voltage of the diode, the diode is on, and the current flowing through the diode is proportional to the voltage applied across the diode. When the voltage UD across the diode is less than the on-voltage of the diode, the diode is off and the current 0; the diode is equivalent to a controlled switch. The control voltage is the voltage UD across the diode.

When Ucm >> Um and Ucm is a large signal (> 0.5V), it can be further considered that the on / off of the diode is mainly controlled by Uc. Generally, the turn-on voltage UP of the diode is relatively small, and there is Ucm >> UP, which can make UP approximately 0 or add a fixed bias voltage in the circuit to cancel UP. Ignoring the reaction of the output voltage, we can get

The corresponding spectrum diagram can be obtained as follows:

After passing it through a band-pass filter with c as the center and 2 as the passband, an AM wave can be obtained.

The analysis here ignores the reaction of the output voltage. This is because the output voltage is small relative to Uc. If the reaction is considered, the output voltage has little effect on the voltage across the diode, and the frequency component does not change, which may reduce the output signal amplitude (rDàrD + RL).

In addition, if the large-signal condition is not met, the switching function analysis method or linear time-varying analysis method cannot be used, but the power series analysis method can be used to know that the circuit can still complete the linear shift function of the frequency spectrum.

2) Diode balanced modulator

In the single-diode circuit, the frequency component generated by the diode is greatly reduced because it works in a linear time-varying working state, but there are still many unnecessary frequency components in the generated frequency components, so it is necessary to further reduce some frequencies Weight.

Diode balancing circuits can meet this requirement. The principle circuit is shown below.

This circuit consists of two diodes with the same performance and a center-tapped transformer Tr1, Tr2 connected into a balanced circuit. The upper and lower parts of the circuit are exactly the same. The control signal (carrier signal) is added at the center taps of the two transformers, and the input signal (modulation signal) is connected to the input transformer, that is, the carrier signal is added to D1 and D2 in phase; the modulation signal u2 is added to D1 and D2 in opposite phases. The output transformer is connected to a filter to filter out unwanted frequency components. The load resistance seen from Tr2 times to the right is RL. This circuit can be equivalent to the following principle circuit form.

Since the control voltages applied to the two diodes are in phase, using the switching function analysis method, the total current on the load can be obtained as

Its spectrum diagram is as follows:

Compared with the single-diode circuit, i contains spectrums: , 1 ± , 31 ± , ..., and after the LC bandpass filter with a 3dB bandwidth of 2 and a center angular frequency of c, the spectrum c can be obtained at load RL ± voltage component, it can be seen that DSB modulation is realized. This is not difficult to understand, because the control voltage uC is applied in-phase to both ends of the two diodes. When the circuit is completely symmetrical, two equal C components cancel each other, so there is no longer C and its harmonic components in the output. That is, in the output, unnecessary frequency components are further reduced. (DSB AM)

3 ) Diode Double-Balance Modulator-Diode Ring Modulator

In the diode-balanced modulation circuit, through the symmetrical connection of two single-diode circuits, the unnecessary frequency components are greatly reduced, and the amplitude of the useful frequency components is doubled. However, there are still unnecessary frequency components such as the frequency component of the modulation signal, and the amplitude of the obtained useful frequency component is still not very large. So, is it possible to further reduce the unnecessary frequency components and double the amplitude of the useful components by rebalancing?

Diode double-balanced circuits can meet this requirement. The principle circuit is shown in the figure.

This circuit is composed of two dual-diode balanced circuits. Since four diodes are connected in a loop, this circuit is also called a diode ring modulator. The carrier is connected from the transformer T1, the modulation signal is connected between the center taps of the two transformers, and the transformer T2 outputs the modulated signal.

The analysis conditions are the same as those of the single diode circuit and the diode balance circuit.

The working conditions of each diode are as follows:

You get,

Its spectrum diagram is as follows:

i contains frequency spectrums: c ± , 3c ± ... After a band-pass filter with a center of c and a 3dB bandwidth of 2, the spectrum c ± voltage spectral components can be obtained on the load RL, which realizes DSB modulation.

It can be seen from the spectrogram that the ring circuit eliminates the frequency component of the low-frequency modulation signal on the basis of the balanced circuit, and the output DSB signal amplitude is twice that of the balanced circuit. Its unmodulated signal component is the result of two balanced cancellations. Each balanced circuit itself cancels the carrier and harmonic components. The two balanced circuits cancel the modulated signal components, so the performance of the loop circuit is closer to the ideal multiplier.