AM Transmitter

AM (Amplitude Modulation) is a type of analogue modulation where the information of the message signal is stored in the amplitude of the carrier signal. The frequency range of AM is 535 to 1605 kHz which is less as compared to other modulation techniques. Any modulation technique requires two waves which are the message signal and carrier wave. The carrier wave is a high-frequency sinusoidal (sine or cosine) wave having a constant frequency. A message signal is a low-frequency wave usually in the audio frequency range which contains information on the amplitude of the signal.

In AM both the message signal and carrier wave are analog waves. AM was the first type of modulation invented to transmit signals to longer distances, have better immunity to interference and reduce the height of the antenna. This technique is relatively simple and cost-effective. In a communication system, there are three main components which are transmitter, receiver and channel. In this article, we will discuss the transmitter part of Amplitude Modulation.

Amplitude Modulation Waveform

What is AM Transmitter ?

In order to transmit the message signal to a receiver that is far away from the sender, the message signal goes through various steps such that the signal is sent successfully to the receiver. These steps form the different blocks of a transmitter which helps to achieve wireless transmission of signal.

The message signal cannot be directly fed through an antenna since it’s frequency is quite low and it would require a huge antenna. Hence it is modulated with a high-frequency wave called a carrier in which the amplitude of the message signal determines the amplitude of the carrier wave. This reduces the length of the antenna and provides good immunity to electromagnetic interference.

Amplification is also necessary since the modulated signal gets attenuated due to reflection, scattering, etc. These blocks forms the transmitter part of Amplitude Modulation technique.

Block Diagram of AM Transmitter

There are two types of AM transmitter based on the output power at which the modulated signal is transmitted. The range of communication also depend on the power of the transmitted wave and if it is not adequate then the entire wave will get attenuated according to transmission formula.

• For long range communication, the transmitted power is in kW (kilowatt) and for this purpose a high level transmitter must be used.
• For short range communication, the transmitted power is in W (watt) and a low level transmitter is used to transmit such signals.

The main difference between high level and low level transmitter is that the message signal and carrier wave are amplified before passing it to the modulator block and amplified again in high level transmitter whereas in low level transmitter, message signal and carrier wave are only amplified after modulation process.

High Level Transmitter

Given Below is the Block Diagram of the High Level Transmitter

Block diagram of high level AM transmitter

The high level AM transmitter consists of the following blocks:

Microphone: Microphone is a transducer which converts sound waves into electrical waves. A transducer is a device which has the ability to convert one form of energy into another form of energy. Human voice is converted to message signal with the help of microphone. The resultant electric signal produced by the microphone is in mV (milivolt) or few V (volt) and contains noise. Hence it is passed through the next block which processes the signal.

Processer: An audio processing unit is employed to filter out noise using filter and increase the amplitude of wanted signal using amplifier. Amplifier amplifies the electrical signal received from the microphone to a intermediate level. This amplification is necessary because in order to increase the magnitude of actual signal to increase signal to noise ratio. Noise are unwanted electromagnetic interferences generally having low amplitude. If such noise gets mixed with our signal then it becomes difficult to retrieve actual information stored in the signal. Hence it is usually connected just after the microphone so that the output signal provided by the device has less interference.

Audio Amplifier: This block is only present in high level transmitter and provides additional amplification to the message signal. This amplified signal is given as one of the inputs to the modulator block. The other input is carrier signal.

Carrier Oscillator: An oscillator circuitry is used to provide a sinusoidal wave of constant frequency. But this frequency is less than the required frequency for modulation process because such circuits cannot provide stable oscillation at such high frequencies. The output of this block is provided to buffer amplifier

Buffer Amplifier: A buffer amplifier is a amplifier having unity gain. This type of amplifier is used to isolate the carrier oscillator from the frequency multiplier block so that the multiplier block does not draw large amounts of current which will affect the stability of the oscillator. The output of this block is supplied to frequency multiplier.

Frequency Multiplier: The frequency of the carrier wave is increased in this stage. The specific frequency required for modulation purpose is achieved through generation of higher harmonics of carrier frequency. Hence this block is also called as harmonic generator. The output of this block is given to the modulator.

Modulator: This block takes two inputs, one from frequency multiplier and another from audio amplifier. It multiplies both the signal which each other resulting in the modulated wave. This block is also called as mixer. There are multiple types of Amplitude Modulation techniques which are Double Side Band-Full Carrier (DSB-FC), Single Side Band (SSB), Vestigial Sideband (VSB), etc. In this article we will be looking at DSB-FC.

Power Amplifier: This block takes its input from the modulator block and amplifies its voltage and current substantially so that the transmitted power is very high. Usually a class C amplifier is used for this purpose since it provides high gain and efficiency. The carrier and message signal were amplified before modulation too, making it a two stage amplifier having their gains multiplied to make a large total gain.

Usually the modulator block and power amplifier are combined together to form a single block which does both the tasks. For generation of DSB-FC, a modulated class c amplifier is used. This type of amplifier has highest efficiency of 80% to 90% and highest distortion. Hence it is only used for amplification of radio frequency signals instead of audio frequency since the effect of distortion is less noticeable in higher frequency.

Circuit diagram of modulated class c amplifier

The efficiency of class c amplifier is high because the transistor Q conducts for only 180° of the input signal. The transistor remains off for the negative half cycle of the input signal. The output still produces full sine wave due to LC oscillator circuit connected to the collector of the transistor. This oscillator requires periodic pulses to generate sustainable oscillations. Hence this type of amplifiers are also used for frequency multiplication where the values of L and C in the oscillator circuit determines the frequency of the output wave.

In the above diagram, it is used as modulator where the carrier wave is passed through coupling capacitor C₁ which blocks DC element and low frequency components. R₁ and R₂ are resistors connected for biasing of the transistor. Transformer T₁ is used to modulate message signal with the carrier wave. The transistor is responsible for amplification by increasing the amplitude of thee modulated signal based on the supply voltage V_CC. The LC circuit is used to maintain the oscillation and it is tuned to resonate at carrier frequency which provides coupling to the antenna. Transformer T₂ blocks DC components in the output and provides low output impedance so that it’s impedance matches with the antenna to minimize power loss due to reflections caused by impedance mismatch.

Antenna: Antenna is a device used for wireless transmission and reception of signals. It is device which converts electrical energy into electromagnetic radiation. Some examples include dipole, yagi uda, horn, etc. In this case the antenna is used for transmission of modulated signal since this is the transmitter part of overall communication system. The output impedance of the power amplifier must be equal to the impedance of the antenna for radiating maximum power otherwise some part of the input signal of the antenna is reflected back towards the power amplifier.

Low Level Transmitter

Given Below is the Block Diagram of Low Level Transmitter

Block diagram of Low Level AM Transmitter

The low level AM transmitter is similar to high level with the only difference being that both carrier and message signal are not amplified before modulation process. Hence carrier amplifier and audio amplifier blocks are not present in low level transmitter.

Advantages of AM Transmitter

Given Below is Some of the Advantages of AM Transmitter

• Simple to generate modulated wave and transmit it and receive original message from the modulated wave at the receiver side.
• Cost effective to develop transmitter and receiver devices since circuit is not complex and requires less components.
• Highest range of communication is possible using a single transmitting tower as compared to other modulation techniques.
• This modulation technique allows omni-directional transmission of signal since it’s frequency is low as compared to other modulation techniques and the transmitted wave becomes more directional as frequency goes on increasing.

Disadvantages of AM Transmitter

Since Amplitude modulation is oldest method of transmitting signals over longer distance, there are many disadvantages and many techniques were developed to overcome it’s drawback

• Low signal to noise ratio as information was stored on the amplitude of the carrier wave which is easily affected by the surrounding electromagnetic interference causing fluctuation in amplitude resulting in deterioration of actual information.
• Poor efficiency resulting in high power usage in turn increasing the cost of running the system.
• Due to low frequency of operation, bandwidth is less as compared to other modulation techniques.
• It does not provide any encryption features. Hence the actual message signal can be easily accessed by anyone. The intruder can access it by only knowing the frequency at which the communication was going on and they can easily tune in to this frequency.
• It requires huge antenna to transmit and receive signals due to low frequency as the length of the antenna is inversely proportional to frequency of the wave.

Application of AM Transmitter

Some of the Applications of the AM Transmitter are

• It is still used in AM radio.
• Air Traffic Control Stations for half-duplex communication.
• It was used to broadcast video signals for television.
• AM transmitters are used by amateur radio operators for communication and experimentation.
• AM transmitters are commonly used in radio broadcasting to transmit voice and music signals.
• AM transmitters are used in non-directional beacons (NDBs) for aircraft navigation.

Conclusion

In conclusion, the AM transmitter plays a Important role in transmitting information over long distances using amplitude modulation. By modulating a high-frequency carrier wave with a low-frequency message signal, it allows for efficient transmission of audio signals for applications such as radio broadcasting, navigation systems, wireless communication, and more.

AM Transmitter – FAQs

What is transmission formula?

transmission model was developed by Harald in 1946 to determine the power of the received signal which travels through free space. It states that the received power is directly proportional to the transmitted power, gain of the transmitting and receiving antenna, square of the wavelength and inversely proportional to the distance between transmitter and receiver.

P_R = P_TG_TG_Rλ²/(4𝜋d)²

What is link power budget?

The connection between transmitter and receiver is called as link. We already know that the modulated signal must be amplified to overcome various losses by the time it reaches the receiver end. But we must know that how much amplification should be done so that the receiver can still recognize the signal and demodulate it to gather the original message so that less power is wasted for amplification. The ability of the receiver to sense minute changes in the signal is called as sensitivity. The required amplification is done by taking account of various attenuation factors in the region such as mountains, buildings, foliage, etc and receiver sensitivity.

What is gain of an antenna?

The gain of an antenna is different than the gain of an amplifier because the antenna does not amplifier the input signal. In terms of antenna, gain is defined as the ratio of intensity of power transmitted by the antenna in a specific direction to the same power transmitted by an isotropic antenna. Ann isotropic antenna radiates electromagnetic waves in all directions. It is also called as omni-directional antenna. Gain increases as the directivity of antenna increases.