Transmitter vs. Receiver: Differences and Types

This page compares Transmitters and Receivers, highlighting their differences and various types.

Transmitter Types

• AM Transmitter
• FM Transmitter
• SSB Transmitter
• Direct Conversion Transmitter
• Superheterodyne Transmitter

Receiver Types

• Direct Conversion Receiver
• Superheterodyne Receiver
• Direct RF Sampling

The differences between transmitter and receiver types are also discussed below.

Transmitter Definition

A transmitter is a device used to transmit a signal from one place to another. The signal contains information in the form of voice, video, or data. Transmitters use antennas to send signals into the air and employ modulation techniques to transmit signals over specific distances according to system design. Amplifiers are used to boost the amplitude of the signal, ensuring it reaches the required transmission distance.

Typical modulation schemes used in transmission systems are broadly categorized into analog and digital:

• Analog Modulation: AM, FM, PM, SSB, etc.
• Digital Modulation: ASK, FSK, PSK, QPSK, QAM, etc.

After the transmit signal travels a certain distance, it experiences attenuation and impairment due to channel characteristics. This attenuated signal is then received by the receiver.

Receiver Definition

A receiver is a device that decodes the transmitted information from the received signal. Like transmitters, receivers use antennas to receive signals from the air. Similar to power amplification in transmitters, receivers also amplify the received signal, focusing on low-noise amplification.

Difference Between Transmitter Types: AM, FM, SSB, Direct Conversion, Superheterodyne

The following are different types of transmitters based on modulation schemes and conversion techniques.

• AM Transmitter
• FM Transmitter
• SSB Transmitter
• Direct Conversion Transmitter
• Superheterodyne Transmitter

AM Transmitter

Figure 1 shows a typical block diagram of an AM transmitter system. AM radio systems use a frequency range of 540 to 1700 KHz with an intermediate frequency (IF) of about 455 KHz. Frequencies are spaced at 10 KHz intervals.

AM transmitters use amplitude modulation to convert audio information into an AM modulated signal. AM modulation uses audio as the modulating signal and a high-frequency signal as the carrier. The amplitude of the carrier signal is varied according to the amplitude of the modulating audio signal to achieve an AM modulated output.

FM Transmitter

Figure 2 depicts an FM Transmitter system block diagram. FM radio systems use a frequency range of 88 to 108 MHz with an IF of about 10.7 MHz.

FM transmitters use frequency modulation to convert audio information into an FM modulated signal. FM modulation uses audio as the modulating signal (Fm) and a high-frequency signal as the carrier. The frequency of the carrier signal (Fc) is varied according to the amplitude of the modulating audio signal to achieve an FM modulated output.

SSB Transmitter

AM transmitters transmit both the upper and lower sidebands. The upper sideband is the sum of Fc and Fm, while the lower sideband is the difference between Fc and Fm.

SSB (Single Sideband) transmitters transmit only one sideband (either upper or lower) and not both. Due to this, SSB transmitters save bandwidth and power compared to AM transmitters.

Direct Conversion Transmitter

Let’s understand the working operation of a direct conversion transmitter. The signal constellation obtained using this transmitter type is known as QPSK (Quadrature Phase Shift Keying).

• First, the digital data to be transmitted is split into I and Q signals.
• The I and Q signals are passed through DACs (Digital-to-Analog Converters).
• The output of the DACs are fed to mixers via low-pass filtering.
• The architecture uses a local oscillator (LO). The LO signal is provided with a 90-degree phase shift to one of the mixers before the mixing process.
• The mixed I and Q components are summed up to obtain the QPSK modulated signal.
• The QPSK modulated signal is amplified using a PA (Power Amplifier) before transmission into the air.

Superheterodyne Transmitter

This architecture uses an additional mixing component after the modulated signal is obtained using a direct conversion transmitter. Before and after mixing, the signal is band-pass filtered. This requires one more LO (Local Oscillator) in the design.

Like other transmitter systems, this type also uses PA (Power Amplification) before transmission. Automatic Gain Control (AGC) is used to achieve variation in the amplitude of the output signal through gain control.

Difference Between Receiver Types: Direct Conversion, Superheterodyne, Direct RF Sampling

The following are different types of receivers based on the transmitter architecture.

• AM, FM, or SSB receiver based on the modulation scheme
• Direct Conversion or Heterodyne Receiver
• Superheterodyne Receiver
• Direct RF Sampling

Direct Conversion Receiver

The figure depicts a simple architecture of a direct conversion receiver. As shown, it uses one mixer to convert the received modulated signal to a baseband signal. The demodulated baseband signal is given to an IQ demodulator to retrieve the I and Q signals. This I/Q demodulator is also known as a QPSK demodulator due to the 90-degree phase shift between the I and Q signals.

Superheterodyne Receiver

The figure depicts a superheterodyne receiver architecture. As shown, it uses two mixers before the baseband information is retrieved.

Direct RF Sampling

In this type of receiver, the received signal, after low-noise amplification, is passed to an RF ADC. Very few components are needed in the design of a direct RF sampling receiver, making it a simple and low-cost architecture.

AM Receiver and FM Receiver

An AM receiver receives the signal transmitted by the AM transmitter. It processes the AM modulated signal and provides audio as output. Similarly, an FM receiver processes and decodes the signal transmitted by the FM transmitter. The figure depicts an AM/FM receiver block diagram.