Signal Chain Discussion Series: Communications Transceiver

A communications transceiver performs the task of converting digital or analog signals to RF, transmitting those signals, and receiving incoming signals for conversion back to digital or analog signals. Hence, the key components of a communications transceiver are the modulation/demodulation circuitry, transmitter, and receiver. Depending on the architecture of the communications transceivers, direct digital synthesizers and direct digital converters may take the place of distinct modulation/demodulation circuitry, upconverters, and local oscillators common to heterodyne transceiver architectures.

Communications transceivers often have to transmit signals over vast distances, especially in space applications, and receive signals that are very weak and distorted by natural phenomenon. Hence, there are many key performance parameters that are used to describe the quality and other characteristics of transmitted and received signals delivered to the demodulator or converter.

Key Communication Transceiver Performance Parameters:

• Dynamic range (DR)
• Sensitivity
• Linearity
• Noise figure (NF)
• Phase noise
• Bandwidth
• Channel selectivity
• Power consumption
• Spurious frequencies effect
• Leakage effects
• Image band rejection
• Power efficiency
• Power control capability
• Transmitted signal power
• Working frequency and bandwidth
• Out-of-band emission, Adjacent Channel Power/Adjacent Channel Power Ratio (ACP/ACPR)
• Intermodulation distortion (IMD)

Receiver Discussion
A transceiver’s receiver takes potentially very weak incoming signals and amplifies those signals to desirable power levels for the demodulator or converter to process. Given that received signals are also naturally weakened and distorted by physical phenomenon innate to a radio link, a goal of communications receiver design is to minimize any additional distortion, noise, or phase noise caused by the receiver circuitry. This is why devices such as low noise amplifiers, low-phase noise amplifiers, and low noise/jitter/phase noise oscillators are commonly used in receiver signal chains.

Heterodyne Receiver Architecture Signal Chain Components

• Band Pass Filter
• Limiter
• Low Noise Amplifier (LNA)
• Image Reject (IR) Filter
• Mixer
• Local Oscillator
• Intermediate Frequency (IF) Filter or Channel Select Filter
• Power Splitter
• 90 Degree Hybrid
• Low Pass Filter
• IQ Demodulator

Low-IF Receiver Architecture Signal Chain Components

• Mixer
• 90 Degree Hybrid
• Band Pass Filter
• IF Filter
• Local Oscillator
• Analog To Digital Converter (ADC)
• Power Splitter
• Power Combiner
• Low Pass Filter
• Low Noise Amplifier

Direct Digital Conversion Receiver Architecture (Homodyne or Zero-IF) Signal Chain Components

• Low Noise Amplifier
• Band Pass Filter
• Limiter
• Power Splitter
• 90 Degree Hybrid
• Analog To Digital Converter

Transmitter Discussion
A Transciever’s transmitter section performs modulation, power amplification, and possibly upconversion depending on the frequencies and design. Most modern transmitters amplify quadrature baseband signals for transmission to an antenna. The key component of a communications transmitter is the power amplifier (PA) which greatly increases the signal power of the modulated transmission signal. The main considerations of the transmitter section are linearity, phase noise, out-of-band emissions, intermodulation distortion, power efficiency, spurious effects, transmitted signal power, and power control capability.

Transmitter Architecture Signal Chain Components

• Duplexer
• Switch
• Circulator
• Matching Network
• Power Amplifier (PA)
• Combiner
• Digital To Analog Converter (DAC) or IQ Modulator
• Antenna

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