Zero IF Transceiver Architecture Explained

This page delves into the Zero IF Architecture-based Transceiver. It explains the advantages and disadvantages of Zero IF Architecture based transceivers used in the RF chain.

There are three common types of receiver architectures: heterodyne, super-heterodyne, and homodyne.

• Heterodyne: Uses one mixer to convert the modulated RF signal to a modulated IF signal, and later the modulated IF signal is converted to I/Q signals with zero frequency using an I/Q demodulator.
• Super-heterodyne: Uses two mixers. The first mixer converts RF to a high IF, and the second mixer converts the high IF signal to a low IF signal, which is then converted to an I/Q signal using an I/Q demodulator.
• Homodyne: Uses no mixers. The RF signal, after passing through the LNA, is directly applied to the I/Q demodulator. This I/Q demodulator converts the modulated RF signal into I/Q signals.

Refer to the comparison between Heterodyne vs Super-heterodyne vs Homodyne receiver architectures for more information.

Figure 1: Zero IF Architecture based transceiver

Figure 1 depicts the homodyne receiver architecture. As this architecture produces zero IF (Intermediate Frequency), it is also known as zero IF architecture.

As shown, the LO (Local Oscillator) frequency is set equal to the frequency of interest, and hence the received signal is directly converted to the baseband I (in-phase) and Q (quadrature phase) signals. In this architecture, both the DAC and ADC operate at baseband sampling frequencies.

The transceiver based on this zero IF architecture is known as the zero IF transceiver.

Advantages of Zero IF Architecture

The following are the advantages of Zero IF Architecture:

• All the filtering is carried out at baseband, and hence analog filtering used in heterodyne architectures is alleviated here. Digital filters are easier to design and are less expensive compared to RF/IF analog filters.
• As the ADC and DAC operate at baseband sampling rates, which are lower compared to other architectures, this saves a significant amount of power.
• Due to the reduction in analog circuitry and components in the front-end stage, zero IF transceivers can be developed with considerable SWaP reduction. The term “SWaP” stands for Size, Weight, and Power.

Disadvantages of Zero IF Architecture

The following are the disadvantages of Zero IF Architecture:

• It is difficult to obtain and maintain a perfect 90-degree phase offset between the I signal and the Q signal. This imperfection results in a degradation in image rejection specification.
• Moreover, imperfect LO isolation in the mixer results in carrier leakage.
• Both carrier leakage and image issues result in a degradation in the sensitivity of the receiver. Moreover, these produce undesired spectral components known as spurious signals.

Hence, both carrier leakage and image frequencies should be addressed (i.e., removed or reduced) in the zero IF architecture. This is done with the help of advanced signal processing techniques at baseband. These techniques are applied on the converted I/Q signals after the ADC in the receiver.