Zero IF Architecture | Zero IF Transceiver
This page covers Zero IF Architecture based Transceiver. It explains advantages of Zero IF architecture and disadvantages of Zero IF Architecture based transceivers used in RF chain.
There are three common types of receiver architectures viz. heterodyne, super-heterodyne and
• Heterodyne uses one mixer to convert modulated RF to modulated IF and later the modulated IF is converted to IQ signals with zero frequency using IQ demodulator.
• Super-heterodyne uses two mixers, first mixer converts RF to high IF and second mixer converts high IF to low IF signal which is converted to IQ signal using IQ demodulator.
• Homodyne uses no mixers and RF signal after passing through the LNA, is directly applied to the IQ demodulator. This IQ demodulated convert modulated RF signal into I/Q signals.
Refer Heterodyne vs super-heterodyne vs Homodyne receiver architectures.
The figure-1 depicts the homedyne receiver architecture. As this architecture produces zero IF (Intermediate Frequency), it is also known as zero IF architecture. As shown LO (Local Oscillator) frequency is set equal to frequency of interest and hence received signal is directly converted to the baseband I (inphase) and Q (quadrature phase) signals. In this architecture, both DAC and ADC operates at baseband sampling frequencies. The transceiver based on this zero IF architecture is known as zero IF transceiver.
Advantages of Zero IF Architecture
Following are the advantages of Zero IF Architecture:
• All the filtering is carried out at baseband and hence analog filtering used in heterodyne architecture is alleviated here. Digital filters are easier to design. Moreover they are less expensive compare to RF/IF analog filters.
• As ADC and DAC operates at baseband sampling rate which is less compare to other architectures and hence will save significant amount of power.
• Due to 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
Following are the disadvantages of Zero IF Architecture:
• It is difficult to obtain and maintain perfect 90 degree phase offset between the I signal and Q signal. This imperfection results into degradation in image rejection specification.
• Moreover im-perfect LO isolation in the mixer results into carrier leakage.
• Both carrier leakage and image results into degradation in sensitivity of the receiver. Moreover these produces undesired spectral components known as spurious.
Hence both carrier leakage and image frequencies should be attended (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 ADC in the receiver.
Refer IQ imbalance compensation>>.
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