DC offset calibration and correction methods are often required in any baseband sampled system. A residual dc offset is equivalent to an interfering signal within the analysis bandwidth of the signal. Several techniques can be applied to mitigate the problem, including dc tracking and cancellation, ac coupling at baseband, or simply by selecting components with good dc characteristics including high even order distortion performance.
Quadrature Imperfections and Image Rejection
I/Q amplitude and phase mismatch can cause degraded SNR performance. In an ideal I/Q demodulator, the baseband I/Q signals share a perfect 90 degree phase relationship between I and Q vectors, and are said to be in perfect quadrature. Under such conditions the discrimination of symbols in digital domain can be easily determined by the instantaneous I/Q vector trajectories. When I/Q mismatch plagues the system, the I/Q symbol vectors will suffer from amplitude and phase errors that will degrade the recovered SNR for the signals of interest. Static I/Q impairments may be corrected using digital techniques. It is important to study the effective image rejection performance of the direct conversion receiver versus signal level and offset from carrier frequency. An understanding of the single tone I/Q impairments of the receiver helps simplify the process of interpreting the measured performance when a modulated signal is applied.
Modulation Error Ratio (MER) Performance
Modulation error ratio (MER) is a measure used to quantify the modulation accuracy of a digital radio transmitter or receiver. In a perfectly linear and noiseless system, the signal received by a receiver would have all I/Q symbol trajectories mapping to the exact ideal signal space constellation locations, but various imperfections in the implementation (such as magnitude imbalance, noise floor, and phase imbalance) cause the actual measured symbol vectors to deviate from the ideal locations. The direct conversion receiver shows exemplary MER performance levels for various modulation schemes. Figure 3 and figure 4 show MER performance over input power for 10 MHz wide OFDMA, WiMAX, and WCDMA signals, respectively.
In general, a receiver exhibits three distinct MER limitations versus received input signal power. At strong signal levels the distortion components falling in-band due to nonlinearities in the receiver will cause a strong degradation to MER. At medium signal levels where the receiver is behaving in a linear manner and the signal is well above any noise contributions, the MER reaches an optimum level that is dominated by the quadrature accuracy of the demodulator, filter network and variable gain amplifier (VGA) , and the precision of the test equipment. As signal levels decrease such that noise is a major contribution, the MER performance versus signal level will exhibit a dB-for-dB degradation with decreasing signal level. At lower signal levels, where noise proves to be the dominant limitation, the decibel MER proves to be directly proportional to the SNR.
3. MER vs. RF Input power for a 10MHz OFDMA WiMAX signal.
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A closer look at Figure 4 reveals the resilient performance of the receiver under various scenarios. One expects the 5 MHz low-IF case to be the most promising since it is sheltered from any dc offsets and flicker noise contributions that are associated with a zero-IF case. At lower power levels, the noise performance of the receiver is fairly constant. Even in the presence of a single tone or two tone blockers (a common test case for W-CDMA Base-Station requirements), the noise figure deviation is well within 1 dB.
4. MER vs. RF Input power for a WCDMA signal for Zero IF, Low IF and Blocker cases.
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The image rejection ratio is the ratio of the intermediate frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. The image rejection ratio is expressed in decibels. Appropriate image rejection is critical because the image power can be much higher than that of the desired signal, thereby plaguing the down conversion process. Figure 5 below shows the image rejection vs. multiple IF frequencies for W-CDMA. The receiver offers excellent uncalibrated image rejection performance. Through additional digital correction it is reasonable to achieve more than 75 dB of image rejection, allowing direct conversion receivers to simultaneously capture several adjacent signals of severely different power levels (a key feature for multi-carrier receiver designs).
5. Image Rejection vs. RF Frequency for various WCDMA IFs.
Modern day direct conversion receivers have the ability to offer very high instantaneous dynamic range and wide RF frequency coverage. Using advanced RF integrated circuits, it is now possible to build high performance cellular base-station receivers that can be field programmed to address multiple cellular standards using a fixed hardware solution. System designers need to pay special attention to higher order nonlinearities in order to ensure a robust receiver solution. By studying single-tone and two-tone behavior the challenge and mystery of direct conversion can be better understood, and commonplace shortcomings associated with classic direct conversion systems avoided.
About the Authors
Rakesh Soni is an RF applications engineer in the RF and Networking Components Group at Analog Devices Inc.. Previously, he worked as an RF/Wireless applications engineer at Teradyne Inc. Rakesh received his Bachelors and Masters degrees in Electrical Engineering from Georgia Institute of Technology. Rakesh.firstname.lastname@example.org.
Eric Newman is an applications systems engineer in the RF and Networking Components Group at Analog Devices Inc. Prior to his work on wireless system design with ADI he worked as a hardware designer at Innovative Imaging Systems Inc. He holds a Masters of Science in Electrical Engineering, focusing on wireless communications, from the University of Massachusetts at Lowell. Eric.email@example.com