Bandpass Sigma-Delta Modulator
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Transcript of Bandpass Sigma-Delta Modulator
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Bandpass Sigma-Delta Modulator
Michael Vincent
Brian McKinney
ECEN5007
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Wireless Communications
• Crowded voice/data frequency bands (think 2.5/3 G cellular and 802.11/Bluetooth wireless) require wide dynamic range front ends and precise IF filtering to receive weak signal with strong interface
• Momentum in industry to push A/D conversion as close to antenna as possible to allow most of the process to be done digitally (soft-radio)
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Bandpass Sigma-Delta Modulator
• Direct conversion technique first proposed in 1928, however, this technique leads to an image component folded into the baseband as a result of 90 phase error and path mismatch gain error
• Sigma-Delta bandpass approach offers no image problem and the possibility of direct downsampling from IF at frequencies higher than the sampling frequency via the mixing property of the sampling process
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Bandpass Sigma-Delta Modulator
• Song proposed switched capacitor direct quadrature demodulation technique
• Double loop bandpass modulator proposed requires same number of op amps as standard double loop lowpass modulator without path mismatch (sampling @ 4x the passband center frequency (fc))
• Identical in structure to low-pass modulator except simple integrator is replaced with a two-delay resonator having a gain of –z^-2/(1+z^-2)
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Bandpass Sigma-Delta Modulator
• Resonator can be realized with two-delay integrators that are time multiplexed between the I and Q channel to avoid path mismatch and save space/power
• Bit stream from modulator is bandpass, I and Q lowpass streams can be obtained in DSP hardware by multiplication by {1,0,-1,0} and {0,1,0,-1} yielding a direct quadrature conversion to dc
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Fully Differential Op Amp
• In order to time multiplex the I and Q channels, the amplifiers inserted into a switched capacitor network.
• Fully differential op-amps are used to implement the amplifiers.
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Basic Fully Differential Op Amp
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Simulation Setup
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Transient Analysis (1 MHz input)
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AC Analysis
Corner Frequency: 15MHz
Phase Margin: 30°
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Future Op-Amp Improvements
• Performance needs little improvement:– 40dB Gain Sufficient for simple gain stage– 15 MHz BW Center frequency of 2 MHz
• Replace current sources with cascode or other approach
• Implement CMFB stage as switched capacitor network.