Martin Snelgrove

A complex bandpass /spl Delta//spl Sigma/ converter for digital radio

A new architecture is proposed for a single-chip digital radio receiver. This architecture has several advantages over the direct-conversion architecture that has been gaining interest for use in single-chip designs. A non-zero quadrature IF reduces the image problem and eliminates concerns about self-EMI and 1/f noise. A complex bandpass filter embedded in a delta-sigma loop allows the A/D conversion to be performed directly on the pair of quadrature outputs from the mixer.<<ETX>>

A filter designer's filter design aid: filtorX

An interactive filter design program suitable for both nonexperts and experts is described. A system library provides the necessary functionality for most design problems. Its language basis allows extensibility for expert users. Examples of code to generate biquads which are automatically paired, ordered, and scaled for dynamic range, and to determine capacitor-ratios for switched-capacitor biquads are provided to illustrate filtorXs potential.<<ETX>>

A CMOS biquad at VHF

A differential transconductance-C biquad implemented in the digital subset of a 0.9- mu m CMOS process operates at frequencies up to 450 MHz and Q-factors to approximately 100 with SNR (signal-to-noise ratio) in the range of 35-45 dB. By switching in capacitors and adjusting control voltages it can be tuned to below 30 MHz, demonstrating the capability of operating over the entire VHF range. The active area is 0.029 mm/sup 2/ and power consumption is 30 mW.<<ETX>>

Stability in a general Sigma Delta modulator

The stability of a single-quantizer Sigma Delta modulator in terms of its noise transfer function is studied. It is shown that stability is conditional upon the input signal, and an explicit formula for the set of all inputs which result in stable behavior is given. In addition, the authors compare the set of modulators stable with zero input to the sets given by several existing rules of thumb and find that these criteria are neither necessary, nor in some cases sufficient, to ensure stability.<<ETX>>

Mismatch cancellation for double-sampling sigma-delta modulators

The performance of double-sampling sigma-delta A/Ds can suffer due to capacitor mismatch as well as uneven clock phases. Improving the resulting SNR is essential. This paper presents a method that uses DSP techniques to filter out the mismatch. Simulation results of applying an adaptive LMS filter to a fourth-order double-sampling bandpass sigma delta modulator are presented that show an improvement in SNR of more than 30 dB.

Multi-rate switched capacitor filter design with aggressive sampling-rates: filtorX in action

A methodology to accurately design multirate switched-capacitor (SC) filters is derived. Multirate SC filter design is an effective means of minimizing silicon area for bandpass systems. The theory derived allows designs to achieve maximal area savings by employing sampling rates approaching the minimum allowed by sampling theory. Designing filters to simultaneously operate at different sampling rates can require numerical optimization to correct for the effects of multirate operation, filtorX, a computer-aided filter design tool, was employed to design a CCITT V.22 high-band modem filter as a test of the derived design methodology. The result was an improvement of the uncorrected multirate filter, which had exceeded passband tolerances, to one which was within tolerances. The multirate filter, operating at sampling rates of 128 kHz and 8 kHz, required 70% less capacitor area than the equivalent single-rate filter operating at a sampling rate of 128 kHz.<<ETX>>