Sampling shift spurs digital receiver gains

Sampling shift spurs digital receiver gains
DALLAS — A new sampling technique may lead to simpler single-chip radio receivers that can be produced without special filters. The circuit technique, described at the International Conference of Signal Processing Applications and Technology here, allows sampling data close to the antenna and will enable the same receiver to process signals from different wireless standards.

The technical paper was delivered Thursday, Oct. 19 in ICSPAT's communications track by Efim Poberezhskiy, senior R&D engineer at Datron World Communications (Vista, Calif.). Co-authored with his son George Poberezhskiy, a research engineer at SiRF Technology (Los Angeles), the paper describes how a sampling technique that uses internal antialiasing filtering can overcome the usual obstacles in conventional digital receivers.

In a conventional digital receiver the analog portion occupies a large area in silicon because it provides significant signal gain before the analog-to-digital conversion. The traditional antialiasing intermediate frequency (IF) filters like SAW, crystal, ceramic and mechanical limit the receiver flexibility and its ability to process the signals of different standards with different bandwidths.

Compatibility problem

The incompatibility of the IF filters' technology with the IC technology complicates the single-chip implementation of digital receivers. In addition, antialiasing filtering performed by IF filters is not effective enough. Although it rejects the out-of-band noise and interference coming from the antenna, it does not reject the out-of-band noise and nonlinear products of the IF amplifiers, which can fall within the desired signal spectrum as a result of sampling.

The sampling procedure used in the Poberezhskiy paper is derived from the sampling theorem and can be performed by a multichannel sample-and-hold amplifier with weighted integration (MSHAWI). The implementation of the new sampling technique in digital receivers changes the design of their analog and mixed-signal portions. The analog portion performs antialiasing filtering, amplification of received signals and conversion of these signals to the frequency that is most convenient for sampling and quantization. The mixed-signal portion performs sampling, quantization and analog signal reconstruction.

In the analog portion of modern digital receivers incoming analog signals are processed, instead of processing and filtering only their samples. In the new sampling technique, the analog front end processes only one frequency conversion and the conventional IF filter is removed.

Simple operation

Poberezhskiy claims that the internal antialiasing filtering performed by the MSHAWI is more effective than conventional ones because it rejects all the out-of-band noise and nonlinear products of the IF amplifiers in addition to the noise and interference that are rejected by conventional IF filters. Removing conventional IF filters makes the analog front-end technology uniform and simplifies the VLSI implementation of the receiver.

The simplicity of the MSHAWI transfer function modification is achieved by changing the filter weights and allows processing the signals of different standards with different bandwidths by the same receiver, making the receiver adaptable to different conditions of reception. It also allows varying the receiver IF to avoid spurious responses.

The ease of IF variations also simplifies the design of multiband receivers. The accumulation of the signal energy in the MSHAWI storage capacitors significantly reduces the required gain of the analog part of the receiver and allows sampling much closer to the antenna than was possible before.

This procedure changes the whole concept of sampling band-limited signals, said Poberezhskiy, who worked on these problems for 30 years in Russia before he immigrated with his family five years ago.