CMOS mixers and polyphase filters for large image rejection

Abstract

Abstract-This paper presents an in-depth treatment of mixers and polyphase filters, and how they are used in rejecting the image in transmitters and receivers. A powerful phasor-based analysis is used to explain all common image-reject topologies and their limitations, and it is shown how this can replace complex trigonometric equations commonly found in the literature. Practical problems in design and layout that limit the performance of image-reject upconversion and downconversion mixers are identified, and solutions are presented or limits explained. This understanding is put to work in a low-IF CMOS wideband, low-IF downconversion circuit, which repeatedly rejects the image by 60 dB over the wide band of 3.5 to 20 MHz without trimming or calibration.

New layout strategies with improved matching performance

“…Second, at the time of designing the circuit prototype at Silicon Labs Inc, one communication circuit required more than 70dB image rejection ratio in one pair of resistors. This IRR requires better than 64dB matching performance in these two resistors [10]. More product, circuit schematic, layout and fabricated die details could not be disclosed due to the intellectual property concern.…”

“…Equation (9) can be rewritten as (10) where k,l is the coefficient of (x-x 0 ) k (y-y 0 ) l assuming x 0 and y 0 constant. Notice that the order of the 2 nd term in (8) and the 2 nd term in (10) are both no greater than (n-1).…”

De-Interleaved Subsampling Architecture for Homodyne Receivers

“…This topology is similar to low-IF technique except that the mixer is replaced by a sample and hold circuit. Therefore, similar to a low-IF receiver, this topology suffers from the signal image rejection problem [15].…”

High-image-rejection wireless-receiver architecture with a 3-phase active RC complex filter

“…Therefore, the residuals of IM1 at the filter output is determined by the mismatches of RC values in the complex filter. The RC mismatches can be reduced with careful-layout techniques [14,15] and the image rejection of more than 60 dB could be obtained [14,15].…”

“…The gain and phase mismatches of the second IF signals depend only on the product of the relative errors of the first IF and the second local signals. This error reduction mechanism resembles the 4-phase double-conversion receiver [14,15], but the architecture is quite different. The final image-rejection ratio is mainly determined by the mismatches of RC values in the 3-phase complex filter.…”