Implementation of a W-CDMA Direct-Conversion IQ Modulator Module Including Evaluation of Chip-Package-Board Interactions

Abstract

This paper presents a W-CDMA direct-conversion IQ modulator MMIC design that employs a new technique to generate the 90 o phase shift with low implementation loss. The package and PCB effects on the implemented IQ modulator MMIC when further developed as a board module are studied. The package and PCB interconnects are analyzed using the 3-D EM simulation tool and transformed into the equivalent-circuit elements for co-simulation with the IQ modulator MMIC. The degradation of error vector magnitude and sideband suppression due to the presence of package and PCB can be well predicted by the co-simulation results and then verified by the final measurement results. IntroductionError vector magnitude (EVM) and sideband suppression are often used to evaluate the modulation accuracy of an IQ modulator. Both parameters highly depend on the amplitude and phase balances of a 90 o phase shifter component inside the IQ modulator. Many techniques have been reported to design a 90 o phase shifter with low amplitude and phase errors. They are summarized to include the (1) frequency divide-by-two scheme [1]-[3], (2) RC-CR network [4],[5], (3) LC high-pass and low-pass filter [6],[7], (4) RC all-pass filter [8],[9], (5) ring oscillator [10],[11], and (6) poly-phase filter [12]-[14]. The first technique generates an accurate 90 o phase shift but requires high DC power consumption. For the second technique, it can be done with simplicity and low DC power consumption but has high implementation loss inherently. The third technique has less implementation loss but is difficult to integrate. It is broadband but complicated in design for the fourth technique. The fifth technique offers a relatively imprecise 90 o phase shift and also consumes much DC power. For the sixth technique, it has a large implementation loss of approximately 3 dB for one stage and may need at least 2 to 3 stages for an accurate 90 o phase shift.A new technique to generate the precise 90 o phase shift has been proposed in this paper. It adopts a similar configuration of differential amplifier pair shown in [4], but has the distinguishing feature that one differential amplifier with an emitter-degeneration inductance and the other one with an emitter-degeneration capacitance create the 90 o phase shift. In further analysis, this phase shifter is based on an RL-RC network. The resistances in this RL-RC network are the intrinsic resistances of transistors and differ from those in the above-referred RC-CR network that uses real resistors. Therefore, the proposed 90 o phase shifter is advantageous to have a relatively low implementation loss. A 1.9 GHz direct-conversion IQ modulator featured with the proposed 90 o phase shifter was implemented for W-