Equalization of Third-Order Intermodulation Products in Wideband Direct Conversion Receivers

Abstract

Abstract-This paper reports a SAW-less direct-conversion receiver which utilizes a mixed-signal feedforward path to regenerate and adaptively cancel IM3 products, thus accomplishing system-level linearization. The receiver system performance is dominated by a custom integrated RF front end implemented in 130-nm CMOS and achieves an uncorrected out-of-band IIP3 of 7.1 dBm under the worst-case UMTS FDD Region 1 blocking specifications. Under IM3 equalization, the receiver achieves an effective IIP3 of +5.3 dBm and meets the UMTS BER sensitivity requirement with 3.7 dB of margin.

Analysis of Internally Bandlimited Multistage Cubic-Term Generators for RF Receivers

“…Many early predistortion circuits utilized the nonlinear impedance characteristic of back-to-back diodes [10]- [12], while an active implementation of such a circuit with higher cubing gain using bipolar junction transistor devices was reported in [13]. Such architectures were not considered for the project in [9] as they are not easily implemented in CMOS processes. CMOS cubic-term generators that utilize the third-order Taylor series coefficients of the MOSFET have also been presented [14]- [16].…”

“…Furthermore, the cubing operation heavily attenuates the IM3 output signal with respect to the noise of the IM3-producing devices. As will be discussed in Section V, the noise of the cubic-term generator is of prime importance in the receiver of [9] and renders the use of these architectures unattractive in such an application, although they may represent competitive design options when noise is not an issue.…”

“…3 was implemented for the receiver in [9] and will be the focus of this paper. Like those of [17]- [19], it utilizes multiple nonlinear operations to generate third-order distortion, and hence, much of the analysis in this paper applies to these architectures as well.…”

“…2 was recently proposed [9], in which IM3 products are generated at analog RF by a CMOS cubic-term generator circuit. This scheme was shown to dramatically improve the receiver third-order intercept point (IIP3) at the frequencies of the large out-of-band blocking signals which dictate the receiver linearity requirements for frequency-division duplexing (FDD) UMTS (Region 1).…”

“…Placing the cubic-term generator at this point then guarantees that the output reference IM3 products will be as large as possible for a given cubic-term generator topology, reducing the noise requirements of subsequent circuitry. Adaptive feedforward error-cancellation concept applied to RF receiver third-order distortion in [9].…”