Dc Offset Suppression in Double Balance Direct Conversion Receivers

The double balance mixer scheme in-phase signal input and double-quadrature Local Oscillator (LO) is suggested for application on IQ direct conversion receivers. The influence of the gain and phase mismatches of splitters is investigated. It is shown that a similar scheme using square-law detectors is promising on optical applications. Introduction The increasing interest in direct conversion receivers (DCR) is based on several qualities of this type of reception which makes it very suitable for integration as well as multi-band multi-standard operation [Parssinen (1)]. The main advantage of DCR, contrary to the most widely used heterodyne reception, is an achievement of high image rejection avoiding the use of expensive bulky off-chip filters. Among the problems existing in DCR, limiting their wide application, DC offset is one of the most serious. DC offset caused by various phenomena contribute to the creation of DC signals. These phenomena can be separated on the three main groups, such as a) LO leakage to the LNA and mixers input due to substrate coupling, ground bounds, bond wire radiation etc., b) LO leakage to the antenna through the mixers and LNA due to their non-sufficient isolation, c) even-order nonlinearity of LNA and mixer. The strength of DC caused by the phenomena of group a) is influenced by chip technology and can be reduced by careful layout or by suitable post processing digital signal processing (DSP) at baseband. DSP removes the DC offset in a way that using cannot be duplicated in the analog domain. For the cases of DC caused by the phenomena of groups b) and c) reductions in DC signal can be achieved in the analog domain by special circuit design. This paper presents one such circuit. Four-quadrant multiplier DCR The basic diagram of the IQ DCR is given in Fig. 1. The possible DC offset due to LO self-mixing can be estimated using LO level in the mixers, LO-to-RF isolation and reflection from the RF interface mismatch. Take into account the typical value for LO-to-RF isolation of the mixer -20dB, reverse gain of the LNA -20 dB, corresponding to mismatch SWR=1.1 reflection -20 dB and required LO level approximately 0 dBm, the LO leakage -60 dBm at the LNA input can be obtained. This value of LO leakage power is 30 dB higher than required sensitivity threshold of the receiver. Leakage power after amplification of LNA and self-mixing with LO produced at the output of the mixer DC offset on the order of 10 mV which is enough high to saturate the following circuits. To solve this kind of DC offset problem various compensative and balance architectures have been suggested [(1), Razavi (2)]. To reduce the LO leakage and DC offset we suggest to use the scheme of analogue double balanced mixers presented in Fig. 2. As distinct from well known scheme with double-quadrature division of input RF and LO, [Crols and Steyaert (3)] here we use in-phase division of RF and quadratureantiphase division of LO. Such construction allows to suppress LO leakage to the front end, because at the entry of in-phase splitter the two equal antiphase components has been summed. Presence of differential pairs ( I I ′ , ) and ( Q Q ′ , ) enables suppression of residual DC offsets at the I and Q outputs. Double Quadrature LO generation is however a critical part of this system due to gain and phase mismatch. To create influence of the gain ( ∆ ) and phase (Θ ) I