Elsevier

Signal Processing

Volume 85, Issue 2, February 2005, Pages 441-448
Signal Processing

Internal calibration techniques for quadrature receiver mismatch errors

https://doi.org/10.1016/j.sigpro.2004.09.015Get rights and content

Abstract

The performance of systems requiring the use of quadrature receivers is significantly degraded in the presence of mismatch errors between the In-phase (I) and Quadrature (Q) branches. This paper discusses the mismatch problem and introduces promising solutions. These solutions are independent of communication format, which improves the generality and flexibility of the techniques.

Current literature emphasizes estimation of either local oscillator (LO) mismatch or frequency dependent path mismatch using an external calibration signal. This paper develops techniques to estimate both LO mismatch and path mismatch using internally generated calibration signals derived from the system local oscillator. Once the mismatch errors are obtained, LO mismatch can be corrected by analog adjustment and path mismatches are corrected digitally.

Section snippets

Organization

Section 2 provides background and discusses the mismatch problem. Section 3 introduces an internally generated calibration signal to estimate LO mismatches. In Section 4, the internal signal is chopped to allow estimation of the path mismatch. Conclusions and future work are given in Section 5. Simulations are presented throughout the text.

Background and mismatch problems

Modulation and demodulation processes are necessary in communication systems. A variety of receivers exist to accomplish demodulation, among which the quadrature/coherent receiver can provide good noise performance making it attractive for high-quality reception [1].

For a typical quadrature receiver, the front-end receives the modulated signal and splits it into two paths. A local oscillator (LO) is used to demodulate the in-phase (I) branch, while a 90° phase-shifted version of the LO output

An internal calibration signal

Application of external calibration signals, while sometimes effective, can suffer greatly from LO drift. Thus, a calibration signal that accommodates LO drift is desirable. This section proposes an internally generated calibration signal that is less susceptible to LO drift. In the next section, a chopped version of this signal is used to estimate the frequency-dependent path mismatches.

In many cases, LO mismatch constitutes the primary source of imbalance. One way to estimate the LO mismatch

Chopped LO signal

As discussed in Section 3, feeding back the LO output can accurately estimate the LO mismatch even in the presence of LO drift. Although LO mismatch is usually the primary concern, there are cases where frequency-dependent path mismatch is also of interest. To obtain a multi-frequency test signal, a chopped LO signal is proposed in this section.

A chopped LO signal can be represented asm(t)=Acos(ωLOt+d(t))schop(t),where schop(t) is a rectangular pulse train with period T0 and duty cycle τ/T0, A

Conclusion

I/Q mismatch is an important concern in implementing quadrature receivers. This paper presents techniques using internal test signals for quadrature receiver mismatch calibration. These techniques show robustness in the presence of both Doppler shifter and LO frequency shift.

Two classes of receiver errors are treated: LO mismatch and path mismatch. To calibrate LO mismatch, an internal calibration signal formed by direct LO feedback is presented. A chopped version is presented to estimate and

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