Analog Circuit Design

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Measuring phase and delay errors accurately in I/Q modulators

P. Stroet

Introduction

This chapter describes a method to accurately measure internal and external phase and timing errors for a high performance direct I/Q modulator. A direct I/Q modulator, such as the LT5528, translates baseband I and Q signals to RF, and combines them to produce a modulated single sideband signal with (ideally) minimal residual carrier (LO feedthrough) and image signals (undesired sideband). In an ideal I/Q modulator, with perfect 90° phase shift between the I mixer and Q mixer local oscillators (LOI and LOQ), and with no other undesired phase and gain impairments, the modulator output will contain only the desired sideband. In practice, this is very difficult ...