AN0001 Datasheet, PDF (1/6 Pages) RF Micro Devices – Optimization of Quadrature Modulator Performance

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AN0001
AN0001
Optimization of Quadrature Modulator Performance
Optimization of Quadrature Modulator Performance
Introduction
Quadrature modulators are common building blocks in a communications link. By their nature they are capable of send-
ing virtually any modulation scheme. They can send both analog, such as AM and FM, and digital modulation schemes,
such as BPSK, QAM, and QPSK.
A common set of specifications for quadrature modulators is the carrier suppression and sideband suppression. The
question often arises as to how they correlate with amplitude and phase error for the same device. This article presents
a derivation showing the relationship between these four parameters.
Because the overall performance of the system can be affected by the modulator, its performance is important. There-
fore, the need to optimize the carrier suppression and the sideband suppression of a quadrature modulator often arises.
For those who may not be experienced with quadrature modulators, this can be a confusing task. This article will make
this optimization easier to understand and perform.
Discussion
The first question that arises is why optimization is required at all. The primary reason is that there are imbalances in the
Gilbert cell mixers and phase error introduced by the phase-shifting network. These imperfections are caused by slight
differences in devices on the same die. There are also imbalances and offsets between the in-phase and quadrature sig-
nal paths as a result of process variations. These errors are not present in an ideal device, however they cannot be elim-
inated in practice.
The second question then would be how to compensate the device to optimize the performance. To understand how
compensation can be achieved, it is helpful to understand how sideband suppression, carrier suppression, amplitude
error and phase error are related.
The block diagram of a typical quadrature modulator is shown in Figure 1.
In-Phase Signal
Quadrature Signal
LO Signal
0°
90°
Σ
RF Output Signal
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Figure 1. Quadrature Modulator Functional Block Diagram
The circuit multiplies the in-phase signal by the local oscillator and the quadrature signal by a 90° shifted version of the
local oscillator. These signals are then summed to form the RF output signal.
Copyright 1997-2002 RF Micro Devices, Inc.
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