AN-899| Application Note
Operating and Evaluating Quadrature Modulators for Personal Communication Systems
Operating and Evaluating Quadrature Modulators for Personal Communication Systems
ABSTRACT The quadrature modulator is a fundamental radio component for worldwide digital wireless communication standards such as Global System for Mobile Communications (GSM) Digital Communications Systems 1800 MHz (DCS1800) Digital European Cordless Telecommunications (DECT) American and Personal Digital Cellular (ADC PDC) Japanese Personal Handy Phone (PHP RCR28) A typical system application and the theory of operation of the quadrature modulator are described along with two practical evaluation methods Radio system considerations are discussed for the quadrature modulator including phase and amplitude imbalances DC offsets time dependent phase errors modulated carrier harmonics and nonlinear impairments INTRODUCTION Worldwide standards for the mobile communications environment are demanding spectrally efficient digital modulation techniques Low power low cost integrated quadrature modulators can be used in wireless personal communications for digital modulation (i e QPSK (Quadrature Phase Shift Keying) q
4DQPSK (Pi over 4 Differential Quadrature Phase Shift Keying) MSK (Minimum Shift Keying) GMSK (Gaussian filtered Minimum Shift Keying)) The quadrature modulator has the advantage that any parameter of a carrier frequency (amplitude phase or frequency) can be simultaneously manipulated to represent information Other modulators do not have this flexibility For example a single mixer can be used to modulate a carrier In this case an information signal is limited to modulating a carrier by altering its amplitude (or phase by 180 ) or its frequency Figure 1 illustrates a mixer performing a simple digital modulation technique binary phase shift keying (BPSK) that consists of shifting the phase of a carrier by 180 (or changing the amplitude of a carrier between a positive and a negative state) The result is two information states where one could represent bit 0 and the other state could represent bit 1 The constellation diagram of Figure 1 illustrates the two states along one axis
National Semiconductor Application Note 899 Ruth Umstattd Wireless Communications October 1993
In comparison for a quadrature modulator information is modulated onto a carrier with full freedom to manipulate the amplitude phase or frequency of the carrier A quadrature modulator is implemented with a phase shifter two mixers and a signal combining stage Figure 2 shows a quadrature modulator performing QPSK
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FIGURE 2 Quadrature Modulator Performing QPSK Even bits of the information signal are directed to one mixer and the odd bits are directed to the second mixer The LO is shifted by 90 at one mixer For QPSK the result at the output of the modulator is four states of information where each state represents two bits Compared to Figure 1 the quadrature modulator transmits the same information in half the time QPSK is only one modulation scheme that can be employed by the quadrature modulator The information bearing signal inputs can vary so that a vector from the origin of the constellation diagram can have any amplitude and phase value at any point in time The quadrature modulator is a flexible modulator that is used with spectrally efficient modulation schemes Historically a discrete version of the quadrature modulator has been used in sophisticated communication systems Currently the cellular and cordless telephony market demands an integrated quadrature modulator suitable for high volume production In this doc
ument the description operation and evaluation of the quadrature modulator is given from the current commercial applications perspective TYPICAL SYSTEM APPLICATION The quadrature modulator provides the necessary interface between digital data and the RF transmission channel Figure 3 shows a typical transmit system block diagram using a direct conversion quadrature modulator
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FIGURE 1 Mixer Performing BPSK
AN-899
C1995 National Semiconductor Corporation
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RRD-B30M75 Printed in U S A
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