Microwave Engineering Europe - September 2008 - (Page 24)

24 TEST — RF AMPLIFIER PERFORMANCE next-generation RF instrumentation needs innovative new digital architectures including software-deﬁned radio (SDR) implementations to test the new signaling schemes. New instrumentation must have the ﬂexibility to generate and analyze many types of modulation and must be able to switch between them quickly. New RF instrumentation must, therefore, be able to make fast and accurate measurements of EVM for a number of different modulation formats. Instrumentation with this capability can substantially lower the cost of test, both the capital cost of the equipment and test times. Here we examine how these new instruments can be used to make accurate EVM measurements for fully characterizing RF ampliﬁer performance. RF power ampliﬁers Figure 1 shows a simpliﬁed communications system where the input signal could be either voice or data. Most modern systems digitize all analog signals, such as voice, so the communications system is virtually all digital. The power ampliﬁer is the last stage of the transmitter. Any amplitude or phase distortion here directly impacts the communications quality of the entire system. For optimum performance, which is particularly critical for mobile devices, power ampliﬁers are normally operated as close as possible to their maximum, linear power output levels. Doing so optimizes power efﬁciency and battery life. Above the maximum linear output is the gain compression region where output power no longer varies linearly with the input power. When a power ampliﬁer is driven into the gain compression region, amplitude and phase distortion will occur. Modulation schemes such as OFDM create signals with high peak-average ratios. This forces designers to “back off” the average power operating point for the power ampliﬁer to ensure the peak power levels do not put the ampliﬁer into gain compression. Keeping the power amp out of the gain compression region can be difﬁcult with a multi-signal modulation scheme and a multi-path, external environment. Signals reaching the receiver can have a wide dynamic range based on constructive interference, generating very large signals and destructive interference that creates very small signals. Thus, the question is not if there is distortion, but rather how much distortion and what is the impact on modulation accuracy. EVM magnitude error can be caused by the ampliﬁer’s frequency response and noise ﬁgure, as well as gain linearity. EVM phase error is caused by non-linear phase shifts, such as when the power ampliﬁer operates in the gain compression region. This is why it’s important to measure the EVM performance of the power ampliﬁer. However, the power ampliﬁer is not the only component that can affect EVM. The transmitter modulator has magnitude and phase offsets and carrier feed-through, all of which add to the EVM error. In the receiver, the preampliﬁer, downconverter, and demodulator also contribute to EVM error. EVM background EVM is a measure of modulation accuracy and is a key measure of the quality of the digital modulation found in modern wireless communication systems. EVM is the vector difference between the ideal measured components I (in-phase) and Q (quadrature phase) of the transmitted signal, known as the reference signal “R”, and the magnitude of the received I and Q components of the measured signal “M.” The EVM is applicable for each symbol transmitted and received. Figure 1: Simpliﬁed communications system. EVM is given as a magnitude quantity and expressed as a percentage, although both the phase and magnitude error of each of the measurement points is measured. EVM is typically measured for many signals. In fact, the EDGE standard requires that EVM be measured over 200 bursts, so it is common to refer to RMS or peak EVM. The RMS EVM is deﬁned as the square root of the ratio of the mean error vector power to the mean reference power. The peak EVM is the maximum EVM that occurred during the measurement interval. EVM provides an insight into the signal’s quality that other performance measurements such as eye diagrams or BER measurements cannot. EVM is proportional to the bit error rate, yet is a faster measurement and gives more diagnostic insight than eye-diagrams or BER testing. There is also a direct relation between EVM and the signal-to-noise ratio (SNR) and the signal to noise and distortion (SNDR) Figure 2: EVM measurement setup. Figure 3: EVM versus input power measurement. Microwave Engineering Europe ● September 2008 ● www.mwee.com 023-024-025-026-027-028_MWEE.indd 24 2/09/08 15:51:25 http://www.mwee.com

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Microwave Engineering Europe - September 2008

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