Error vector magnitude (EVM) is essentially a scalar measurement of digital modulation accuracy, an important figure of merit for any source of digital modulation. Thus, Linear Technology has shown how a vector signal analyser helps optimise EVM performance of IQ modulators such as the LTC5598, a 5MHz to 1600MHz high linearity direct quadrature modulator.

Low modulator EVM is desired because the EVM degrades farther down the line—transmit up-converters, filters, power amplifiers, the communications channel and the receiver all impair the received signal.

Linear Technology has done a typical 450MHz EVM measurement to a LTC5598, resulting to an EVM of 0.34% rms and 0.9% peak. After the harmonic filter, the output power measures 0.4dBm for the same signal. By comparison, a lab-grade signal generator with the same amplitude, frequency and digital modulation measures 0.28% rms and 0.8% peak, on the same VSA setup. This indicates that the LTC5598 modulation accuracy is nearly as good as the test equipment being used to measure it.

In summary, the LTC5598 provides excellent digital modulation accuracy across many popular VHF and UHF communications bands. In some cases, EVM is comparable to that of a lab-grade signal generator. Where desired or necessary, baseband correction of quadrature phase and/or gain may be implemented for enhanced accuracy.

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