Agilent Measurement Journal - Issue 4 - 2008 - (Page 64)

Considering total harmonic distortion Total harmonic distortion (THD) is the ratio in dB of the total level of the ﬁrst few harmonics to the level of the input signal at the output. THD is usually calculated from an FFT of a test signal where f is the fundamental (input) frequency and f1 through fn are the ﬁrst n harmonic frequencies: Conclusion The handful of key speciﬁcations described here only begins to illustrate the non-trivial task of selecting a product based on published speciﬁcations. For example, ENOB (and SINAD) provide some indication of amplitude measurement accuracy. Clock accuracy and sampling jitter provide an indication of the frequency measurement accuracy and the temporal noise introduced in the measurement. There are additional useful speciﬁcations, each with individual implications for speciﬁc applications. For example, linearity addresses the fact that the discrete measurement levels described by the bit resolution are not necessarily equally spaced, leading to amplitude measurement error. Voltage standing wave ratio (VSWR) describes the reﬂection of the input signal at the input terminals, an effect that diminishes the signal amplitude before it is measured and also introduces signal echoes into the system. VSWR also is frequency dependent, introducing more complexity to the selection process. Noise ratios have been addressed here, but other speciﬁcations such as the Sparkle Code Rate indicate the probability of a sampled point exceeding a speciﬁed deviation threshold. In general, spurious noise could have an effect on any peak-detection routine based on the digitization of a signal. In most cases, obtaining the best measurement system for a speciﬁc application can be ensured by consulting not only the speciﬁcation sheets of the various devices but also the manufacturers themselves. THD f12 + f22 + f32 + … + fn2 f2 Tip: Harmonic distortion values are most meaningful when quoted with the number of harmonics used in the calculation; the input frequencies over which the measurement is valid; the sampling rate of the measurement quoted; the full-scale voltage range; and the input voltage level as a percentage of that full-scale range. Examining spurious-free dynamic range Spurious-free dynamic range (SFDR) is the difference, expressed in dB, between the RMS values of the input signal at the output and the peak spurious signal where a spurious signal is any output signal that was not present at the input (Figure 6). dB Signal Spurious signal SFDR Frequency Figure 6. Spurious-free dynamic range affects the usable dynamic range in a measurement. Tip: SFDR values should be quoted with the input frequencies over which the measurement is valid; the sampling rate of the measurement quoted; the full-scale voltage range; and the input voltage as a percentage of that full-scale range. 64 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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