Dynamic Range & Frequency Accuracy of Spectrum Analyzer

Chapter Dynamic Range

Guide to Spectrum and Signal Analysis
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Summary of Contents

Guide to Spectrum and Signal Analysis

  • 16 | Guide to Spectrum and Signal AnalysisDynamic RangeThe dynamic range of a spectrum/signal analyzer is determined by four key factors.i. Average noise level. This is the noise generated within the spectrum analyzer RF section, and is distributed equally across the entire frequency range.ii. Residual spurious components. The harmonics of various signals are mixed together in complex form and converted to the IF signal components which are displayed as a response on the display. Consequently, the displayed response is present regardless of whether or not a signal is present at the input.iii. Distortion due to higher order harmonics. When the input signal level is high, spurious images of the input signal harmonics are generated due to the nonlinearity of the mixer conversion.iv. Distortion due to two signal 3rd order intermodulation products. When two adjacent signals at high power are input to a spectrum/signal analyzer, intermodulation occurs in the mixer paths. Spurious signals, separated by the frequency difference of the input signals are generated above and below the input signals.The level range over which measurements can be performed without interference from any of these factors is the dynamic range. This represents the analyzers performance and is not connected with the dis play (or measurement) range. The four parameters that determine dynamic range can normally be found in the analyzer specifications.For simplicity, some analyzer specifications state the dynamic range as “Y dB for an input level of X dBm”. The following example shows how these parameters are related to dynamic range:Amplitude Dynamic Range: 70 dB for a mixer input signal level of –30 dBm (Atten. = 0 dB) In order to achieve this value of dynamic range the following conditions are required:a) the IF bandwidth must be narrow enough such that the average noise level is better than –100 dBm.b) the residual spurious components must be less than –100 dBm.c) for an input level of 30 dBm the higher harmonic distortion must be better than –70 dB (i.e. better than –100 dBm).Analyzer manufacturers often relate the above specifications at a particular frequency or over a range of frequencies.Frequency AccuracyThe key parameter relating to frequency accuracy is linked to the type of reference source built into the spectrum/signal analyzer. • Synthesized The analyzer local oscillator is phase locked to a very stable reference source, often temperature controlled to prevent unwanted frequency drifting. In this case, a precision crystal is often used and the overall frequency accuracy and stability, both short term and long term depend on its quality. Portable analyzers, intended for outdoor use, often have GPS receivers that can significantly improve the stability of the internal local oscillator.• Non Synthesized The analyzer local oscillator operates as a stand-alone voltage controlled source