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SR1 Operation Manual 158
© 2014 Stanford Research Systems
Harmonic Selection
These checkboxes select the set of harmonics included in each measurement. Clicking on the boxes
next to the "Odd" and "Even" labels will automatically select (or unselect) all the corresponding odd or
even harmonics. Alternatively, the individual harmonic can be selected or unselected by checking or
unchecking the corresponding box. The amplitude reported by the measurement is the RMS sum of the
harmonic amplitudes for each of the selected harmonics. Note that it is possible to select a harmonic
which is outside the frequency range of the selected converter. It is the user's responsibility to ensure
that each of the selected harmonics is within the analyzer's frequency range.
If the "Ratio" box is checked the summed amplitude will be divided by the amplitude of the fundamental
for the selected channel and the result will be reported as a ratio.
The definitions of THD+N and THD both involve the ratio of distortion products to the amplitude
of the fundamental. However most time-domain based analyzers, including SR1's TDD,
cannot separately identify the amplitude of the fundamental and instead measure the ratio of
the distortion products to the total power in the signal. For small distortion, the difference is
insignificant.
The THD Analyzer, which is frequency selective, correctly displays the ration of the sum of the
harmonics to the fundamental amplitude. The only exception to this is when the THD analyzer
source is set to "Other Analyzer" and the THD analyzer is looking at the notch filtered output
of the TDD. In this case the "Ratio" result is divided by the total power in the signal as
measured by the TDD. Since the "Other Analyzer" input is only needed when measuring the
smallest levels of distortion (<-95 dB), this will not signficantly impact the measurement result.
Measurement Speed
The THD analyzer allows a tradeoff between measurement speed and measurement precision. Internally
this is accomplished by varying both the number of FFT averages performed and the resolution of the
FFT spectra. When using the precise and very precise settings sweep speeds will be noticeably slower,
but the measurement results will exhibit less variability.
Enabling Synchronous Averaging can sometimes be useful when the harmonic amplitudes are close to
the noise floor. When Synchronous Averaging is enabled the THD analyzer uses the averaged Linear
Spectrum to compute the harmonic amplitudes. As discussed in the FFT Analyzer section, averaging
the linear spectrum reduces the amplitude of uncorrelated noise which allows a more accurate
measurement of harmonic amplitudes, which are phase correlated to the fundamental and therefore
maintain their amplitude in the linear spectrum.
Pressing "Clear" clears the current average buffer. This is useful for reducing the transients caused by,