ScopeDSP Time and Frequency Statistics

Time and Frequency Data Statistics

Time Data Statistics

For Time Data, ScopeDSP calculates the Minimum, Maximum, Inphase Mean, Quadrature Mean, and Magnitude RMS.

Frequency Data Statistics

For Frequency Data, ScopeDSP calculates the power of each frequency data point in decibels (dB). Assuming the data is a periodic sine, It uses the frequency spectrum list to calculate common descriptive frequency statistics including Signal To Noise Ratio (SNR), SINAD (Signal to noise-plus-distortion), Spurious Free Dynamic Range (SFDR). (See Part 4 of the A/D Specifications Measurement tutorial for definitions of these statistics.)

Frequency List

The assumption that the "signal" is a sine doesn't apply to every data sample. For these cases, ScopeDSP generates an ordered list of frequency powers which allows the user to calculate his own statistics manually while taking into account the nature of the "signal". This Frequency List is displayed on the screen and can be written as a text file. The length of the list may be specified by the user. Here is a portion of a Frequency List taken from the A/D Specifications Measurement section, Part 4:

Frequency Component List

The first part of the Frequency List repeats the Frequency Data Statistics. The next part describes individual frequency bins, sorted by component power. The columns are described as follows:

"Frequency" is the component frequency in terms of the current frequency units.
"Bin" refers to the DFT "bin number" (index into the Frequency Data array). Bin "0" refers to DC (signal mean).
"Harm" is the harmonic number. The largest component is deemed to be the "fundamental" so it is defined as harmonic number " 1". (The first entry will always be harmonic "1".) Note that analog harmonics which are above the Nyquist frequency reappear as aliases below the Nyquist frequency. In this case, the second harmonic of 3.906 is 7.812; since this is more than half the sample rate, it appears at 2.188 MHz. The problem of aliased harmonics can be confusing to sort out manually, so ScopeDSP does it automatically. Knowing whether an undesired component is distortion (a harmonic of the fundamental) or spurious (an extra component) is quite important because the steps you take to reduce these two kinds of problems might be quite different.
"dB Below Peak" indicates the power, in dB, below the fundamental. In this case, the "Bin Power" and "dB Below Peak " columns are identical because the decibel units are referenced to the peak component (dBc).
"Total Smaller Power" is the power of all components that are smaller than the given component. This information is very useful in calculating signal ratios manually when the assumptions in ScopeDSP's automatic ratio calculations are not appropriate for your signal.

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