Lambda SR1, Руководство по эксплуатации, Страница: 149 / 317

SR1 Operation 149
© 2014 Stanford Research Systems
acquire a time record of a given length will increase. Therefore the FFT display will update more slowly
as the bandwidth is narrowed. Unlike the FFT1 analyzer the FFT2 analyzer has no heterodyne capability
so that when the bandwidth is decreased the frequency range always starts at DC.
Aliasing
Aliasing arises in sampled data systems because the real-world filters used to protect A/D converters
have finite cutoff slopes. Each of the SR1's A/D converters is protected with either analog or a
combination of analog and digital anti-aliasing filtering. For each converter there is some range of
frequency below Fs/2 that is not fully protected to the level of the ultimate attenuation of the anti-aliasing
filter. When using the "zoom" feature of the FFT3 analyzer the analyzer applies digital anti-aliasing filter
after each decimation stage to meaning that there is a small region at either end of the analysis range
that is not alias-protected to the full attenuation of the digital anti-aliasing filter.
When is checked, the analyzer displays all lines in the full analysis range of the
FFT without regard to whether they are fully alias protected. This setting is often perfectly useful for
many typical audio measurements. However, when aliasing is potentially a problem the box can be
unchecked, in which case only FFT lines that are fully alias-protected are displayed. Areas of the
spectrum that are not fully protected are shown in red in the graphical frequency indicator on the FFT
panel.
Resolution
The resolution control determines the number of lines in the computed spectra. Values from 256 lines to
32k lines can be selected. In the resolution control the time to acquire a time record for the selected
resolution and current bandwidth selection is shown alongside the number of lines. Obviously, the higher
the selected spectral resolution, the longer it will take to acquire the time record for that spectrum.
Averaging
The Power Spectrum, Linear Spectrum, and Frequency Response are all averaged quantities.The
amount and type of averaging for both spectra is specified by the controls above. The averaging type
controls determines how each of the averaged spectra is averaged. "None" implies that no averaging is
performed. In the magnitude of the Power Spectrum and Linear Spectrum are the same and reduce to
the magnitude of the last individual FFT. "Fixed Length" averaging means that the analyzer will average
the selected number of spectra and then stop. "Continuous" averaging continuously averages the
spectra weighting more recent results exponentially more than older spectra. The two "Peak Hold"
selections only affect the averaging of the Power Spectra. When Peak Hold is selected instead of