GaryParticipantGary February 2, 2021 at 7:27 am
Good morning! I have a question concerning sweep time. In the display window, there is a “sweep time” shown underneath the spectral display, and there is a “sweep time” setting in the right column. Now, the “sweep time” on the right column is a setting. How is the “sweep time” shown under the spectral display derived? Is it measured? If so, how?
For example, with the attached image, the set sweep time is 1 msec, while the sweep time shown under the spectral display is 8 msec. Does this mean that Spike collected energy for 1 msec, then required 7 msec to process it (window the data, perform the FFT, calculate amplitudes, display the result)?
Further, since the sweep time is typically much longer than the time required to sample the data (which is typically just a couple of hundred microseconds), does this mean that Spike collects multiple time records at each frequency point,which is where the “min/max” points are derived from?
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AndrewModeratorAndrew February 2, 2021 at 9:07 am
The sweep time under the spectrum is a measured sweep time. The resolution on the timer is coarse and provides a rough estimate. This sweep time includes acquisition and processing time. The sweep time control is used to specify a minimum acquisition time. We attempt to acquire data for at least as long as the value provided here. There are situations where we may not be able to accommodate this, and there are other situations where we may have to overshoot this value by a bit. Either way, we try to get as close as the hardware will allow.
Regarding acquisition lengths… The BB60C measures spectrum 20MHz at a time and stitches the results together to form the final sweep. At each 20MHz step we might acquire more data than is truly necessary, either due to minimum acquisition lengths of the hardware, increased sweep times set by the user, or even the default 1ms sweep time combined with a narrow span and large RBW, which results in acquiring more time data than is necessary to achieve the selected RBW. We process this time data with overlapping FFTs and use the detection algorithm selected (minmax/avg) to produce the final result. The BB60C always performs as many overlapping FFTs as possible with its acquired data, unless the “sample” video units are selected, at which point, a single FFT is performed, and no minmax/averaging is performed.
I hope this helps.
GaryParticipantGary February 2, 2021 at 9:46 am
Okay, I think I understand well enough now. Thanks, Andrew!
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