Resampling based on decimation is characterized by the following drawbacks: (i) the selection of the sampling rate is limited to the values that can be obtained dividing the maximum sampling rate by integer values (ii) if the selected sampling rate is less than the Nyquist rate of the analog input, the acquired signal is corrupted by aliasing. All sets have the same size, which is equal to the required decimation factor-for instance, grouping samples into sets with size equal to 2 means acquiring one every other sample, thus halving the input sampling rate. Resampling is performed by means of a digital circuit that interfaces ADC and acquisition memory, and merely consists in decimating the input stream, which involves grouping the samples at the maximum sampling rate into consecutive sets, and acquiring, that is, storing in the acquisition memory, only the first sample of each set.
The user can also select lower sampling rates, which are achieved by seamlessly resampling the ADC output stream. In the majority of digital storage scopes (DSOs) the analog-to-digital converter (ADC) always works at its maximum sampling rate, imposed by an internal fixed frequency clock. Both a field programmable gate array (FPGA) implementation and an application specific integrated circuit (ASIC) design of the proposed circuit are evaluated. The output samples produced by the digital circuit are characterized by a sampling rate that can be incoherent with the clock frequency regulating the memory access. The circuit relies both on the use of a short digital filter with dynamically generated coefficients and on a suitable memory management strategy. Then, it proposes a digital circuit that, if included in the acquisition channel of a digital storage oscilloscope, between the internal analog-to-digital converter (ADC) and the acquisition memory, allows the user to select any sampling rate lower than the maximum one with fine resolution. The paper first discusses digital signal processing based methods that allow changing the sampling rate by means of digital resampling approaches.
#Realflight 7.5 anti aliasing Offline#
Fine resolution selection of the sample rate is not available in digital storage oscilloscopes (DSOs), so the user has to rely on offline processing to cope with such need.
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