Dewetron DEWE-ORION-0824-20 Series, Техническое справочное руководство

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DE-M080801E • DEWE-ORION-0824-20x • Technical Reference Manual • Printing version 1.1.2 • March 19, 2014
DEWE-ORION-0824-20x
When the sampler modulates frequencies out of the Nyquist bandwidth back to the 0 to 500 Hz baseband
it is called aliasing. Signals which are not pure sine wave can have many components (harmonics) above
the Nyquist frequency. These harmonics are erroneously aliased back to the baseband, added to parts of
the accurately sampled signal and produces a distorted data set. To block frequencies out of the Nyquist
bandwidth, a lowpass filter is applied to the signal before it reaches the sampler.
Each input channel has its two pole anti-alias lowpass filter with a cutoff frequency of about 250 kHz. The
very high cutoff frequency allows an extremely flat frequency response in the bandwidth of interest and a
small phase error. The analog filter precedes the analog sampler. The analog sampler operates at 256 times
the selected sample rate for rates below 51.2 kS/s, 128 times for rates between 51.2 kS/s and 102.4 kS/s.
For rates over 102.4 kS/s the oversampling is 64 times. That means, the ADC operates at 13.1072 MS/s if
you select a sample rate of 102.4 kS/s (128 * 102.4 kS/s).
The 1-bit oversampled data is passed to a digital anti-aliasing filter. This filter has no phase error and an
extremely flat frequency response. It also has an extremely sharp roll-off near the cutoff frequency (0.38 to
0.494 times the sample rate) and the rejection above 0.5465 times the sample rate is greater than 92 dB.
The output stage of the digital filter resamples higher frequencies to 24-bit samples.
The digital filter passes only signal components within the Nyquist bandwidth or within the Nyquist bandwidth
at 64, 128 or 256 times (depending on sampling rate) the sample rate and multiples of it. The analog filter
rejects most noise near these multiples. The following diagrams show the frequency response of the input
circuitry.
Sample rate 1 kS/s to 51.2 kS/s
Input frequency response
Input frequency response near the cutoff
Amplitude (dB)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
0.45 0.46 0.47 0.48 0.49 0.50 0.51 0.52 0.53 0.54 0.55
Frequency/sample rate
Amplitude (dB)
Frequency/sample rate
Sample rate 51.2 kS/s to 102.4 kS/s
Input frequency response
Input frequency response near the cutoff
Amplitude (dB)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
0.40 0.43 0.45 0.48 0.50 0.53 0.55
Frequency/sample rate
Amplitude (dB)
Frequency/sample rate