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SDS6000L User Manual
int. s i g l e n t . c o m 1 8 9
Adapt Sample Rate
Unlike an ideal “brick wall” filter, the actual FIR filters have their roll-off from the passband to the
stopband because of the limited number of taps. The 200-tap FIRs applied in the SDS6000L provide
roll-off as fast as 0.01*fs (where fs is the sample rate), this means some limitations in the setting of
corner frequency at a specified sample rate. The figures below show the limitations in detail:
≥0.01fs ≥0.01fs
fs/2fc
≥0.01fs ≥0.01fs
fs/2fc
Low pass
High pass
≥0.01fs
≥0.01fs
fs/2
fl
≥0.02fs
fh
≥0.01fs ≥0.01fs
fs/2
fl
≥0.02fs
fh
Bandpass
Band reject
Take a Low pass filter as an example. It requires that the corner frequency (fc) is not less than 0.01*fs
(i.e. fc ≥ 0.01*fs), and the 0.01*fs wide roll-off does not exceed the 1st Nyquist zone (i.e. (fs/2) - fc ≥
0.01fs). If fs = 5 GSa/s, then the legal range of the corner frequency (fc) is 50 MHz~2450 MHz
If the expected fc is out of the legal range at some sample rate, then the sample rate needs to be
changed. For instance, when fs = 5 GSa/s, the setting fc = 10 MHz is illegal, so we must lower fs to 1
GSa/s to satisfy the limitation (fc ≥ 0.01*fs).
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