i have completed a filter that 3db frequence is 400M and Au is -40db in the frequence of 16M. But the signal from the DAC through the filter Au decreased only -30db in the 16M. why? thank you for your help!
The filter specifications do not appear to be consistent. Did you mean to say that the 3db frequency is 4.00M? With a corner frequency of 400MHz, the gain at 16MHz should be aprroximately 0db.
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What type of filter did you design? If it is a digital filter, you could be having aliasing problems, depending on what your sampling rate is.
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How about some more details?