Vaisala RVP900 - Figure 29 Example of a Filter with Poor DC Rejection

Example
The oset error of the IFDR A/D converter is at most 10mV, that is, -27 dBm into its 50 Ω
input.
To achieve 90 dB of dynamic range below the converters +8 dBm saturation level, we
expect usable I and Q values to be obtainable from a (sub-LSB) input signal at -82 dBm.
This is 55 dB below the interference that would result from the worst-case A/D
oset.
But a weak input signal at -82 dBm would still be damaged by even an equal level of DC
interference. Therefore, adding another 10 dB safety margin, we get -65 dB as the
recommended maximum DC gain of the matched
filter. This DC gain should be reduced
even further if it is known that coherent leakage is present in the receive signal at a level
greater than the -27 dBm worse-case A/D oset.
The following figure shows a 60 MHz filter with particularly poor (-42 dB) DC rejection.
The frequency range of the plot is 36 MHz to 72 MHz, therefore, DC appears aliased at the
right edge and we can see a peak in the
filters stopband at DC. Contrast this with the
filter shown previously that has a true zero at DC. In general, a poor filter can be
converted into a "nearly" good filter by making only incremental changes to the impulse
response length and/or desired bandwidth.
Figure 29 Example of a Filter With Poor DC Rejection
RVP900 User Guide M211322EN-J
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