Complex Filter bandwidth - newbie question

A DDC requires an output bandwidth of 80% of the output sample rate.

A complex mixer generates I and Q signals from the real input, I and Q
are decimated and filtered.

To achieve the 80% complex bandwidth requirement are the individual I 
and Q the filter passbands at Fs-out * 0.8 or 0.4?

Thanks!

On Sep 23, 8:26 am, System Alchemist <dotter@nospam_btconnect.com>
wrote:

> To achieve the 80% complex bandwidth requirement are the individual I
> and Q the filter passbands at Fs-out * 0.8 or 0.4?

Each of I and Q has a passband from negative 0.4 to positive 0.4,
it's a lowpass filter from zero to +/-0.4 FS.

Having both I and Q lets you tell if a given signal is in the positive
or negative half,
but for I and Q taken individually as the seperate filters see them,
there is no difference.