thanks LvW
if i want to design baseband filter ,then what bandwidth i have to consider
For my opinion, this is not really a question that can be answered.
Some basics:
A group of frequencies (e.g. a modulated signal) consists of some frequencies that are wanted (they belong to the signal) - and, unfortunately, in most cases also of some unwanted components (e.g. harmonics due to non-linearities).
This is the case of a filter - and you have to decide: What is the task of the filter?
In detail: Which range of frequencies shall pass and which range has to be suppressed?
This leads to one of the basic filter types: Lowpass, highpass, bandpass, bandstop.
So, it's up to you (the user) to specify the requirements to the filter (transmission bandwidth, frequencies to be attenuated by how many dB?)
You see, it's to early to give you a specific answer, OK?