tunable Intermediate frequency filter

hi all ,
i want to design tunable IF filter in direct conversion receiver .so what i want to ask, output of mixer in direct conversion recevier is base band signal that is filtered out from the mixer why we want to design filter after mixer.please can anybody tell

thanks&regards
ramakrishna

i want to design tunable IF filter in direct conversion receiver

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why we want to design filter after mixer

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Sounds like "tell me what I'm doing here"?. IF filter is mostly related to receiver selectivity, I think.

hi FVM,
In direct conversion receiver channel selection depends on LO & mixer, output coming from the mixer is desired signal, why selectivity is required for filter

How should LO and mixer introduce any selectivity. They just mix down the input signal spectrum. Typically, you have some input stage selectivity provided by a tuned circuit. But it has at least MHz bandwidth.

In a direct conversion receiver, most selectivity is achieved by a low pass after the mixer.

Maybe you do not really mean "direct conversion" receiver. A direct conversion reciever has a baseband output. You can lowpass filter it to remove higher frequency components, and that lowpass can have a tunable corner frequency. Such a lowpass filter can be an analog filter, or a DSP digital filter, or a combinaiton of both. Also, you can highpass filter the baseband (with a series capacitor) to remove DC. But that is about it for filtering.

For communicaitons filters, typically you downconvert the rf signal to a "low IF frequency", instead of downconverting it to 0Hz center, and that allows you to bandpass filter the signal's width. For communicatioins systems, you set that bandpass filter to be just a little bigger needed to send the data rate through it, and that helps to reject "out of band" signals that might jam your reception.