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ADC Type of filter ? - RF Cafe Forums

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gui076
Post subject: ADC Type of filter ? Posted: Thu Jun 12, 2008 5:20 am

Captain

Joined: Mon May 05, 2008 9:56 am
Posts: 15
Location: France
Hello,

I have an IQ demod before an ADC. After the IQ demod i would like to use direct conversion or low-if (70/140 MHz). My question is what type of filter i have to insert between the IQ demod and the ADC ? LPF,BPF ? SAW for 70/140 MHz ?

Thank you.


Guillaume


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IR
Post subject: Posted: Fri Jun 13, 2008 4:06 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello gui076,

The filter has to be LPF to limit the frequency of the sampled signal and by this to prevent aliasing. I would use a lumped LC filter and not SAW, which has a high insertion loss and thus will reduce the ADC's dynamic range.


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gui076
Post subject: Posted: Sat Jun 14, 2008 7:56 am

Captain

Joined: Mon May 05, 2008 9:56 am
Posts: 15
Location: France
So if i have to put a filter it's better to have a LPF until 160 MHz instead of a bandpass at 140 MHz and about 40 MHz of bandwidth. The noise won't be too high ?

Guillaume


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IR
Post subject: Posted: Sun Jun 15, 2008 6:19 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
The choice of filter also depends on your system architecture: Do you plan to use undersampling? If yes, then you will require a LPF to allow sampling in lower frequencies. If not, then a BPF is adequate.

Both filters can be implemented by lumped components which will provide lower insertion loss than SAW technology.


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gui076
Post subject: Posted: Sun Jun 15, 2008 8:18 am

Captain

Joined: Mon May 05, 2008 9:56 am
Posts: 15
Location: France
Hello,

Maybe i will try to implement some switche if i want to test some kind of sampling.

So thank you for your answer

Guillaume




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