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ADC Type of filter ? - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


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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