[USRP-users] LFRX antialiasing

Marcus D. Leech mleech at ripnet.com
Tue Apr 30 15:37:51 EDT 2013


On 04/30/2013 03:31 PM, mepard wrote:
>
> In the "Receive FPGA pipeline overflow?" thread, on Apr 30, 2013, at 
> 12:12 PM, Marcus D. Leech <mleech at ripnet.com 
> <mailto:mleech at ripnet.com>> wrote:
>
>> Also, if you aren't band-limiting your input to handily-below the 
>> Nyquist frequency (50Mhz, in the case of the N210, because the ADC 
>> samples at
>>  100Msps), then you'll get aliases folded into the signals after 
>> they're sampled.
>
> So we need external filtering for the LFRX board? I knew this was true 
> for the BasicRX (1 - 250 MHz), but I had the impression this wasn't 
> necessary for the LFRX (0 - 30 MHz).
>
> -Marc
>
There is a 30Mhz low-pass on the output of the LF_RX differential 
amplifiers, but I'm not sure that the out-of-band attenuation is all 
that terribly deep.

If there's a possibility of strong signals above 30Mhz, I'd put in an 
external filter.

The main "draw" of the LF_RX is that it goes down to DC on the input, 
whereas on the BASIC_RX, its response starts to roll off quite a bit 
below about
   1Mhz.




-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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