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Re: [Discuss-gnuradio] Bypassing CIC and half-band filter on N2x0

From: Juha Vierinen
Subject: Re: [Discuss-gnuradio] Bypassing CIC and half-band filter on N2x0
Date: Wed, 26 Feb 2014 13:01:51 +0000


I did manage to sample two real channels at 25 MHz! I set the NCO to 0 and the mux to "A:AB". I don't know why I didn't think of this trick earlier. 

Now the real part is channel 0 and and imaginary part channel 1. I will next try to do the fpga mod. Can you point me to the correct file in the fpga? I'd like to take a stab at this. 

The reason why I'm doing this, is I'm exploring the possibility of using the N200 as a phase comparator. We are looking for picosecond scale phase variations, and I'm trying to do all the calculations in double precision floating point to avoid phase inaccuracies caused by integer arithmetic.  


On Wed, Feb 26, 2014 at 8:30 AM, Matt Ettus <address@hidden> wrote:

You could only do that by modifying the FPGA.  It would be a very minor mod, though.  Hook up the undecimated ADC values to where the output of the decimators go, and leave everything else intact.  Then just set the decimators as normal and you will get the rate you request.


On Tue, Feb 25, 2014 at 12:20 PM, Juha Vierinen <address@hidden> wrote:
Is there any way to bypass the CIC and the HBF on the USRP N200 to just stream decimated (no integration) real samples off the 100 MHz ADC? I'd like to eg., record every 25th sample arriving on the ADC. I'd like to avoid compiling my own fpga is necessary. 

Is there any configuration of  calc_cic_filter_word  and calc_cordic_word_and_update in the firmware that would magically do this? 


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