# Bypassing CIC and half-band filter on N2x0

**URL:** <https://www.ruby-forum.com/t/bypassing-cic-and-half-band-filter-on-n2x0/239427>\
**Category:** GNU Radio\
**Created:** [February 25, 2014, 9:22pm UTC](https://www.ruby-forum.com/t/bypassing-cic-and-half-band-filter-on-n2x0/239427 "2014-02-25T21:22:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\
**Post date:** [February 25, 2014, 9:22pm UTC](https://www.ruby-forum.com/t/bypassing-cic-and-half-band-filter-on-n2x0/239427/1 "2014-02-25T21:22:23Z")

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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?

juha

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**Author:** ![Juha\_V](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4af34b/32.png) [@Juha\_V](https://www.ruby-forum.com/u/Juha_V)\
**Post date:** [February 26, 2014, 9:31am UTC](https://www.ruby-forum.com/t/bypassing-cic-and-half-band-filter-on-n2x0/239427/2 "2014-02-26T09:31:59Z")

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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.

Matt

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<div class="post-metadata">

**Author:** ![Juha\_V](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4af34b/32.png) [@Juha\_V](https://www.ruby-forum.com/u/Juha_V)\
**Post date:** [February 26, 2014, 2:03pm UTC](https://www.ruby-forum.com/t/bypassing-cic-and-half-band-filter-on-n2x0/239427/3 "2014-02-26T14:03:24Z")

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Thanks,

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.

juha