# Possible to change RFX400 LO tuning step size?

**URL:** <https://www.ruby-forum.com/t/possible-to-change-rfx400-lo-tuning-step-size/177694>\
**Category:** GNU Radio\
**Created:** [October 22, 2009, 3:34am UTC](https://www.ruby-forum.com/t/possible-to-change-rfx400-lo-tuning-step-size/177694 "2009-10-22T03:34:14Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![NewmanS\_Timothy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/96bed5/32.png) [@NewmanS\_Timothy](https://www.ruby-forum.com/u/NewmanS_Timothy)\
**Post date:** [October 22, 2009, 3:34am UTC](https://www.ruby-forum.com/t/possible-to-change-rfx400-lo-tuning-step-size/177694/1 "2009-10-22T03:34:14Z")

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Is it possible to change the tuning step size on the RFX400 to something  
different than the 2 MHz step size it’s on now? I’ve been looking  
through db\_flexrf.cc and have had no luck.

Tim

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**Author:** ![NewmanS\_Timothy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/96bed5/32.png) [@NewmanS\_Timothy](https://www.ruby-forum.com/u/NewmanS_Timothy)\
**Post date:** [October 23, 2009, 2:10am UTC](https://www.ruby-forum.com/t/possible-to-change-rfx400-lo-tuning-step-size/177694/2 "2009-10-23T02:10:07Z")

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Newman, Timothy wrote:

> Is it possible to change the tuning step size on the RFX400 to  
> something different than the 2 MHz step size it’s on now? I’ve been  
> looking through db\_flexrf.cc and have had no luck.

I assume you mean on the USRP1. Change the R divider in constructor of  
the db\_flexrx\_400\_{tx,rx}\_mimo\_b in db\_flexrf\_mimo.cc

An R divider of 16 (the current setting) make the LO step size 64 MHz/16  
= 4 MHz, but the following stage divides it by 2 again for a step size  
of 2 MHz. If you want 1 MHz, use 32, if you want 500 kHz use 64. You  
can also use numbers which aren’t a power of 2. In-band phase noise is  
better for bigger step sizes.

Matt

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<div class="post-metadata">

**Author:** ![NewmanS\_Timothy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/96bed5/32.png) [@NewmanS\_Timothy](https://www.ruby-forum.com/u/NewmanS_Timothy)\
**Post date:** [October 23, 2009, 6:17am UTC](https://www.ruby-forum.com/t/possible-to-change-rfx400-lo-tuning-step-size/177694/3 "2009-10-23T06:17:43Z")

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Matt E. wrote:

> I assume you mean on the USRP1. Change the R divider in constructor  
> of the db\_flexrx\_400\_{tx,rx}\_mimo\_b in db\_flexrf\_mimo.cc
> 
> An R divider of 16 (the current setting) make the LO step size 64  
> MHz/16 = 4 MHz, but the following stage divides it by 2 again for a  
> step size of 2 MHz. If you want 1 MHz, use 32, if you want 500 kHz  
> use 64. You can also use numbers which aren’t a power of 2. In-band  
> phase noise is better for bigger step sizes.  
> This is slightly different, but I recently brought out the control to  
> change the LO offset via GRC, so that in my testing I could change  
> between say, 4 and 8 MHz. (It actually didn’t dawn on me to change the  
> step size too.) The trouble I have is with amplifying this signal. I  
> have need of a 4 watt or more output, and so will run the ~100mW signal  
> through an external amplifier. However, I don’t want to go splattering  
> the LO all over, since it is within the bandpass the bandpass of the  
> amplifier (hence the need to move the LO away)… An external bandpass  
> filter is an obvious step, but then ties me directly to a specific band  
> that is smaller than the operating frequency of the rfx400. Is there  
> anything else we can do to reduce the LO? I currently get about -30db  
> of attenuation.

Thanks for your input!  
Glenn