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Re: [Discuss-gnuradio] USRP2 Filtering/Decimation Question

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP2 Filtering/Decimation Question
Date: Fri, 4 Sep 2009 14:56:59 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Fri, Sep 04, 2009 at 05:01:59PM -0400, Bruce McGuffin wrote:
> Matt Ettus wrote:
>> Bruce McGuffin wrote:
>>> Hi
>>> When I specify a particular receiver decimation rate
>>> for the USRP2, the task of decimating is presumably
>>> split up between the CIC and two half-band filters.
>>> My question is how is the task split up,
>>> and where in the code does this happen?
>> If the requested decimation rate is a multiple of four, we use the two  
>> halfband filters, and the rest is CIC.  If it is only a multiple of 
>> two, we use one halfband filter and the rest in CIC.  If it is an odd 
>> number, we use only CIC.
>> Matt
> When the decimation rate is divisible by 2 but not 4, which half-band  
> filter do you use?
> And where in the code is this decision made?
> Bruce

Hi Bruce!

The decision is made in the firmware.  See
usrp2/firmware/apps/app_common_v2.c line 247, in the function

The actual control register (dsp_rx_regs->decim_rate) is written on line 265.


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