[USRP-users] Purpose of 24-bit CORDIC in DDC chain?

Ian Buckley ianb at ionconcepts.com
Tue Apr 2 12:39:05 EDT 2013


Florian,
Think of the CORDIC as a DCO and Quadrature Mixer combined.
So to expand on Marcus's answer further: There's typically 40MHz of useful bandwidth coming from the more popular Ettus radio's such as WBX. The two half band FIR filters plus the CIC filter create a low pass filter with programable decimation. (Centered on DC obviously). The CORDIC allows that 40MHz of bandwidth to be arbitrarily repositioned w.r.t the DC of the downstream lowpass filter, thus you can in combination with the programmed decimation select a much narrower channel from anywhere within that 40MHz.
Some example reasons why you might want to do this: 
* Offset the LO of the analog mixer in the upstream analog radio from DC so that it falls in the stop land of the low pass filter.
* Instantaneous frequency hopping
* Multi-channel receiver from a single RF analog front end.
* Extremely high precision tuning.
etc etc
-Ian

On Apr 2, 2013, at 6:57 AM, mleech at ripnet.com wrote:

> On 02 Apr 2013 08:59, Florian Schlembach wrote:
> 
>> I am just analysing the FPGA code of the DDC chain. I just got stuck 
>> when trying to find out the purpose of the CORDIC that comes right after 
>> the ADCs.
>> According to my understanding, the only purpose of the CORDIC is to add 
>> a certain phase increment using an efficient multiplier-free 
>> implementation. The subsequent decimation of incoming, 100 MHz sampled 
>> data (dealing with N210) is then performed via the CIC and Halfband filters.
>> 
>> Is that correct?
>> 
>> 
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>> 
> It's used to do arbitrary up/downconversion, with high precision, of the sample stream coming off the ADC.
> 
> Many daughtercards have fairly coarse tuning step size, so the CORDIC in the FPGA is used in those cases to tune to a precise frequency, as requested by the user.
> 
>  
>  
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