Regarding Digital NCO design
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From: nowsheen on 23 Sep 2009 07:22 Hi, everyone. I need to design a digital NCO for implementing costas loop for carrier recovery. What I know is that NCO is driven by the loop filter output, but I don't know how? I have designed a NCO with a counter and a sin cos lookup table that generates digital sin and cos wave depending on counter value. But how can i adjust counter value according to the loop filter output? Loop filter output is a digital value and how can i use this value to adjust NCO counter. I mean what is the adjustment step? How frequently it is adjusted? So on... Can anyone here give me some ideas? Thanks in advance. Thanks Nusrat
From: Vladimir Vassilevsky on 23 Sep 2009 10:35 nowsheen wrote: > Hi, everyone. > > I need to design a digital NCO for implementing costas loop for carrier > recovery. What I know is that NCO is driven by the loop filter output, but > I don't know how? Absolutely idiotic question. It is all in your textbook. VLV
From: Tim Wescott on 23 Sep 2009 21:21
On Wed, 23 Sep 2009 06:22:47 -0500, nowsheen wrote: > Hi, everyone. > > I need to design a digital NCO for implementing costas loop for carrier > recovery. What I know is that NCO is driven by the loop filter output, > but I don't know how? I have designed a NCO with a counter and a sin cos > lookup table that generates digital sin and cos wave depending on > counter value. But how can i adjust counter value according to the loop > filter output? Loop filter output is a digital value and how can i use > this value to adjust NCO counter. I mean what is the adjustment step? > How frequently it is adjusted? So on... > > Can anyone here give me some ideas? Thanks in advance. Without knowing what you've done it's hard to tell you what to do. Most NCO's with sine outputs use a phase accumulator approach, where the phase increment (per-sample frequency) is added to the accumulator at each sampling interval. To change the frequency you just change the value in the phase increment register. If all you need is a square wave or a pulse you can use a counter that counts from zero up to a limit, or down from a pre-loaded value to zero; with one of those you change the limit/pre-load to change the period -- but it would be exceedingly difficult to get a sine wave out of such a counter arrangement. Is it starting to make sense? -- www.wescottdesign.com |