Digital costas loop design
in [DSP]
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From: nowsheen on 7 Oct 2009 06:45 Thank You very much for your suggestion. But i am working with a digital costas loop design.So,can you help me to get idea about the cut of frequencies and order of the low pass filters used in I and Q phase of costas loop? The output of these filters are multiplied together and pass through a loop filter to strobe the NCO. Since all the values are digital; i cant decide how to send the loop filter's output to the input of NCO. How does this value decide the Phase increment of NCO to lock the phase and frequency? According to your response i do understand that we should check whether the loop filter's output is non zero or not? but if it is nonzero then how can i use this value to adjust NCO?I am also worried about the design of loop filter though i am using an averaging filter as a loop filter? Thanks Nusrat >On Tue, 06 Oct 2009 23:50:02 -0500, nowsheen wrote: >(top posting fixed) >> >> >> >>>nowsheen wrote: >>>> I am actually designing a digital costas loop and now at the stage of >>>> feeding loop filter's output to phase accumulator of NCO. The loop >> filter >>>> produces a 32 bit digital data. The loop filter output is fed back to >> NCO. >>>> But how can i decide the phase increment value from the loop filter's >>>> output? I have designed my phase accumulator and Sinlookup table. But >>>> unable to feed the loop filter's output to NCO. Because i can't derive >> the >>>> phase increment value needed to accumulate the phase counter? Can you >>>> suggest me an approach to adjust phase increment value depending on >> loop >>>> filter's output? >>> >>>What is the lhe loop filter's outout when the phase is dead on? That is >>>the offest. Subtract that and the signed result is an indication of >>>which way the phase increment needs to change, and how much. Ssuitably >>>scale that result and add it to the existing phase increment to make a >>>new one. Don't let saturation screw up the result. >>> >>>Jerry >> >> The loop filter output is the multiplied form of data coming from I and >> Q arm of Costas loop. So, after multiplying these two data i am getting >> the input of loop filter. How can i know when the phase is dead on? >> Which value is this subtracted from? How does the loop filter's output >> decide phase increment value?When is the phase locked? > >Have you studied phase locked loops much at all? That doesn't sound like >the loop filters that I know and love. > >Any PLL should give you an estimate of phase error, often with some >mangled form of the signal mixed in. You can lock phase because the >mangled signal that is mixed in has an average value of zero, so any non- >zero average value that's left must be phase error. > >_Normally_ in a real PLL the loop filter has at least one integrator in >it to insure that the average phase error is zero, and will often have a >low-pass filter in it to cut down (sometimes dramatically) on the amount >of signal that gets applied to the variable oscillator and thus cut down >on the phase jitter of the oscillator. > >-- >www.wescottdesign.com >
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