Digital costas loop design

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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