Re: Regarding Digital NCO design

On Oct 10, 5:35 am, rickman <gnu...@xxxxxxxxx> wrote:
I'm not sure what your phase detector is doing, but I assume the two
states indicate if the phase is leading or lagging with no indication
that the phase is actually aligned.  That means you need to integrate
this signal.  You can use it to increment/decrement a counter and use
the counter output as the phase step in your NCO.  This will be slow
to lock in and you will need to analyze your entire loop for
stability.  Adding the counter as an integrator makes the loop a bit
more complex to analyze.  But the integrating counter will allow the
loop to settle in a state where the error output is equally in both
states indicating an average phase error of zero.  It sounds like your
phase detector is analog, no?  If so, be sure to isolate all digital
noise from the analog phase detector or you will have problems with it
settling and stability.

Rick

On Oct 7, 7:51 pm, "nowsheen" <Nusrat.Nowsh...@xxxxxxxxxxxxxxxxxxx>
wrote:

Thanks. The multiplied form is just (I*Q). This is sent to a loop
filter(Low pass filter) to get the phase error? But i cant decide the
design idea of this filter. What is the relation between the loop filter's
output and phase increment value when the frequency is locked? Since the
output of loop filter is binary, does it imply a phase error between the
input received signal and reference signal? I am using an averaging filter
as loop filter but can't decide the phase increment value from the filter's
output.

Nusrat

nowsheen wrote:
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?

By "multiplied form of data coming from I and Q" do you mean
sqrt(I^2+Q^2)? The phase increment is right when the frequency is
locked. Then the phase remains steady. When the phase is locked, the
phase comparator output is zero.

Jerry
--
Engineering is the art of making what you want from things you can get..

I am designing the costas loop with every component as digital. Input
signal is BPSK modulated data which is multiplied by local carrier
used to modulate the signal.
I have multiplied the digital samples of incoming signal and local
carrier to get I data and the digital sample with 90 degree shifted
local carrier to get Q data. now what can be my next step?I have
designed a Low pass filter to discard twice the frequency component
from data. But after low passing both I and Q data what can be my next
step? How can i decide the input of NCO? Can anyone give me an example
or detailed steps to do this? I have found costas loop designs where I
is multiplied with Q. What is the benefit of this multiplication? How
does it generate phase increment value of NCO?

Thanks
Nowsheen
.

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