Problem With Implementation of costas loop In MATLAB

   I am trying to implement a costas loop in matlab.The code which i
have written is based on blockwise most of it using the available
functions of the matlab...the signal frequency 600bits/sec and the
carrier frequency is 3khz.In trackin the error the the lowpass filter
used is butterworth filter...after removing the carrier frequency
component....the left value should be just a dc componet which should
be a constant value theoretically....but in realtime i has some
settling time which is almost double the symboltime...so tracking the
carrier with this  loop is very tough with what i did....is there any
other alternative way in which you have implemented this system....if
you can post your code it will be very helpful and evena general pll
code will be useful (but the implementation should be block wise rather
than using the loop equations)

As i have mentioned above the carrier frequency is not 3khz.but it is
9khz.can some one please comment on the filter design in the above
system...i have been using butterworth filter of order 8th due to which
ther eis unstability in the dc response...can someone suggest an
alternative filter which might improve the filter stability...

Neophyte wrote:
> As i have mentioned above the carrier frequency is not 3khz.but it is
> 9khz.can some one please comment on the filter design in the above
> system...i have been using butterworth filter of order 8th due to which
> ther eis unstability in the dc response...can someone suggest an
> alternative filter which might improve the filter stability...

I like to use baseband loops with full complex multiplies, so that the
image term mathematically cancels and there are no "arm" filters at
all.

John