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Sat Dec 8 13:25:20 CET 2007

first packets: pll and reversals

apart from some problems related to gain (probably too much drive
which kicks the PLL out of sync: moving the things apart gives better
results.) it seems to work just fine.

looking at an I,Q plot i suspect the slow rise of the I signal is not
due to filtering, but due to loss of sync: Q gets thrown off, and the
PLL needs to re-sync. maybe it's more important to filter the error
feedback..

aha! it seems as if the PLL switches to the negative frequency
attractor. indeed. with wide spaced reversals it is clear that Q moves
from around -13 to +13

the problem is that by suddenly moving from subtract to add changes
the frequency of the oscillator from bias+corr to bias-corr. how to
solve this? aha.. maybe it's not necesary to flip the sign? since a
phase reversal in the I plane doesn't change the Q component?
actually, it does. switching of the sign compensation resynchronizes
the oscillator on transition to I = +A.

it looks like i need a controller with a zero error, which effectively
means a PI instead of a P controller. note that i already had an I
controller, but that's unstable.

i'm measuring an error of about #x10 / #x2000 = 0.2 % -- the spec
sheet says 0.5 % max. looks normal.

thinking about this PI controller: P + lowpass can't work, because
there is no zero-error. so i need an integrator. the problem is the
time constant / gain factor.

the error (Q) does seem to go to zero now. however, there is stil a
transition at the reversal.

now that i have a zero error, it's maybe best to multiply the I and Q
to obtain the error signal? for after dinner: i'm stuck with yet
another scale problem.. fixed point without a barrel shifter is
madness.. it might have been better to just implement the tools
necessary, even if they are inefficient: it is definitely doable
(which is what i wanted to prove really..) but it's difficult.

NEXT:
    - I * Q
    - AGC

preferrably combined such that I * Q and error feedback become simple.

i just saturated the error output to +-127.. i get nice results for I
amplitude around 100-150. but still: the Q component wiggles when the
phase transforms.

reading the costas-loop paper mentioned above: the 3rd multiplier is
called a phase doubler. it's only point is to make +-180deg both
stable lock points.

so, i'll write up the problem below.




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