On 3/15/12 7:49 AM, J. Forster wrote:
Suppose the modulation is not present. The output of the phase detector
that steers the local standard ot indicator works correctly.
Now reverse the 60 kHz carrier. The phase detector works exactly thye
opposite way... wrong.
Now alternate between 0 and 190 degrees.
The loop alternate works between exactly right and exactly wrong... it
dithers around and the output is a measure of the ratio of 1's to 0's and
is utterly useless.
and the cleverness of the Costas loop is that it uses (an estimate of)
the current data bit (the output of the I arm) to flip the sign of the
error signal from the quadrature arm.
There's a lot of scope for modification of the basic linear Costas loop.
Hard/soft limiters in either or both arms, you've got three filters
(the two arm filters and the loop filter) to fool with, plus all sorts
of schemes using "data aiding" where you get feedback from your symbol
slicer to help do a better job on the carrier tracking.
You can also run your loop with hard limited signal input (makes the
"mixers" turn into XOR gates).
If you don't need the bits in real time (i.e. you can tolerate some
latency), then you can also build tracking loops that effectively "look
into the future"; i.e. make decisions on carrier and bit at time t using
future data from t>now, as well as t=[-infinity, now].
Enormous literature out there on this, and it's been grist for many a
Master's or PhD dissertation.
All in a quest to get ever closer to the Shannon limit...
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