costal loop question

[viswanath]

Hi,
I have come across a lot of literature saying that costas loop is used
for phase error correction. I had few questions regarding that. After
multiplying and generating a difference signal either cos(phi) or
sin(phi) where phi is the phase difference between the incoming and
the locally generated signals, how is this translated to an angle
which can be given as an input to the VCO??
The output of the carrier discriminator which can be a multiplier is
sin(2*phi). I wanted to know what the loop filter does that converts
this value to an appropriate value as an input to the VCO.
If we consider another discriminator such as arctan then we directly
can get a phase difference angle as the input to the VCO. However what
is the use of a loop filter in this case??
Are there any higher frequency terms that need to be filtered out?
Could you please reply to my questions?
I would greatly appreciate your response.
Thanking You,
Viswanath

 

[Rajeev]

daita@eng.usf.edu (viswanath) wrote in message news:<791e9679.0405051135.17e8df30@posting.google.com>...

Hi,
I have come across a lot of literature saying that costas loop is used
for phase error correction. I had few questions regarding that. After

The beauty of the Costas loop is that it is a relatively simple circuit
that allows you to extract both the clock and data from a modulated
data stream eg BPSK.

multiplying and generating a difference signal either cos(phi) or
sin(phi) where phi is the phase difference between the incoming and
the locally generated signals, how is this translated to an angle
which can be given as an input to the VCO??

That's the beauty of feedback. It is not necessary to calculate an
angle, just to nudge the VCO in the 'right' direction. You can use
+/- cos/sin of the phase difference, the system will settle at a
'stable equilibrium' phase, with the VCO 0/90/180/270 degrees from
the incoming data.

The output of the carrier discriminator which can be a multiplier is
sin(2*phi). I wanted to know what the loop filter does that converts
this value to an appropriate value as an input to the VCO.
If we consider another discriminator such as arctan then we directly
can get a phase difference angle as the input to the VCO. However what
is the use of a loop filter in this case??
Are there any higher frequency terms that need to be filtered out?

The multiplication you've mentioned above gives will give you terms
at twice the clock frequency, in addition to the low frequency term
you want. On paper, the 2F terms are disposed of by averaging over
a clock period, but in hardware they are eliminated by the loop filter.

Could you please reply to my questions?
I would greatly appreciate your response.
Thanking You,
Viswanath

What kind of Costas loop do you wish to implement ? In what kind of
hardware ?

Good luck,
-rajeev-