Re: [Discuss-gnuradio] A strange question involving costas loop

[Jason Uher]

On Thu, Feb 4, 2010 at 4:37 PM, Mir M. Ali <address@hidden> wrote:
> In GRC I have a complex sine source with amplitude 10 and frequency 2000Hz
> sampled at 4000Hz as a input to the costas loop through a throttle block.
> The max and min frequency used is +/- 0.157rad/sample which is 100Hz
> /sample. When I see the output on scope and FFT scope i see peak at 0Hz in
> the FFt scope but on the oscilloscope I see a sine wave with amplitude 10 on
> the Inphase component and 0 in the Q component. Why?
> The output is shown in the figure.
> http://img690.imageshack.us/img690/7879/snapshot1u.jpg
> Thanks

I assume you are using a bpsk costas loop (m=2).  In bpsk all of the
data is in the real portion of the signal, therefore any amplitude in
the Q is considered error.  In order to recover the data, the loop
rotates the data so the Q component to 0, and the I component is
presumably +/-1.

If you were to do the same with a qpsk loop (m=4), you should see both
components looking like a saw wave.

Additionally, a standard costas loop assumes that the data has already
been sampled.  Your samples per 'symbol' is higher than 1 in this
case, and the loop output would not be predicable if you were sending
data.

Jason