# A strange question involving costas loop

Hi,
What is expected if I input a costas loop block with a complex sine wave
signal? Will the Costas track the sine and give me a baseband output
containing nothing?

On Thu, Feb 4, 2010 at 09:23, Mir M. Ali [email protected] wrote:

What is expected if I input a costas loop block with a complex sine wave
signal? Will the Costas track the sine and give me a baseband output
containing nothing?

You will get a complex DC value. I will be the magnitude of the input
signal and Q will be 0, plus a small amount of “noise” due to round
off error.

Johnathan

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

On Thu, Feb 4, 2010 at 2:52 PM, Johnathan C. <

On Thu, Feb 4, 2010 at 4:37 PM, Mir M. Ali [email protected] 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.

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

Thank you very much.

The output is shown in the figure.
http://img690.imageshack.us/img690/7879/snapshot1u.jpg

In unrelated help… you can install the icons and menu items with:

sudo grc_setup_freedesktop install

-Josh