[Discuss-gnuradio] Lissajous figure of two cosines. Transmitted (basic tx); Received (basic rx)

[Jorge Miguel]

Dear all,

I am trying to understand why my Lissajous plot between two cosine signals is not stable.

I transmit a cosine signal with the basic tx. I receive the signal with the basic rx. Interpolation and decimation rates are 500 in both cases.

What I get is an ellipse (that means that frequency signal a is the same that frequency signal b, great!), but it is not stable! Sometimes is a circle and sometimes becomes a line. It is like an ellipse rotating around the center point with its axis changing. That means that the phase difference between the two signals is not constant! How is this possible if the received signal should be a delayed version of the transmitted signal?

My command line looks like this:
usrp2::tx_raw: FIXME: short packet: 4 items (44 bytes)
Susrp2::tx_raw: FIXME: short packet: 7 items (56 bytes)
usrp2::tx_raw: FIXME: short packet: 7 items (56 bytes)
usrp2::tx_raw: FIXME: short packet: 7 items (56 bytes)
usrp2::tx_raw: FIXME: short packet: 7 items (56 bytes)

Am I wrong if I say that I should get a fix ellipse as Lissajous figure since the two cosines are different amplitude and a fix different phase?
Furthermore when I see both channels in the oscilloscope both signals are not fix. It only remains fix the signal I trigger with.

Any ideas?

Many thanks,
Jorge