How to correct the phase offset at the RX part of one USRP 2930 ?

[Danan]

Hi, everyone.

I want to use one USRP 2930 to measure an AM signal transmiited by other devices, I have known that the AM signal is  generated by two different frequency ( 70Hz and 190Hz) but the same phase ( 0°), the carrier frequency is 119.1MHz.

After received and AM demodulated the AM signal, I find that the phase difference between the 70Hz and 190Hz signal is random but not the ideal value 0.

The phase offet knowledges in this community are mostly about multi-receivers,  it can't resolve my quenstion.

 ANY HELP WOULD BE REALLY APPRECIATED !

My receive vi is as follows:

[muffin.vi]

Danan,

I'm a little confused--are the 70 Hz and 190 Hz signals the modulated signals on the 119.1 MHz carrier? And when you demodulate and look at the phase difference, is it constant, or is it changing?

[Danan]

The phase difference is changing.

[Danan]

The 70 Hz and 190 Hz signals the modulated signals on the 119.1 MHz carrie, there relationship is :

S=A*[1+a1*sin(2pi*70*t+b1)+a2*sin(2pi*190*t+b2)]*cos(2pi*fc*t)