Hello everyone, let me explain my question:

Let the input signal,Fc=cos(2*pi*fc*t+a(t)),fc is the carrier’s
frequency,and a(t) is the information which modulated into the carrier.
But the SDR(software define radio)doesn’t processes the input signal Fc
directly, it just deal with the I/Q signal.
Multiplying the input signal Fc by cosine and sine wave, and putting
result
into low pass filter, the output of this low pass filter are I/Q
signals.
The frequency of the cosine and sine wave is fs. Usually,fs should equal
to
carrier’s frequency fc.
So the I/Q signals can be expressed as follows:
I signal SI=1/2*cos(2

*pi*(fc-fs)

*t+a(t));*

Q signal SQ=-1/2sin(2

Q signal SQ=-1/2

*pi*(fc-fs)

*t+a(t));*

If the fs is equal to fc, then fc-fs is equal to 0, and SI and SQ just

contain the modulated information a(t).

But in the real time project, fs isn’t equal to fc, there may be little

difference.

Older oscillator and doppler effective will cause the fs having little

difference with fc.

so my question is that, how to remove the errors in SI and SQ which

called

by fs-fc not equal to zeros, and get SI and SQ as follow:

I signal SI=1/2cos(a(t));

If the fs is equal to fc, then fc-fs is equal to 0, and SI and SQ just

contain the modulated information a(t).

But in the real time project, fs isn’t equal to fc, there may be little

difference.

Older oscillator and doppler effective will cause the fs having little

difference with fc.

so my question is that, how to remove the errors in SI and SQ which

called

by fs-fc not equal to zeros, and get SI and SQ as follow:

I signal SI=1/2

Q signal SQ=-1/2*sin(a(t));

thanks.