TKS Leon and Alberto for the help. So the working must be something close to: * analytic mixing of I (with cos(wt) and sin(wt)) * analytic mixing of Q (with cos(wt) and sin(wt)), At this level there are 4 components. * sum et difference of components (I don't go into details) will give two new components in quadrature I' and Q', with two major frequencies (the base band (around w=0) and the image band (around -2w).
At this level, either you do a FFT or you do a decimation. In the second case, you must apply two low pass filters (one for I' and for Q') before the decimation and the FFT. 73 Patrick ----- Original Message ----- From: i2phd To: [email protected] Sent: Sunday, August 26, 2007 1:08 AM Subject: [soft_radio] Re: Mix the time domain Q & I signal with a NCO to make a near to zero signal --- In [email protected], "Leon" <[EMAIL PROTECTED]> wrote: > >> Hello Fred, Alberto and all, >> > > I'm investing an interesting time in the SdR theory. One particular message of this group makes me problem. >> >> The problematic sentence is the following: >> "1° Mix the time domain Q & I signal with a NCO to make a near to zero signal"(this before the FFT) >> It seems that the "NCO" must mean something as "numerical oscillator". I don't suppose that it is an analytic oscillator >> (cos wot / sin wot) but a simple oscillator (cos wot). This simple oscillator applies to I and Q. > > An NCO (numerically-controlled oscillator) is the same as a DDS. > > 'I' means 'in-phase' and 'Q' means 'quadrature'. > > Leon > Yes, correct, the NCO in Winrad is a numerically controlled oscillator that mimics the working of a DDS, and that generates two components, sin(wt) and cos(wt), which are then used for the full complex mixer that brings to zero IF the signal. 73 Alberto I2PHD
