--- In [email protected], "Daniel Jackson" <[EMAIL PROTECTED]> wrote:
> The Ways: > > There are two ways we use in radio to take the baseband audio off of > a signal carrier. > > 1. The beat frequency mixer method such as is used with Sideband > and in the Quadrature demodulator ideas. > > 2. The audio is demodulated off of the RF signal carrier from the > i.f. by means of a diode. Where the diode is not being used as a > mixer but rectifier. Keep this one in mind now..... In method 1, if we know the local oscillator frequency, the beat frequency tells us how far a signal is above or below the local oscillator. With the help of the I and Q signals, the SDR software can determine whether the signal was above or below the LO. So if there are multiple frequencies present, the software can sort them all out. To see what happens with method 2, take the simplest case of a constant carrier signal. The output of each diode detector (I and Q)is a DC voltage. There is no way to determine the frequency or phase of the signal. If multiple frequencies are present, the detector outputs will be a DC component plus a mishmash of all the mixing products between the various frequency components. Simply put, method 2 won't work... Lyle, K0LR
