Generalized Receiver Explanation
Generalized Receiver
Human voices are heard near DC frequencies which mean that the input signal from the antenna is a baseband signal. The job of the generalized receiver is to transform the base band signal into a base band signal that has an upper and lower part. The benefit of transforming the baseband signal to a band pass is that it requires only half the power to transmit it as band pass signals rather than a baseband signal.The circuit elements needed to create the receiver are an oscillator, two low-pass filters, and two multipliers.
Theoretically, Electronics class notes demonstrates how to create a band pass signal from a baseband signal. Lets called the baseband signal g(t). First, the baseband signal g(t) is multiplied by a function with both sine and cosine with frequency fc, . The results is the signal in the frequency domain shifted to the location of fc which now looks like G(f-fc). Next we take the real part of this signal, half the conjugate and the normal, is what we now call the band pass signal.
Experimentally, as you will be doing in lab, to create a band pass signal the process is as follows. First using your oscillator, you will need to create an original oscillating signal and one that is delayed by 90 degrees, this will be your sine and cosine like the one in the class notes. Next, you will use your two multipliers to perform the multiplication separately on the baseband signal. The next stage in your receiver is to pass the result of your multiplier into a low pass filter that has bandwidth of frequencies only close to the one of your oscillator to reduce the signal to noise ratio. Each signal from the low pass filter is output x(t) and quadrature y(t). In the soundcard that your receiver will be connected to will sum both signals together and would match the result we found theoretically.