The Receiver front end is usually customized as if it were a Direct Conversion Receiver design. Be cautious of Direct Conversion designs as they don't constantly exercise an RF Power Amplifier. The RF Amplifier is essential as it provides the Receivers Selectivity, and Signal to Noise Ratio (SNR) characteristics. Simpler designs will alternate a Band Pass Filter (BPF) and rely on Intermediate Frequency Amplification in an afterward stage to amplify the receiver sensitivity (by using Low noise amplifier)
For separate applications the Processor circuit will typically consist of a Digital Signal Processor (DSP) and two Audio Codec's. An alternative to this advance can be accomplished by a Personal Computer (PC) running a functional program to resolve the transmitted signals audio component. The interface to the SDR utilizes the PC's soundcard.
The Quadrature Sampling Mixer offers two output signals called 'I' and 'Q'. The I signal is the in-phase modulated signal. The Q signal is the Quadrature modulated signal. The DSP samples these signals and creates I and Q data values. These can be thought of as X, Y Cartesian coordinates. The I signal is the Real component (X axis), and the Q signal is the Imaginary component (Y axis).
The Processor has the job of filtering the IQ data (successfully providing the Audio Bandwidth characteristics), and processing the Signal information (demodulation) in order to pick up the functional audio.
To demodulate an AM signal the DSP calculates the Amplitude of the functional signal by captivating the square root of (I squared + Q squared).
On the other hand, to demodulate an FM signal the DSP calculates the Phase of the functional signal by captivating the opposite Tan of (Q / I).