Radios have increased in complexity and component count since the simple crystal set of the early days. Problem is, every component used, even the humble resistor can add to the noise generated within the receiver. Every stage used in a receiver adds noise to the wanted signal.
With Superhet techniques came the harmonics and image responses. With Frequency Synthesised injection for mixers came phase jitter noise.
The SDR takes away many of these problems. The Aerial goes directly into a low noise Mixer the output of which goes via balanced low noise Audio Amplifiers to a Computer which takes over all the work of selectivity and signal processing.
The Crystal controlled Mixer Oscillator injection is from a divider which provides I and Q signals, In and Quadrature (out) of phase. The resultant audio output, I and Q components are accurately balanced by the computer software; producing a spectrum, as wide as the sample rate of the sound card, either side of the central point, as seen in the first screen shot below.
A small frequency segment at the centre point, 0Hz +/- is unusable for obvious reasons. So if you wish to use that frequency what do you do.
(Note the 'hole at the centre point, poor sound cards and earth loops produce a hump.)
Answer, change the Crystal frequency, there are 2 on board.
Transmit function is the exact opposite process using another identical Mixer chip this time to mix I an Q audio signals from the computer with the Oscillator to produce the RF envelope.
Thus in theory any mode conceived or yet to be thought of can be received and transmitted by the SDR system, gone is the Mode switch on the Hardware.
