Page 26 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

In SDRs, though, rarely is any variable tuned filter used at the front end. Instead, a

wide-band filter is used. To address the image problem, the low-frequency IF signal
is processed in a way to provide two channels of low-frequency IF. The two
channels are 90 degrees out of phase with each other, forming an I channel and a
Q channel. The I channel is defined as the a-degree phase channel, and the Q
channel is defined as the channel that is 90 degrees phase shifted from the I
channel. It should be noted that having the I and Q channels allows for easy
demodulation for AM signals via a Pythagorean process without the use of envelope

detectors.
Block Diagram of a Software-Defined Radio Front End

Figure 1-5 shows a front-end system for a software-defined radio. An antenna is
connected to a fixed (not variable) wide-band RF filter. The output of the
wide-band RF filter then is connected to a two-phase quadrature mixer. This mixer
generates two channels of low-frequency IF signals of 0 and 90 degrees via the
oscillator, which also has 0- and 90-degree phase signals. The two channels
(channel 1 and channel 2) form I and Q channels, which are amplified and sent to

the stereo (audio) inputs to a computer or computing system. The computer then
digitizes the I and Q channels, which contain a \\block" of radio spectrum to be
tuned to. For example, if the sound card in the computer samples at a 96-kHz rate,
a bandwidth of 48 kHz of radio signals can be tuned into via the software-defined
radio program in the computer.