Page 172 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Figure 12-18 shows a Imore common way of generating I and Q signals at low IFs

for coupling into the computer. The analog phase-shift network is replaced by an I
and Q m,ixer system~, which consists of two mixers. So in Figure 12-1A there is one
mixer and an analog phase-shift network. And in Figure 12-18 there are two mixers
instead to provide the phase-shifting function that replaces an analog
phase-shifting network. A first mlixer down-converts the RF signal to a low IF signal
at reference 0 degree, whereas the second mixer down-converts the same RF
signa:1 to the same low IF but with a phase shift of 90 degrees. The low IF signals

from both mixers are amplified by relatively low-bandwidth operational amplifiers,
and the outputs of the amplifiers then are sent to the sound card input of the
computer for digital signal processing.

o Degree Output ( I Channel)
o Degree
land Q
Filter
Mixer 90 Degrees Output (Q Channel)
90 Degrees L-----------,1I A2

FIGURE 12-18 Generating I and Q signals via a mixing method.
The I and Q mixer shown in Figure 12-1B consists of two mixer circuits. The first
mixer circuit receives a O-degree phase signal from the local oscillator, whereas the
second mixer circuit receives a 90-degree phase signal from the local oscillator. By

multiplying at 0 and 90 degrees, two (Iow-frequency) IF signals are generated such
that one of the IF signals is 90 degrees shifted from the other IF signal.
Figure 12-1C illustrates some of the 5DR functions within a computer that is
running the SDR software program. The computer's sound card channel 1 and
channel 2 inputs digitize the I and Q signals to further filter them. Also, the
software program (e.9., Winrad) allows for fine-tuning of the phases and

amplitudes of the I and Q signals for maximum image rejection through the
image-reject filter in the computer.