Page 173 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

I Channel Input --)~

Corn puter with SDR Program

Ilmage Rejection Mixer

Tuning and IF Filtering

Demodulation ( AM FM SSB CW)
Q Channel Input
/

FIGURE 12-1C SDR functions within a computer or system on a chip.

With the I and Q signals, demodulation of broadcast AM signals is achieved by
multiplying the I signal by itself and multiplying the Q signal by itself, summing the
squares of the I and Q signals to provide a summed signal, and then taking the
square root of that summed signal. Although this process may seem mysterious
when compared with envelope detection, it is not.

But here is a hint on how this demodulation works: Use the trigonometric identity
that [sin(Btjf + [cos(Bt)f always is equal to 1 and that the I signal is [1 + Ili(t)]
cos(Bt) and the Q signal is [1 1 m(t)] sinCBt), where rrt...t) represents the audio
information. Chapter 16 provides a detailed explanation on how AM demodulation

works via I and Q signals.
Other functions included in an SDR are demodulation of single-sideband signals and
frequency-modulation (FM) signals. And depending on the sampling rate of the
sound card, the SDR software program allows for tuning to a bandwidth of half the
sample rate or up to the sampling rate of the sound card. For example, if the sound
card samples at a rate of 192 kHz, an SDR software program allows tuning over a

range of at least 96 kHz or up to 192 kHz. Figure 12-1D shows a screen capture of
the Winrad SDR software program running. As seen in the figure, the local oscillator
is set for 7,150 kHz in this particular example to tune into the amateur-radio
40-meter band. The tuning spans from 7,054 kHz to 7,246 kHz for a range of 192
kHz. Since amateur radio traditionally has single-sideband transmission on the

lower sideband for the 40-meter band, lower sideband (LSB) is chosen for the
demodulation.