Page 27 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

To Olan 1 Sound Card
1------------1 I Amplifier
Quadrature
Wide-Band RF Filter
t-------I
Mixer To Chan 2 Sound Card
Q 1------------1 Q Amplifier

Two-Phase
Oscillator
+Y

1 To Chan 1 Sound Card

C-,n Phase J R4A R5A

L1
U28 100 10 K
74HC4066
Y1 XTAL T C3A
~---~D~----~ rh' uf
+v 7 To Chan 2 Sound Card

U3B
R2 Flip Flop R4B R58
Oock In 100 10 K
C2 1K T C3
T 22Pt C3B
rl-/2 pt rh T 11Uf
1Meg m
FIGURE 1-5 Block diagram and schematic for a front end of an SDR.
A practical example would be listening into the SO-meter amateur radio band for a

continuous-wave (CW) signal (Morse code), which spans from 3,675 kHz to 3,725
kHz (50 kHz of bandwidth). Most of this 50-kHz block of radio spectrum can be
mixed down to about 100 Hz to 48 kHz. And the computer's software-defined radio
program then can tune into each of the CW or Morse code carrier signals and
demodulate them for the listener.

Description of Front-End Circuits for a Software-Defined
Radio System

Figure 1-5 also shows an antenna (e.g., a long wire or whip antenna) connected to
a fixed, nonvariable wide-band RF filter consisting of capacitor Cl and inductor Lt.
The output of this wide-band filter is connected to two analog switches that form a
two-phase mixer via U2A and U2B. One switch, U2A, is toggled by a O-degree

phase signal from a flip-flop circuit, whereas the other switch, U2B, is toggled by a
gO-degrees phase signal from another output terminal of the flip-flop circuit. The
O-degree switch U2A samples the RF signal from the RF filter and produces a low IF
frequency. The sampling capaCitor C_InPhase forms a low-pass filtering effect and
thus provides a low-frequency IF signal to the I amplifier U3A. Similarly for U2B,
the sampling capacitor C_Quadrature forms a low-pass filtering effect and also