Page 187 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

I !rh Loop Antenna

RF Filter Amp
VC1 RF

T1 U2
Computer

Audio Input
/~ ~1RF \ ISig. ,
270 pI Mixer and I and Q
SDR Program
IF Amplifier Phase Circuit Image Reject
QSig. Tuning
U3 U4 U5,U6,U7
AM/FMlSSB Det.
Audio Output
Oscillator
VC1 osc -

T2 U1

P
/ 2701
VCl Osc

FIGURE 12-7 A front-end block diagram for an SDR that tunes the
standard AM band.
The switch-mode mixer also is connected to a local oscillator tuned via VC1 Osc
that is set to about 15 kHz above the tuned RF signal such that the output of the
mixer circuit provides a low IF of about 15 kHz, The 15-kHz IF signal from the
mixer is connected to a phase-shifting circuit similar to the one in Figure 12-5, And

the I and Q outputs of the phase-shifting circuit then are coupled to the audio
inputs of a sound card in a computer. As stated previously, tuning is accomplished
mostly by the variable capacitor, so the entire 1-MHz range of the broadcast AM
band can be accessed, Thus the 192-kHz tuning range of the computer via Winrad
and the sound card is insufficient for tuning across the entire AM band,

The SDR program Winrad then is used to detect the I and Q signals, which then
provide audio signals to the speakers in the computer,
Parts List

• Cl, C2, C3, C4, C6, CS, C24: 0,01 uF
• CS, Cll, C12, C1S, C22, C2S: 33 ~F, 16 volts

• C7, C9: 0,001 ~F, 5% film
• C10, C23: 0,01 ~F, 5% film
• C13: 5 pF for R22 = 220 k
n

; or 47 pF for R22 = 22 k
n