Page 190 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

R1i1 R13 R14 RI6 1K 1% RH R19 1K1%
C13
5 pt 1K 11% 1K 1% l K1%
U7A
U5A
R20
R22
100
220K
+9
500 pr C18
R15 33~T
R12 76.8K C29
14K 1% +4.5 C27
1620 C15 11ut
R21 l' 11ur o Degree Out
22K u t C26 C28
C25
11Uf I Signal Out
l1ur
l33ur
R23 R25 R26 R28 R29 R31 1K1%
1K1 % lK1%
R32
7 100
500 pr
500 pt
500 pt cn
Toe7 33utT
of Previous Page R24 R27 R30
+4 .5 +4.5 +4 .5 90 Degrees Out
5900 32.4K 280K
Q Signal Out
FIGURE 12-88 Schematic diagram of an AM radio front-end circuit with
the analog phase-shifting circuit.
Before proceeding with building this circuit, with an inductance meter, adjust the
inductance value of Tl's primary winding to 322 IJH and T2 to 310 IJH. Radio

signals are tuned via VCl RF and Tl to provide the RF signals to amplifier U2,
which has noninverting and inverting analog signal outputs to the commutating
mixer, U3. The mixer U3 is switched via the local oscillator signal from U1B, whose
frequency is between 10 kHz to 20 kHz above the frequency of the tuned RF signal.
The output of mixer U3 then is low-pass-filtered via R9 and C7 to provide a

low-frequency IF signal in the range of 10 kHz to 20 kHz. This low-frequency IF
signal is coupled to an IF amplifier, U4A, to further amplify the IF signal. The
amplified IF signal then drives two banks of phase-shifter circuits to provide I and Q
signal outputs. The I signal via U7 A and the Q signal via U7B then are connected to
the line input of the sound card of a computer with an SDR software program
running. With the SDR program running, select "AM" for the demodulation and a
demodulated bandwidth of 3 kHz to 5 kHz. Then, via the software program, tune

for a radio station between 10 kHz and 20 kHz. To tune throughout the AM band
range of 540 kHz to 1,600 kHz, adjust variable capacitor VCl.
The value of R22 may be selected to adjust the signal amplitude to match the
requirements (e.g., input sensitivity) of individual sound cards. For some sound
cards, R22 set to 220 k

may cause overloading. Thus R22 can be replaced with a 22-k