Page 197 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

+9 +5
C24
C2 lo01 ut
rT 47 pr R7 Re
33 U~ R3 3,3K J 0,0047uf lK R9 100
Long Wire Antenna
I 47 U1 B 74HC4066 U2B C9 100K C1l

T1
C15 21F129 LowSide Tap R16 2N3906 1 ut 33~T
.01 uf t 2 C5
~: R6 0,0047 u~ I Output
100
100
U'l C 74HC4066
R5
0.0047 u;f; +9
R4 22 C6
470 Cl0 Rl0
~Uf lK
U1D
I Rl1
74HC4066
CB
1 100K
I /0 11
o Degree Phase 12 1 ut
C13 - --
33~T
180 Degrees Phase
90 Degrees Phase
Q Output
270 Degrees Phase
Note: U2 and U3 are NE5532 or LM833
To Ground D3 ToC3
2 To Base of 01
ToC15 4 1 ToClIRl
T1 Bottom Side
FIGURE 12-118 Schematic diagram of the front-end circuit including

front-end input filter and mixing circuit.
An antenna is coupled to a low-impedance winding of an RF transformer Tl, which
is really a lO.7-MHz I F transformer with an added capacitor C3 to lower the
resonating frequency to 7.22 MHz. Tl as configured in this circuit has an input

impedance of about 50

for matching to an antenna. The other side of T1 at the low-side tap has about a
l: 10 turns ratio, which then provides about a lD-fold increase in RF signal when
compared with the signal at the antenna input terminal. To drive the analog switch
mixer, the signa'l from the low-side tap of Tl is buffered by a compound
comp lementary transistor amplifier Ql and Q2. This amplifier provides a
high-impedance input resistance so that the Q of Tl is not affected and so that the

amplifier also provides a low output resistance capable of driving the mixing circuit
consisting of four sample-and-hold switches in Ul.

To adjust Tl, connect a 7.22-MHz signal to C1S, and measure with an oscilloscope
the signal output at the emitter of Ql [MPSHlD, and note the pin-out for this