Page 165 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

• U3: 74AHC14 or 74HC14

• U4: 74HC74 or 74HCT74
• US: 74HC4053
• U6, U8: NESS32 or LM833
• U9: LM78LOS or LM780S

• VR1: l-k

pot
• VR2: 10-k

pot

• D1: lN270 or lN34 or lN914
Figures 11-6A and 11-6B provide schematic diagrams for the inductor-Iess radio. A
short wire or telescopic antenna is connected to a high-pass filter circuit consisting
of Rl, R2, Cl, and C2, which has a cutoff frequency at about 280 kHz. This
high-pass filter removes or attenuates low-frequency signals that would be
demodulated as hum or extraneous noise. The output of the high-pass filter at C2

then is coupled to a five-pole l,600-kHz active low-pass filter consisting of
capacitors from CS to Cl0, U1B, and U2A. The output of the low-pass filter is
inverted in phase via U2B so that the inputs of the mixer USA have a balianced or
push-pull signal. Thus the inputs of the balanced mixer USA are connected to a
noninverting output of the low-pass filter and an inverting output of the low-pass

filter. By commutating between the noninverting and inverting outputs of the
low-pass filter, multiplication occurs with the local oscillator signal from the flip-flop
frequency divider circuit U4A.