Page 157 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Figure 11-2 is a block diagram of a radio with 455-kHz IF ceramic filters. Antenna

coil Ll and variable capacitor VCl provide an RF signal to the converter circuit Q1.
Oscillator coil Tl provides a local oscillator signal for converting the incoming RF
signal to an IF signal. Ceramic filters extract the IF signal from the output of the
converter circuit. The IF signal is filtered and amplified by an IF amplifier circuit.
The output of the last IF amplifying transistor Q3 is fed to a diode detector D3 for
demodulation ..

Converter IF Filter
RF Ant1enna Filter Audio Signal \.
~ VC Dsc -j CF1 ,CF2,CF3 -j: Det 03 /
VC RF/L 1
T1 Dsc/Q1 Q2,Q3 I
I

VC10SC
+ +
VC1 RF
/ 140pf / 60pf

FIGURE 11-2 Block diagram of a radio using ceramic filters.

Parts list
• Cl, C3, C7: 0.01 lJF

• C2, C4, CS: 1 lJF, 35 volts
• CS, C6: 0.15 lJF
• Rl: 2,200

• R2, RS: 3,000

• R3, R6, RlO: 22 k

• R4, R7, RS: 2,200

• R9: 47 k

• L1 antenna coil: 600 I-IH to 680 I-IH, 10 to 20 turns for secondary winding
• VCl: two-gang variable capacitor 140 pF and 60 pF

• Dl, D2: IN914
• D3: IN34 or IN270
• Tl oscillator coil: 42IF100

• CFl, CF2, CF3: Ceramic filter SFU455A, Murata or equivalent
• Ql, Q2, Q3: 2N3904
Figure 11-3 shows a schematic for an AM radio that uses ceramic filters to replace
IF transformers. Received radio station signals are tuned by Ll and VCl RF to
provide RF signals to the base of converter oscillator transistor Ql. Oscillator coil Tt