Page 24 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

both cases, the difference between the oscillator frequency and the tuned RF

frequency is 455 kHz.
Although the superheterodyne circuit is probably the most complicated system
compared with other radios, it is the standard bearer of radios. Every television
tuner, stereo receiver, or cell phone uses some kind of superheterodyne radio
system, that is, a system that at least contains a local oscillator, a mixer, and an IF
filter/amplifier.

Block Diagram of a Superheterodyne Radio
One of the main characteristics of a superheterodyne radio is that it has a local

oscillator that tracks the tuning for the incoming RF signal (Figure 1-4).

RF Ant. Filter Converter/Osc Two-Stag.e IF Amplifier and Filter tc----- Det

I

455KHz 455KHZ
1 Cl
-3v 22ut

R1
2700
22k

R9
2700
~I · LPrimary vc RF .01 ut Re
240uh
R6
365pf
10k
R4 47
R2 .01 ut 2700 C10
ITI • LSecondary 1200
11 2.2uh C9 ·°'1
22uf T
Note: 2N1224 is a Germanium Transistor
Output
FIGURE 1-4 Block diagram and schematic of a superheterodyne radio.
The tunable RF filter is connected to an input of the converter oscillator circuit. The

converter oscillator circuit provides an oscillation frequency that is always 455 kHz
above the tuned RF frequency. Because the converter output has signals that are
the sum and difference frequencies of the oscillator and the incoming tuned RF
signall, it is the difference frequency (e.g., 455 kHz) that is passed through the IF
filter and amplifier stage. So the output of the IF amplifier stage has an AM

waveform whose carrier frequency has been shifted to 455 kHz.
To convert the 455-kHz AM waveform to an audio signal, the output of the IF
am,plifier/filter is connected to a detector such as a diode or transistor for
demodulation.