Elenco AM/FM Radio Kit - Manual (Page 5)

Section 9
-4-
FM RF
AMPLIFIER
FM
OSCILLATOR
1ST FM IF
AMPLIFIER
AFC
Figure 1
Section 8 Section 7 Section 6
Section 1 Section 5 Section 4 Section 3 Section 2
FM MIXER
2ND FM IF
AMPLIFIER
FM
DETECTOR
AM MIXER
AM
OSCILLATOR
1ST AM IF
AMPLIFIER
2ND AM IF
AMPLIFIER
AM
DETECTOR
AGC
IC or
TRANSISTOR
AUDIO
AMPLIFIER
Speaker
FM RADIO
AM RADIO
The purpose of Section 1, the Audio Amplifier Stage, is to
increase the power of the audio signal received from either
detector to a power level capable of driving the speaker. The
audio amplifier is IC or transistor version. Section 2 includes
the AM detector circuit and the AGC (automatic gain control)
stage. The AM detector converts the amplitude modulated IF
(intermediate frequency) signal to a low level audio signal.
The AGC stage feeds back a DC voltage to the first AM IF
amplifier in order to maintain a near constant level of audio at
the detector. Section 3 is the second AM IF amplifier. The
second AM IF amplifier is tuned to 455kHz (Kilohertz) and
has a fixed gain at this frequency of 50. Section 4 is the first
AM IF 2 amplifier which has a variable gain that depends on
the AGC voltage received from the AGC stage. The first AM
IF amplifier is also tuned to 455kHz. Section 5 includes the
AM mixer, AM oscillator and AM antenna stages. When the
radio wave passes through the antenna, it induces a small
voltage across the antenna coil. This voltage is coupled to the
mixer, or converter, stage to be changed to a frequency of
455kHz. This change is accomplished by mixing
(heterodyning) the radio frequency signal with the oscillator signal. Section 6 is the FM ratio detector circuit. The FM ratio
detector has a fixed gain of about 20. Section 7 is the second
FM IF amplifier. The second FM IF amplifier is tuned to
10.7MHz (Megahertz) and has a set gain of approximately
20. The 3dB bandwidth of this stage should be approximately
350kHz. Section 8 is the first FM IF amplifier. The first FM IF
amplifier is also tuned to 10.7MHz and has a set gain of
approximately 10. It also has a 3dB bandwidth of 350kHz.
Section 9 includes the FM mixer, FM oscillator, FM RF (Radio
Frequency) amplifier, AFC (Automatic Frequency Control)
stage, and the FM antenna. The incoming radio waves are
amplified by the FM RF amplifier, which is tuned to a desired
radio station in the FM frequency bandwidth of 88MHz to
108MHz. These amplified signals are then coupled to the FM
mixer stage to be changed to a frequency of 10.7MHz. This
change, as in AM, is accomplished by heterodyning the radio
frequency signal with the oscillator signal. The AFC stage
feeds back a DC voltage to the FM oscillator to prevent the
oscillator from drifting. Each of these blocks will be explained
in detail in the Theory of Operation given before the assembly
instructions for that stage.
GENERAL DISCUSSION
INTRODUCTION
The Elenco ® Superhet 108C AM/FM Radio Kit is a
“superheterodyne” receiver of the standard AM (amplitude
modulation) and FM (frequency modulation) broadcast
frequencies. The unique design of the Superhet 108 allows
you to place the parts over their corresponding symbol in the
schematic drawing on the surface of the printed circuit board
during assembly. This technique maximizes the learning
process while keeping the chances of an assembly error at a
minimum. It is very important, however, that good soldering
practices are used to prevent bad connections. The Soldering
Guide should be reviewed before any soldering is attempted.
The actual assembly is broken down into 9 sections. The
theory of operation for each section, or stage, should be read
before the assembly is started. This will provide the student
with an understanding of what that stage has been designed
to accomplish, and how it actually works. After each
assembly, you will be instructed to make certain tests and
measurements to prove that each section is functioning
properly. If a test fails to produce the proper results, a
troubleshooting guide is provided to help you correct the
problem. If test equipment is available, further measurements
and calculations are demonstrated to allow each student to
verify that each stage meets the engineering specifications.
After all of the stages have been built and tested, a final
alignment procedure is provided to peak the performance of
the receiver and maximize the Superhet 108’s reception
capabilities.