Elenco SEE AMFM108CK - Manual (Page 4)

Section 9
-3-
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
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.
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 108 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.