Circuit Operation - Elenco Electronics AK-710 Assembly And Instruction Manual

CIRCUIT OPERATION

Figure 5 shows a block diagram of the FM wireless
microphone circuit.
Block 1 acts like a resistor that changes when
exposed to sound waves. The change in resistance
causes current through the microphone element to
change when sound waves apply pressure to its
surface. This action is similar to squeezing a garden
hose and watching the water through it decrease.
When the hose is released, the water through it will
increase. When sound waves hit the microphone
element, the electrical current through the element
will increase and decrease according to the pressure
(loudness) of the sound.
Microphone Audio
Element Amplifier
Q1
Block 1 Block 2
Figure 5
Block 2 is a transistor (Q1) used as an audio
amplifier. The signal from the micro-phone element
is increased in amplitude by a factor of 3.
electronics, this action is described as transistor Q1
having an audio gain of 3.
Block 3 is a transistor (Q2) used as an oscillator. An
oscillator is an electronic circuit similar to the
pendulum in a grandfather clock.
pendulum is started in motion, it will use only a
small amount of energy from the main spring to
keep it swinging at the exact same frequency. It is
this stable frequency rate that sets the time
accurately. If the weight is moved down the stick on
the pendulum, the swing takes longer if the
frequency is lower. If the weight is moved up the
stick, the frequency increases. This is called tuning
the frequency of the pendulum. In electronics, an
oscillator circuit also has tunable elements. The
inductor in a tuned circuit is equivalent to the length
of the pendulum (see Figure 6).
Pendulum
Length
Weight
Figure 6
The microphone element in
Radio Radio Antenna
Frequency Frequency
Oscillator Amplifier
Q2 Q3
Block 3 Block 4 Block 5
Once the
Electronic Tuned
Circuit in Oscillator
C
L
C = Capacitance
L = Inductance
By changing the position of the iron core in the
inductor, the inductance can be changed to tune the
oscillator to a desired radio frequency, just like
changing the weight of the pendulum would change
its frequency. When sound strikes the microphone
element, it is converted to an electrical signal,
amplified and used to change the capacitance
(length of the pendulum) of the electronic oscillator's
tuned circuit. This causes the frequency of the
oscillator to make slight changes at the same rate
as the sound striking the microphone. This effect is
known as frequency modulation.
Block 4 is a transistor used as a radio frequency
amplifier. This block amplifies the modulated signal
from the oscillator and acts as a buffer stage
between the antenna and the oscilator.
antenna were tied directly to the oscillator without
the buffer, any capacitance added to the antenna
(touching it with your finger for example) would
produce a large change in the frequency of
oscillation. The receiver would not be able to follow
this large change in frequency and would lose the
transmission.
In
Block 5 is the antenna. The antenna is also a tuned
element since the length of the antenna determines
how well it will radiate the modulated signal. An
antenna acts much like a piece of string tied to a
wall and stretched tight. If you tap the string, a wave
will travel to the wall and part of the energy will go
into the wall and part will be reflected back (see
Figure 7A). If the length of the string is adjusted to
match the rate of tapping as shown in Figure 7B, the
wall receives all of the energy because it is at a
node or proper multiple of the wavelength.
electronics, the wall is similar to the space around
the antenna. By properly tuning the antenna, all of
the available power in the antenna will be radiated
into the space around the antenna. None will reflect
back. A term used in electronics to describe the
amount of power reflected back as a ratio of the
amount of power radiated is called "The Standing
Wave Ratio".
Original Wave
Vibrating String
-4-
Figure 7A
Reflected Wave
Wall
All energy goes
into the wall,
none is reflected.
Node
Figure 7B
If the
In