AM/FM Receiver Circuit
- recap Output Transducer
- part of the FM receiver that
interprets the signal processed by
the demodulator
IF Section
- part of the receiver that prevents
interfering signals from entering the
radio tuner
Detector
- other name for demodulator
- separates the audio information from
the carrier wave
- a rectifying diode rectifies the
alternating current signal in the AM
signals
- a direct current signal that feeds to
an audio amplifier circuit is what
remains on the alternating current
once passed into a rectifying diode
- the detector circuit is a bit more
complicated for FM signals
Heterodyning
- process that involves mixing of two
different signals to produce new
ones
Capacitors
- component used in making a tuning
condenser
Antenna
- a piece of or length of wire that
captures radio waves.
- when exposed to radio waves, the
wave induces a minimal alternating
current in the antenna
RF amplifier
- a sensitive amplifier that amplifies
the very weak radio frequency (RF)
signal from the antenna to be
processed by the tuner
Tuner
- extracts signals of a particular
frequency from different frequencies.
- the antenna captures radio waves of
all the frequencies and sends them
to the radio frequency (RF) amplifier,
which amplifies them all
Audio amplifier
- the weak signal that comes from the
detector is amplified by this
component using a simple transistor
amplifier circuit
AM Receiver Circuit
➔ Antenna
- intercepts and collects the radio
signals coming from the radio
broadcasting or transmitting radio
station.
- the receiving antenna in transistor
radio receivers is made of a coil of
small wires. These wires wound
around the soft ferrite core. There
are two wire windings around the
ferrite core. One winding is called
the primary winding, while the other
is called the secondary winding.
- induced alternating current is
generated
- charges oscillates and make the EM
signal
➔ Tuning Capacitor (Tuning
Condener)
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select which of the collected radio
signals at the antenna (filter) should
enter the radio tuner.
only one radio signal should enter
the radio tuner, and that radio signal
is the one selected or tuned-in by
the tuning capacitor.
usually composed of metal plates
that are separated by the insulators.
tuning capacitors are also called
variable capacitors where in the
capacitance or capacity of the tuning
capacitor is varied or changed every
time by turning the knob to select
which radio signal from the radio
station should enter the radio tuner;
the plates of the adjustable tuning
capacitor are called the rotor plates.
The plates that are not adjustable
are called the stator plates.
variable capacitor-adjacent are the
rotor plates
➔ Tuner
- usually employs the combination of
an inductor (for example, a coil) and
a capacitor to form a circuit that
resonates at a particular frequency.
This frequency, called the resonant
frequency, is determined by the
values chosen for the coil and the
capacitor.
- this type of circuit tends to block any
AC signals at a frequency above or
below the resonant frequency. You
can adjust the resonant frequency
by varying the amount of inductance
in the coil or the capacitance of the
capacitor. In simple radio receiver
circuits, the tuning is adjusted by
varying the number of turns of wire
in the coil. More sophisticated tuners
use a variable capacitor (also called
a tuning capacitor) to vary the
frequency.
➔ Local Oscillator
- produce the oscillator signal called a
local oscillator signal. The local
oscillator is in the radio receiver and
is local to the radio receiver. There is
also an oscillator in the radio
transmitting station, and this is called
radiofrequency (RF) or radio
carrier frequency oscillator.
- the local oscillator is made of two
small wires with adjustable ferrite
core. These small wires are wound
around the adjustable ferrite core.
One wire winding is called the
primary winding, and the other is
called the secondary winding.
- The local oscillator is usually
enclosed in a metal shield to prevent
the local oscillator signal from
radiating to other circuits in the radio
tuner. To identify it, the top portion of
the local oscillator is usually painted
red.
➔ Mixer-Converter Transistor
- mix the signals coming from the
antenna
- it is tuned in by the tuning capacitor,
and the signals come from the local
oscillator. After mixing, these two
signals are converted to new signals
known
as
the
intermediate
frequency (IF signal). The standard
frequency of the IF signals for
amplitude modulation (AM) radio
tuner is 455 kHz, (Kilohertz), or
455,000 Hertz, (H2). These are the
same for all AM radio tuners.
- the process of mixing two different
signals to produce a new signal or
signals is called the heterodyne
process or heterodyning process.
This process is used in all radio
receivers, and that is why all radio
receivers
are
often
called
superheterodyne radio receivers.
➔ Intermediate Frequency (IF)
section
- prevents interfering signals to enter
the radio tuner when it is tuned to
one radio station with this action
- like an electronic gate.
- when the 455 kHz IF signal arrives,
it automatically opens to permit this
455 kHz IF signal to pass through
and enter the radio tuner. However,
when other signals whose frequency
is not 455 KHz, attempts to enter,
the IF section automatically blocks
this signal.
- there
are
three
intermediate
frequency transformers (IFTs) used
in the IF section. Between the
second IFT and third IT is the
second IF amplifier transistor.
However, although three IFTs were
used and two IF amplifier transistors
in the IF section, their functions are
the
same;
they
prevent
interference.
- made of two small wires wound
around the adjustable ferrite core
and enclosed in a metal shield. The
adjustable ferrite cores are adjusted
during the aligning process of the
radio tuner.
➔ Detector (demodulator)
- detects and recovers the audio
information from the radio signal
received by the radio tuner. The
radio signal that comes from the
radio station contains the audio
information or audio signal. This
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audio information is the equivalent of
the radio announcer's voice or the
voices that comes from the record
being played at the radio station.
the separated and recovered audio
information or signal is then fed to
the audio amplifier where it is
amplified
or
strengthened
sufficiently; the detector is usually
made of a small crystal diode.
Potentiometer - 10k Ohms
Resistor - 1M Ohms
Capacitor - 100 microfarads
Inductor - 2.2 microhenry
Transistor
FM Receiver Circuit
➔ RF Tuner section
- the modulated signal received by the
antenna is first passed to the tuner
circuit through a transformer. The
tuner circuit is nothing but a LC
circuit, which is also called a
resonant or tank circuit. It selects
the frequency desired by the radio
receiver. It also tunes the local
oscillator and the RF filter at the
same time.
➔ RF Mixer
- the signal from the tuner output is
given to the RF-IF converter, which
acts as a mixer.
- it has a local oscillator, which
produces a constant frequency.
The mixing process is done here,
having the received signal as one
input and the local oscillator
frequency as the other input. The
resultant output is a mixture of two
frequencies [(f1 + f2),(f1 - f2)]
produced by the mixer, which is
called the Intermediate Frequency
(IF).
- the production of IF helps in the
demodulation of any station signal
having any carrier frequency. Hence,
all signals are translated to a fixed
carrier frequency for adequate
selectivity.
➔ IF Filter
- a bandpass filter, which passes the
desired frequency. It eliminates any
unwanted
higher
frequency
components present in it as well as
the noise. IF filter helps in improving
the Signal to Noise Ratio (SNR).
➔ Demodulator
- the received modulated signal is
now demodulated with the same
process used at the transmitter side.
The frequency discrimination is
generally used for FM detection.
➔ Audio Amplifier
- power amplifier stage which is used
to amplify the detected audio signal.
The processed signal is given
strength to be effective. This signal
is passed on to the loudspeaker to
get the original sound signal.
This super heterodyne receiver is
well used because of its advantages
such as better SNR, sensitivity
and selectivity.
Superheterodyne Radio Receiver Block
Diagram
how does the radio signal flow?
We have seen the pre-emphasis network
(high pass filter), which is present before the
FM modulator. It is used to improve the
SNR or High-frequency audio signals. Thus,
in this FM receiver, the de-emphasis
network (low pass filter) is included after the
FM demodulator. This signal is passed to
the audio amplifier to increase the power
level. Finally, we get the original sound from
the loudspeaker.
➔ RF Amplifier
- amplifies the signal received from
the tuned circuit.
-
improves the sensitivity of the radio
allowing the reception of weaker
signals from distant stations.
➔ Mixer Block
- mixing is a way to lower the radio
frequency to one that is manageable
by a silicon transistor,
- the physics of this process is that
when you mix waves together, you
create a diffraction pattern where in
some places the two waves add,
and in others, they subtract.
➔ IF Amplifier
- amplifies the radio signal coming
from the mixer. It rejects all
harmonics produced by the mixer
stage and allows only the lower one.
The lower harmonic still consists of
the carrier and the information we
want. It is neither at radio at
frequency level, nor audio frequency
level but in between or intermediate.
➔ Detector
- the detector block is where the
demodulation process takes place.
Demodulation is the process that
retrieves the original modulating
wave. This is where the carrier wave
is separated from the information it
is carrying, which is usually the
original audio signal.
-
the purpose of the automatic gain
control block is to adjust the gain of
the IF amplifier block so that it
remains within a specific operating
range.
➔ AF Amplifier
- the audio frequency (AF) block is
responsible for amplifying the weak
AF
signal by increasing its
amplitude. When the amplitude is
sufficiently high enough, it can feed
a power amplifier to drive a
loudspeaker.
➔ Loudspeaker
- an
electro-acoustic
transducer,
which radiates acoustic power into
the air with the same waveform as
the electrical input signal. It is a
device, which converts electrical
energy into mechanical energy in the
form of vibrations. It usually consists
of a paper cone diaphragm, which
sets the air in motion to create
sound waves.
Antenna
Tuning Capacitor
Detector
➔ AGC
- In this block diagram, there is an
automatic gain control (AGC) at the
radio frequency (RF) stage, known
as RF AGC. When radio signals are
from nearby sources, the signal is
very strong, and when they are from
distant locations the signal is very
weak in amplitude.
Local Oscillator
(Red)
Mixer