FM SIGNAL RECEIVER
Signal detection and amplification using op-amps
P Jayachandra Naik P Sai Bharath Reddy
Department of ECE Department of ECE
NIT CALICUT NIT CALICUT
Kozhikode-673601, India Kozhikodet-673601, India
Abstract Radio is the reception of electromagnetic wave II. DESCRIPTION
through air. The main principle of this circuit is to tune the
circuit to the nearest frequency using the tank circuit. Data A simple op-amp radio basically consists of a crystal radio
to be transmitted is frequency modulated at the along with a fairly sensitive audio amplifier. The audio
transmission and is demodulated at the receiver side. amplifier is able to receive strong stations around Los
Modulation is nothing but changing the property of the Angeles with a minimal 15-foot antenna. Stronger signals are
message signal with the respect to the carrier frequency. always provided by longer antennas. But the main
Frequency range of FM signal is 87.5MHz to 108.0MHz. disadvantage is that in the case of long antennas the stronger
The output can be heard using speaker. signals provided may produce a noise as it could be heard in
the background of the weaker signals.
Keywords Key words- Modulation, Receiver
In the case of longer antennas, the selectivity will also become
I. INTRODUCTION bad. A remedy can be obtained to the problem of low
sensitivity by connecting the long antenna to one of the taps of
The FM Band transmission has started very recently in India the coil. For this the connection of the antenna to the junction
but its superior technique and quality has attracted the of the capacitor and coil must be changed. For listening to the
listeners. Unlike AM, the FM is a separate band and its local radio news, we just have to stand outside and let the long
frequency ranges from 88MHz to 108MHz. The FM Band headphone leads to lay on the floor. As a result of this setup a
cannot be received by the conventional AM receivers. Each ground is also established. The inductance coil has 200 turns
and every AM receiver does not incorporate FM facility. The and is made of #28 enameled copper wire. The inductance coil
present project is a very low cost project and it can be fitted to can produce 220 micro henry inductances with the wire kept
any radio receiver/audio system to receive FM transmission. on a 7/8 diameter, 4-inch length of PVC pipe. The inductance
The circuit of this project is very simple and can be easily coil was setup in such a way that there were wounds with taps
assembled. A theoretical study of radiation from a linear every 20 turns. This mechanism is provided so as to select a
antenna (length l) Power radiated a (p/l)2 This implies that perfect diode and antenna connection. For a little weaker
for the same antenna length, the power radiated by short wave signals a germanium diode such as 1N34A type would be
length or high frequency signal would be large. Hence the more appropriate. An approach would be to reduce the overall
effective power radiated by long wavelength base band signal gain with a smaller feedback resistors. High impedance
would be small for a good transmission, we need high power headphones will probably work best you can also try with
hence, this also point out to be need of using high frequency Walkman headphones.
transmission. Modulation In amplitude modulated
communication, propagation of radio waves from the
transmitting antenna to the receiving antenna takes place in
the following two important ways :1. Ground Wave III. CIRCUIT IMPLEMENTATION
Propagation2. Sky Wave propagation The radio wave induces
current in the ground, over which it passes. It attenuates to A. COMPONENTS REQUIRED:
some extent due to partial energy absorption by the ground.
Basic concepts involved:
a. Signal receiving (Antenna) R4 -10K
b. Demodulation R5 -100K
c. Amplification R6 -100K
R7 -470K
R8 -470Ohm
c1 -30-360 Pf Variable
C2 -0.1 uF ceramic
C4 -100uF 15 V Electrolytic feed back path come into existence, it amplify the present
IC1&2 -741 opamp signal at the non inverting input.
T1 -Sl 100 NPN Transistor
Z1 -Head phone c) Output stage
A1 -See notes
D1 -1N34 Germanium Diode The output taken across the resistor followed by a
capacitor has the amplified version of the input. We can give it
to any speaker systems/ head phone socket to listen the
particular station tuned from the tuner circuit.
C. Units
B. INDIVIDUAL BLOCK WORKING:
a) LC tuned circuit: Omega - hertz
Amplifier gain - volt/volt
This circuit is used to tune a particular frequency (station)
to listen. Here we used tuned C (capacitor) circuit to achieve Voltage milli volt
this. By changing capacitor value in tuner circuit we set the
resonance frequency to a particular value that we would like to Current milli ampere
listen. Resistance -ohms
b) Amplifier circuit: Capacitor -faraday
OP-AMPS:
D. Equations
An operational amplifier is an amplifier circuit which
=1/L1C1^1/2;
typically has very high open loop gain and differential inputs.
Op amps have become very widely used as standardized "gain
VOLTAGE DIVIDER:
blocks" in circuits due to their versatility; their gain,
Voltage at STAGE2 non -inverting
bandwidth and other characteristics can be controlled
terminal=9*R6/R5;
by feedback through an external circuit. Though the term
STAGE2:
today commonly applies to integrated circuits, the original
Amplifier Gain = -R7/R4;
operational amplifier design used valves, and later designs
STAGE1:
used discrete transistor circuits.
Voltage at input = Voltage at output ;
NEED FOR AMPLIFICATION:
As we know radio signal we are receiving are have low power E. Ciruit schematic
signals. They undergone through so many obstacles, so their
energy might decrease as a result. So we need to amplify the
those signals. Here we make use of the op-amps to amplify the
signals.
AMPLIFIER WORKING:
STAGE1
The signal from the tuner circuit output is given to the voltage
follower circuit to get the signal amplification more
efficiently. The input impedance of voltage buffer is very
high , so the signal fully delivered from the tuner and the
output impedance of buffer is very low , so the amplifier get
complete signal. By doing this we are increasing signal quality
of FM signal.
STAGE2
In this stage we are amplifying the FM signal by a operational Fig 1. FM receiver circuit
amplifier using it in inverting configuration. If the input signal
is more than certain value the amplifier do not need to amplify
that. To achieve this we make use of the voltage divider at the IV. ACKNOWLEDGMENT
non inverting terminal of amplifier.
If the voltage is less than that value, at the positive terminal, This project was partially supported by my seniors. We
the output is high, it makes the transistor to ON. Then the thank our professors from National institute of technology
who provided insight and expertise that greatly assisted the [1] Adel S Sedra, Kenneth C.Smith Microelectronic circuits theory and
applications, 6th ed.
project, although they may not agree with all of the
[2] Sergio Franco, Design with operational amplifiers and analog integrated
interpretations of this paper. circuits, 3rd ed.
We would also like to show our gratitude to the [3] Basic linear design, hand book,RF AND IF BASICS,.
electronic circuits 1&2 course professors for sharing their [4] http://www.instructables.com/id/Simple-OP-Amp-radio/
pearls of wisdom with us during the course. [5] https://en.wikipedia.org.
REFERENCES
