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AASCIT Journal of Physics 2017; 3(4): 18-27 http://www.aascit.org/journal/physics ISSN: 2381-1358 (Print); ISSN: 2381-1366 (Online)
Keywords Balanced Full-Wave Detector,
Micro Controller,
Demodulation,
Phase Locked Loop
Received: April 29, 2017
Accepted: July 30, 2017
Published: September 20, 2017
Development of Am Field Strength Meter
Babalola Micheal Toluwase1, *
, Akeredolu Bunmi Jacob2, *
,
Ewetumo Theophilus3
1Department of Physics, Afe Babalola University, Ado Ekiti, Nigeria 2Department of Pure and Applied Physics, Federal University, Wukari, Nigeria 3Department of Physics Federal University of Technology, Akure, Nigeria
Email address [email protected] (A. B. Jacob) *Corresponding author
Citation Babalola Micheal Toluwase, Akeredolu Bunmi Jacob, Ewetumo Theophilus. Development of Am
Field Strength Meter. AASCIT Journal of Physics. Vol. 3, No. 4, 2017, pp. 18-27.
Abstract The Amplitude Modulated field strength meter circuit consists of an antenna, AM
demodulation unit with signal indicator output, tuning circuit comprising of a phase lock
loop (PLL), integrator, differential amplifier, frequency divider, microcontroller output
with serial communication interface and a varactor. The AM demodulator IC uses
balanced full-wave detector method for demodulation. Internally, it has a full wave
rectifier circuit that gives signal strength indication output, and frequency oscillation that
can give audio output as well. The output signal is fed into an analogue to digital
converter (ADC) of a microcontroller (PIC 16F877) to display the measured value. This
system will display the frequency received by dividing the frequency from the oscillator,
and this will be fed into PIC16F877 to give station frequencies that will be locked into.
To be able to confirm the stations, an audio output is incorporated so that the station
programmes can be heard. Tuning to different stations is done using PLL circuit which
generates a local oscillation via a varactor for balanced full-wave demodulation. The
system is equipped with facility to interface with computer via comport using visual
studio2008.net program to store the data at every two seconds time interval.
1. Introduction
Communicating over long distances has been a challenge throughout history. In
ancient times, runners were used to carry important messages between rulers or other
important people.
Modern telecommunications began in the 1800s with the discovery that electricity can
be used to transmit a signal. For the first time, a signal could be sent faster than any other
mode of communication e.g. transportation. Yet there are many challenges which face
the so called modern communication system, such as a barrier that reflects the signal
away from reaching the targeted audience. More importantly the strength of signal
transmission which conveys information to people varies with the time of the day
because of topography and weather [1]. Addressing this problem leads to development of
monitoring equipment to express the quality or value of a particular signal strength at a
certain distance from its transmission station [29]. The paper will assist in the
development of field strength meter.
In Telecommunications, a field strength meter is a measuring device which measures
the intensity of the electric field caused by a transmitter. A field strength meter in its
simplest form is a simple radio receiver. The tuner circuit is at the front end; then the
19 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter
signal is detected and fed to a micro-ammeter, which is
scaled in dBųv [29]. The frequency range of the tuner is
usually within the terrestrial broadcasting bands, though
some field strength meters can also receive signals in the
gigahertz range. In an ideal free space, the electric field
strength produced by a transmitter with isotropic radiation is
readily calculated as [27]
E= √
(1)
where E is the electric field strength in volts per meter, P is
the transmitter power output in watts and d is the distance
from the radiator in metre.
Field intensity measurements are made in support of a
reference. Section 12.4.2.1 of the Nigeria broadcasting code
NBC [22] specifies that a broadcaster shall maintain
specified minimum values of field strength within its
assigned coverage area depending on its mode of
transmission and its location’. Accordingly, the acceptable
minimum values shall be as indicated in table 1:
Table 1. The acceptable minimum values of field strength within assigned coverage area in Nigeria.
Urban Areas microvolt/m Rural Areas
i. AM Sound Broadcasting 72 dB i. AM Sound Broadcasting 66 dB
ii. VHF-FM Sound Broadcasting 60dB ii. VHF-FM Sound Broadcasting 48dB
iii. VHF-Television (Band III) 60dB iii. VHF-Television (Band III) 49dB
iv. UHF-Television (Band IV) 65dB iv. UHF-Television (Band IV) 60dB
v. UHF- Television (Band V) 70dB v. UHF- Television (Band V) 60dB
The AM field strength meter measures the strength level of a
signal that is being received from any AM transmitter. It detects
the electric field of AM radio frequency (RF) signals. The meter
reads the signal strength directly on the display [3, 17, 29]. To
design sensible and sensitive AM field strength meter, it is
useful to understand the amplitude modulation process.
Amplitude modulation (AM) occurs when the amplitude of
a carrier wave is modulated, i.e the process of impressing a
low frequency intelligent signal into a high frequency carrier
signal [19], and it has an equation of the form:
= [ + (t)]cos 2 t+ ∅ ) (2)
where the modulating signal component (t) is added to the
carrier amplitude , then the modulated carrier wave is
, the term [ + (t)] describes the envelope of the
modulated wave. The frequency is the frequency of the
carrier while ∅ is its phase [12]. A device that performs modulation is known as a modulator and a device that performs the inverse operation of a modulator is known as a demodulator (sometimes detector or demodulator). A device that can do both operations is a modem. [2]. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones [28]. A perfect AM transmitter, modulated with a single sine wave tone would radiate a constant average power regardless of modulation level, with spectral power shared among the assigned frequency carrier and two sideband frequencies above and below the assigned centre frequency by the frequency of the modulating tone. A field strength meter, or calibrated power bolometer, would read a constant value, with or without a modulating tone [17]
2. Method
2.1. Basic Description of the Block Diagram
The block diagram of an AM field strength meter is shown
in figure 1. It works as follows. An antenna A1 intercepts an
AM radio wave and supplies the signal to a radio frequency
amplifier S1 of a receiver chip U0. The radio wave is
amplified by the RF amplifier S1 and is fed to a mixer S2
together with oscillator signal from controlled oscillator S3.
The controlled oscillator signals is generated from frequency
synthesiser and fed into the mixer; multiplicative mixing
occurs between the AM rf input and oscillator signals. The
mixer output provides the intermediate frequency (IF) at pin
1 of the receiver U0. The output signal from IF filter is fed to
IF amplifier via pin 3. The output signals from IF amplifier
S4 is internally fed to the balance full-wave detector S5. The
balanced full-wave detector demodulates an audio signal
which corresponds to a modulated broadcast signal. The
detected audio frequency output is passed through an AF pre-
amplifier S6 to audio amplifier S7 through pin 6.
The Local oscillator signal from the local oscillator S3 via
pin 10 of AM receiver U0 is fed to comparator and inverter
which convert the sine wave to square wave before input to
programmable divider U4. The divider divides the frequency
fx of the channel signal with a predetermined ratio N. The
new signal is fed to a phase detector of PLL U19 which is
supplied further with the reference signal from reference
oscillator U21. The detector compares the frequency divided
signal with the reference signal and produces a signal which
represents the frequency difference and/or phase difference.
The signal has its ripple component removed by a low pass
filter (LPF), and is therefore amplified into a control signal.
The DC voltage level of the control signal is supplied to the
local oscillator S3. The oscillation frequency of the local
oscillator is changed according to the voltage level of the
control signal. The local oscillator S3 is therefore called
"voltage controlled oscillator (VCO)". The signal from a
field strength indicator is amplified and fed through the
microcontroller to the display. The control key input the
frequency of the channel station, and both the field strength
readings and the frequency of channel station are displayed
AASCIT Journal of Physics 2017; 3(4): 18-27 20
on the LCD. The audio signal is taken from pin 6 of U1 and re-amplifies before it is fed to the speaker.
Figure 1. Block diagram of AM Field Strength Meter.
2.2. Circuit Description
The circuit in figure 3 can conveniently be divided into
three parts; the AM receiver circuit, frequency synthesizer
circuit and controller circuit.
2.2.1. Receiver Circuit
The sensing unit (antenna) used, was a monopole antenna
of 50 ohms, which was capable to intercept or capture a radio
wave travelling through space and able to deliver them to a
receivers. Other features of the antenna include its length,
power received and antenna factor, made it an ideal choice
for this project. The main component of the receiver is a
TDA1572 AM receiver chip U, around which everything else
was based, TDA1572T is a 16-lead mini pack plastic package,
normally measuring and yet it contains 10 major circuit
blocks, these blocks are integrated on a chip. TDA 1572T
performs all the main function of an AM superhet radio
receiver including RF Amplifier, Local Oscillator, Mixer,
Detector, AF Amplifier, AGC system and Field strength
indicator. The captured signal from an antenna is fed to pin
14 of RF amplifier via capacitor C23. The differential
amplifier in the RF stage of TDA 1572T employs an AGC
negative feedback network to provide a wide dynamic range.
Very good cross-modulation behaviour is also achieved by
AGC delays at the various signal stages. Low noise working
is achieved in the differential amplifier by using transistors
with a low base resistance. A double balanced mixer provides
the IF output at pin 1 of TDA1072. Local oscillator operates
at 455kHz above the wanted incoming signal frequency i.e if
the wanted signal RF is 1000kHz, then the local oscillator
frequency will be 1455kHZ. The oscillation frequency is
generated by a frequency synthesizer with varactor D1, and a
parallel tank (made up of coil L1, and capacitor C20 and C2)
connected between pins 11 and 12 of IC TDA1072. The local
oscillator needs to be tuned to a range 990 to 2100 kHz, such
that, it’s frequency plus the IF frequency (455 kHz) equals
the received frequency, that is a range of 990 to 2100kHz. i.e
from (535 + 455 to 1645 + 455kHz). A varactor diode D1
which gives about 90 to 200pF along with the capacitor C21
set the tuning span and the tank inductor L1 of 95, formed
by a copper wire wound round a ferrite core. Extra oscillator
output from pin 10 of IC TDA1072 is fed to programmable
divider through transistors Q1 and Q2 and a comparator U32
which are used to amplify and convert the analogue signals
(sine wave) to digital (Square wave) to form input signal to
programmable divide The intermediate frequency signal from
21 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter
the mixer output via pin1 of TDA 1072 is filtered through IF
filter. Designing of IF filter was done using the formula,
= 12√ (3)
Where is the centre frequency, L is the inductor and C is the capacitor.
Since 455kHz was selected as centre frequency, the IF
filter, FL1, consists of two pole ladder crystal filter X3 and
X4 of 455kHz, a matching transformer TR1, capacitors C28
and C29. The matching transformer has 13 turns on the
primary coil and 9 on the secondary coil of TR2. A trimmer
capacitor C50 is used to resonate the inductance of 31H of
the primary coil of TR1.
C29 can be calculated as follows: = 12√ , the
value of C29 is given as; = 3.9$%
The TR1 and C29 are used to tune to 455 kHz, giving a
good match for IF filter: a 2 pole ladder filter is from toko.
Inside the TDA 1572T the full- wave balanced envelop
detector is used for demodulation of the intermediate
frequency to audio frequency signal, internally it has a low
pass filter to block residual IF carrier from signal path. The
audio frequency signal is amplified by AF pre-amplifier,
which uses an emitter follower with a series resistor (inside
the TDA1072), together with an external capacitors, provides
the required low-pass filtering for AF signals. The audio
filter at pin 6 of TDA1072 consisting of the capacitor C27,
C26 and resistor R33. The AF output voltage at input signal
50& is 130mV, (Data sheet TDA 1072) so there will be
need for amplification, because a normal audio line is 0.5 to
2 Volts. (Kennick, 2001). LM 386 is used to amplify the
audio signal before fed to speaker.
The AGC amplifier provides a control voltage which is
proportional to the carrier amplitude. Second-order filtering
of the AGC voltage achieves signals with very little
distortion, even at low audio frequencies. The AGC voltage
is fed to the RF and IF stages via suitable AGC delays.
The data sheet of IC TDA 1072 has a graph of field
strength voltage indicator in volts against RF input signal in
dBµV, this graph has good linearity for logarithmic input
signals over the whole dynamic range. A buffered voltage
source from the IC TDA 1072 provides a high-level field
strength output signal. At maximum voltage of 28V, the
corresponding RF input is 120dBµ& and this was used in
calibrating the field strength of the received signal. Philips
semiconductors (1989)
2.2.2. Controller Circuit with User Input and
Output Interface (Microcontroller) and
LCD
The pic16f877 is a controller used in this paper. It is an 8
bit RISC (reduced instruction set computer), central
processing unit. It is a complete computer on a chip, and
entire processor, memory and the I/O interfaces are located
on a single piece of silicon. For this reason, it takes less time
to read and write to external devices. Pic16f877 U1 has three
types of memories which are: flash program memory,
Electrical erasable memory and static random access memory.
Pic16f877 is also has 33 inputs and output pins that can be
individually configured as inputs or outputs
These ports have been assigned to the various tasks in the
circuit;
PORTA bit 0 is configured via firmware as an analogue
input for analogue to digital conversion. One channel is used
in this project to convert received signal strength from pin 9
of TDA1572T, in analogue form to digital, which will be
then be displayed on the LCD unit,
PORTC bits 6 and 7 were used as transmit and receive pin
for asynchronous communication with a user PC (personal
computer) using RS232. PORTA bits 1, 2, 3, 4, 5 and
PORTE bit 2 are used for the user keypad. Each of these pins
was configured as inputs. A 10 kΩ (R1 to R6) pull-up
resistor was used for each pin, while a de-bounced button
serves as the keypad. Via firmware the PIC monitors the state
of each pin. When not depressed the PIC sees logic 1 and
does nothing and when depressed the PIC sees logic 0 and
performs the associated routine attached to it. The keypad
allows channel selection from 500 kHz to 1699 kHz.
PORTB and the lower 4 bits of PORTC were configured
as output and were used to give data for the programmable
divider used in the frequency synthesizer
PICs generally have internal oscillator buffer configuration
option. In the circuit design, the HS (high speed crystal)
option is used with a 16 MHz crystal X1, because 16 MHz
crystals are widely available in the market. Also the circuit
clock is the time base for the generation of the serial
communication baud rate with the PC
2.2.3. Frequency Synthesizer
A frequency synthesizer is a circuit design that generate a
new frequency from a single stable reference frequency. For
frequency span 535 KHz to 1605 KHz, the standard divide
by N value and a fixed divide by M value of 2 are used.
Since N needs to be between 535 and 1605, The ICs required
for the programmable divider is LS74161. The LS74161 is a
4 bit synchronous, pre-settable counter. This counter is fully
programmable; the output may be preset to logic 0 or to
logic1. Pre-setting is synchronous. To achieve the
programmability from 990 to 2060 for this work a 12 bit
counter is needed. Three LS74161 (U33, U32 and U31) are
cascaded to make a 12 bit counter. The output from the VCO
is coupled to the programmable counter via a 2.2uf capacitor
C7 and converted to square wave using the 7414 Schmitt
trigger NOT gate U34.
AASCIT Journal of Physics 2017; 3(4): 18-27 22
Figure 2. Block diagram of frequency synthesizer.
The reference oscillator is 500 Hz, A crystal oscillator of 8
MHz, X2 was obtained and divided down to 500 Hz. The
circuit is shown in figure 2. The NOT gate 7404, (U15, U16
and U34) serves as amplifier (buffer) for the crystal to
oscillate and the two 1kΩ resistors (R9 and R10) are the
feedback resistors for the amplifier. LS74161, a 4 bit counter
divides 8 MHz, X2 by 16 to get 500 kHz, Also three 4017
decade counters (U21, U22 and U23) are cascaded to form a
divide by 1000 counter, the result is that 500 KHz is divided
by 1000 to get 500 Hz, on pins 12 of the 3rd 4017, U38
2.2.4. Phase Locked Loop
The integrated circuit CD4046, U36 is the phase locked
loop used in the circuit. The reference oscillator output signal
of 500Hz is fed on phase comparator II of PLL. The output
of VCO (from TDA1572T receiver) is fed back to the phase
comparator II second input, through a programmable divider
that has been programmed to divide by any number from 990
to 2100. The loop will lock when the frequency fed back
through programmable divider is equal to 500Hz of reference
oscillator frequency. A simple RC filter is employed as the
loop filter. Where C12 is fixed at 1000µf, the value of R
varies via a digital switch between 10 kΩ, 100 kΩ and 330
kΩ. The CD4066 analogue switch is controlled by the PIC
controller. The need for using different loop is to ensure fast
lock and smooth lock. The centre frequency of the low pass
filter is given as
= 12( (4)
This gives a frequency cut-off of 0.015Hz for R = 10 kΩ,
0.0015Hz for R = 100 kΩ and 0.00048Hz for 330 kΩ. The
output of the phase comparator/detector is amplified using
LM 741 OP-AMP. A non-inverting amplifier configuration is
used. The gain of the stage is;
) 1 (*(++
(5)
Where RF is 2.2 kilo-ohms (R17) and 1 kilo-ohms (RV2)
variable in series
R18 is 10 kilo ohms, Therefore,
) 1 2.2 110 = 1 3.2
10 = 1.325
This gives a gain of less than 2
2.2.5. Audio Amplifier
The LM386 is a power amplifier designed for use in low
voltage consumer applications. The gain is internally set to
20 to keep external part count low, but the addition of an
external resistor and capacitor between pins 1 and 8 will
increase the gain to any value from 20 to 200. The inputs are
ground referenced while the output automatically biases to
one-half the supply voltage. The quiescent power drain is
only 24 milli-watts when operating from a 6 volt supply,
making the LM386 ideal for battery operation.
2.2.6. RS232 Serial Port
Serial Ports come in two sizes; there are D-Type 25 pin
connector and the D Type 9 pin connector both of which are
male on the back of the PC, thus you will require a female
connector on your device. RS-232 communication is
asynchronous, a clock signal is not sent with the data. Each
word is synchronized using its start bit, and an internal clock
on each side, keeps tabs on the timing. The first step in
connecting a device to the RS-232 port is to transform the
RS-232 levels back into 0 and 5 Volts. This is done by RS-
232 Level Converters. An example of such a device is the
MAX-232 use in this work.
2.2.7. Power Supply
The circuit is powered from an 18 volts rechargeable
battery source. A transformer is provided to charge the
battery. The voltage is then regulated to 12 volts and 5 volts
for each stage using voltage regulator 7805.
23 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter
Figure 3. Complete Circuit Diagram of AM Field strength meter.
A firmware program was developed to test the working of this circuit, below is the flow chart used.
AASCIT Journal of Physics 2017; 3(4): 18-27 24
Figure 4. Flow Chart for the Microcontroller.
Figure 5. PC graphic user interface.
Plate 1. Real-Time Data Logging with Laptop.
25 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter
The flow chart is then converted to the source codes using
ASSEMBLY program. The Real-Time Data Logging with
Laptop is shown in plate 1. The PIC programmer is used to
“burn” the source codes into the PIC. The serial port program
is written using visual basic dot net 2008 edition. This
program reads the serial port data and displays in a text box
with the TIME information shown in figure 5. The complete
work is shown in plate 2
Plate 2. Complete work of FSM.
3. Operation and Performance of the
System
3.1. Calibration of the Field Strength Meter
The AM field strength meter was calibrated against a
laboratory standard. The calibration was made to prove the
performance of the equipment after the construction. The
meter was also calibrated with the standard and imported
field strength meter lag-gear, model. TC805C. The two
meters were used together for measurement at the same time
at different locations in Ibadan Oyo state. Before the
measurement the two meters were switched on, the
constructed meter was adjusted to give the same reading with
the standard meter at initial reading of 30dBµV before tuning
to the station. On tuning the two meters to the AM broadcast
transmitting station, broadcasting at frequency of 756 kHz,
readings were recorded for five different locations. The result
shown in figure 6, gives 95% accuracy with imported Model
TC805C.
3.2. Cost Analysis
One of the advantages of using the newly constructed AM
field strength meter is cost efficiency of the meter. By
analysing the cost of the constructed AM field strength meter,
the total cost is N35,000.00 ($175), but the minimum cost of
acquiring a field strength meter of the same frequency range
from abroad ranges from N100,000.00 to N300,000.00 ($280
to $840), the rate of conversion was $1 to N360. This clearly
indicates that the newly developed FSM is cheaper since part
were sourced locally. Table 2 shows the cost analysis of the
field strength meter.
Table 2. Cost analysis of FSM.
Materials Quantity Cost (N) Cost ($)
AM receiver 1 2,000 6.0
Battery 3 2,400 7.0
Transformer/ crystal Oscillators 1 1,200 3.3
PIC 16f877. Microcotroller 1 1,000 3.0
Other components 20,000 56.0
Casing 1 3,000 8.0
Transportation 3,000 8.0
Total N35,000 $91.30
3.3. Testing of Am Field Strength Meter
On completion of the design and construction work,
certain performance tests were carried out with the meter and
the available one (imported field strength meter lag-gear,
model TC805C) from the Electrical Engineering Department,
University of Ibadan in Ibadan, Oyo State. The test took
place in five different locations in Ibadan; the two
instruments were used to measure the AM field strength from
AM transmitter of the Broadcasting Corporation of Oyo State
in Ile Akede, Bashorun, Ibadan, transmitting at the frequency
of 756kHz.
4. Result and Discussion
The constructed meter performance was evaluated in terms
of sensitivity and against standard imported FSM. Figure 6
shows the result of the field strength values in decibel-
microvolt -.& against distance of various locations
where the measurement took place in kilometres (km) for
two meters (imported and constructed FSM). It was observed
that, as the two meters were moving away from the
transmitter in kilometres, there was gradual decreased in the
field strength measurements. At Bodija market located 2.9m
away from Akede AM Transmission station, the variation
percentage in the values of the two meters was about 0.1%.
Also at UI (5.8km away), the variation percentage in the two
meters was 0.05%, at N. B (7.0km away) the variation in the
values of the two meters was 0.001%. But at Ibadan airport
(8.0km away) and IITA (10km away) there were no
significant variations in the value recorded in the two meters,
i.e the meters read the same value in decibel-microvolt
((dBµV)). The variations in values of the FSM measurement
of the two meters were as a result of minor errors that might
occur during the development of the constructed meter.
AASCIT Journal of Physics 2017; 3(4): 18-27 26
Figure 6. Graph of field strength (dBµV) against distance (Km).
5. Conclusion
An AM field strength meter capable of measuring AM
signals from any AM broadcasting transmitters with centre
frequency of 500kHz to 1650kHz, bandwidth of 10khz and
gain control from 0 – 120dBµV was designed, constructed
and tested. The system consist of an antenna, AM
demodulation unit with signal indicator output, pre-amplifier,
frequency divider, tuning circuit comprising of a phase lock
loop (PLL), integrator, differential amplifier, microcontroller
output with serial communication interface and a varactor.
The AM field strength meter integrates a digital tuning using
frequency synthesisers, and data logger for interface with a
computer. Frequency synthesiser helps to overcome some
problem of accuracy and stability of signals.
A comparison test was conducted with this meter using the
signal level meter, lag-gear, model TC805C as a standard
meter. The work testing exercise was conducted in five
different locations in Ibadan city, Oyo State. Comparing the
readings of the two meters (imported and constructed FSM),
the result shows that the constructed field strength meter was
specify with accuracy of 99%
Hence, the use of this system is limited to area where AM
Transmission is available
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