INTRODUCTION
In this project we will construct and test an amplitude modulated radio transmitter operating
at a frequency in the AM radio band. This circuit will allow you to transmit the audio (from
the earphone out) of an iPod or other MP3 player to a nearby table top or car radio. we will
also construct an AM demodulator that will demonstrate the means that a receiver uses to recover
information from a received signal. The approach used and the waveforms generated by this project
are similar to those used for the AM radio broadcast service and shortwave radio stations. The
basic underlying analog circuitry is also common to many forms of digital wireless transmission
such as found in Wi and bluetooth systems.
we will make use of the oscilloscope and function generator to observe the signals (waveforms)
that are generated throughout this circuit.
Because of the high-frequency signals that will appear in this project, circuit layout
is very important. Follow the suggestions in this text.
In the following material you will be asked to construct these circuits one at a time. This way,
each can be separately troubleshot! Don't jump ahead and build the remainder of the circuit until
each previous section works or else we may never be able to get it to work in time.
rf oscillaters
The first circuit we will construct is the RF (radio frequency) oscillator, shown in Figure 1. Thiscircuit is responsible for generating a (approximately) 1.4 MHz (1400 kHz on the AM radio display)
sinusoid. A single transistor is used as a current controlled current source to supply the gain for this
oscillator. The 33 H inductor, L, and the capacitors C1 and C2 implement the positive feedback
network. The conditions for oscillation are met at the radio frequency, ! = 2 f, (f is the frequency
in Hz.) at which the following equation is satis ed:
WL=(1/WC+1/Wc)
For a pre-lab you will be asked to calculate the \resonant" frequency f for this circuit by solving
the above equation.
1Circuit layout for this portion of the circuit is very critical. Use minimum lengths of wire
throughout this construction. The components for this entire circuit should lie within an inch of
each other!
Especially important is the use and placement of the power supply bypass capacitors C3, C4,
C5, C6. It may look ridiculous that small :1 F capacitors have been placed in parallel with large
10 F capacitors, but each has its function. The overall function of these capacitors is to provide a
low impedance path for AC currents to low
between the negative, positive and ground lines of the
power supply. The long wires (1 foot) that run back to the power supply are too long to act as the low
impedance paths that we like to think that wire represents. Furthermore, the electrolytic capacitors,
C3 and C4 are not what they seem to be at high frequencies. To achieve their high capacitance, a
fairly large inductance has also been introduced! At lower frequencies this inductance is not a factor
since its impedance Z = j!L and hence close to zero at very low frequencies. However, electrolytic
capacitors will become ine ective as capacitors at higher frequencies, where the somewhat more
\pure" C5 and C6 have a low capacitive impedance.
All the above power supply bypass capacitors should be mounted near the RF oscillator and
should have very short connections for proper operation.
Don't forget to use your DMM to adjust the power supply voltages to +10 and {10 V before
connecting the power supply to your circuit. It will be very easy to destroy the transistor in this
circuit if these are not set properly.
After constructing the circuit, connect your CH1 scope probe tip to the point marked Vrf by
clipping its tip to a wire inserted at this location and of course clip its ground clip to a wire inserted
into your board connecting to the power supply ground bus. Connect the point marked Vin to
ground (temporarily, you will be using this input later.). Set the scope to trigger on CH1 (use the
menu to select to internal trigger on CH1, auto mode). Set the scope input sensitivity to the 5
volts/div scale and the time base to provide 100ns/div. Use the input menu to also set the CH1
input to AC coupling. This will block the 10 V DC component from the oscillator output.
Now apply power to the circuit. You should see a sinusoid on the screen. Adjust the triggering
and sensitivity settings to give yourself a good, stable waveform on the scope. Now read the
frequency of this waveform from the input display on the ride hand side of the scope display. This
should agree approximately with the calculations.
amplitude modulation
Now you will use the function generator to supply a low frequency sinusoid that will be used to
modulate the the amplitude of your RF oscillator. Recall that the output of the function generator
is available at a BNC connector on its front panel. Use a cable with BNC connector and alligator
clips to connect to a piece of wire inserted into your protoboard in the experiments that follow.
In the previous experiment you connected Vin temporarily to ground. Now disconnect this point
from ground and connect this point instead to the red-clip output of the function generator as shown
in Figure 2 (where the RF oscillator is represented by the black box in the diagram as indicated to
the left of Figure 1.) remembering to also connect the black ground clip from the function generator
to your circuit ground. Set the function generator to supply a 1 KHz sine wave without DC o set
(either push in the o set knob on those generators that have one, or select the no o set sinewave
on those generators that have this waveform selection option.)
to Connect the CH2 scope probe to view this signal before connecting it to your circuit. You will
have to change the scope time base to the .1 ms setting, select triggering from CH2, and choose the
scope input selections to show both CH1 and CH2 signals if it is not already doing so. Adjust the
signal generator for about a 5 volt peak-to-peak (voltage di erence from the most negative peak to
the most negative peak) signal.
Now turn on the power to the RF oscillator. You should see a waveform like one of those in
Figure 3. Adjust the amplitude of the function generator until you see a waveform consistent with
about 50% modulation. (Some of the function generators will not be able to achieve su cient
output levels to obtain even 50% modulation. That is alright. In these cases just set the amplitude
of the function generator to max.)
The high frequency oscillations of this signal are now so closely spaced that they would appear
as a blur in between the peak output amplitudes, so the computer in the oscilloscope renders this
region as a diagonal hash pattern. The envelope (the border that lines the top and bottom of the
pattern) formed by these peaks should be clearly visible and can be seen to resemble the input
modulating waveform (though there may be a very apparent phase shift between the two sinusoidal
shapes.)
Ordinarily an AM radio transmitter is operated in such a way that the the amount by which
the envelope modi es the unmodulated waveform, which is called the modulation index, is between
0 and 100 %. At 100 % the peaks of the modulation vary the RF output amplitude from zero to
twice the average amplitude and the envelope thus becomes the largest envelope that still resembles
the modulating wave.
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Figure 1: Switch position for charging the capacitor
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Figure 2: Switch position for making the circuit oscillate
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How is the output of this oscillator being amplitude modulated? The theory goes a little beyond
this course, but for future reference it can be summarized in this way: The audio frequency voltage
you are applying to the base of the transistor a ects the input Thevenin resistance of the controlled
current source model of the transistor. A transistor's input resistance is inversely proportional to
its emitter current. The amplitude of the sinusoid generated by this oscillator is also a function
of this resistance, hence the amplitude modulation. This is not a very good method to generate
amplitude modulation, but it is simple and easy to implement. Why do we say it is not very good?
Look carefully at the envelope of the RF output signal. You will see that it is not quite sinusoidal,
especially at a large modulation index. Furthermore, note that the top and bottom envelopes are
not quite the same. Usually a second circuit, a \multiplier" is used to modulate the constant
amplitude output of an oscillator.
Now connect a 1 foot wire to Vrf and leave the other end of this antenna unconnected to
anything, just sticking up in the air. This is your transmitter's antenna. Have the lab TA bring
a radio over to your lab bench. You should be able to nd the signal you are transmitting on the
receiver. Vary the signal generator frequency to prove to yourself that you have found your signal.
When done, remove the antenna so that your signal does not interfere with the e orts of other
groups in the lab to hear their signal.
Envelope Demodulation
Now you will construct a simple AM demodulator, often called an envelope detector. Construct
the circuit shown in Figure 4. As will be discussed in class, this circuit acts like a peak detector.
Whenever the voltage Vrf exceeds the voltage Vout
, the diode allows the capacitor to quickly charge.
.
to this new value. When the RF voltage swings downwards, the capacitor maintains this voltage
as it only slowly looses its charge through the resistor to ground. Thus the output voltage tracks
any slow changes in the RF amplitude.
Move the CH2 scope probe to the point Vout
. Note that the envelope detector has in fact
extracted the original waveform from the RF signal. Try removing the 510 pF capacitor, C7,
temporarily. Note that Vout
is now simply the positive excursions of Vrf . The transient response of
the output RC transient circuit plays an important role in producing a true envelope detector.
Now replace C7 and temporarily remove R1. Now the time constant of the RC circuit has
become so large that the capacitor voltage can no longer track the changes in the envelope,
..
Music Modulation
Now implement the circuit the Figure 5. This is an audio ampli er driven by your MP3 player.
During the following steps you will nd it necessary to adjust the volume control on the MP3 player
to obtain a modulation index below 100 % so as to obtain an undistorted audio signal from the
radio receiver.
Use the \phone plug" supplied by the TAs to connect your MP3 player to the transmitter as
shown in the circuit diagram. Move your CH2 probe to the output of the audio ampli er Va. You
should be able to observe the signal that is generated when you play your music into the circuit. If
not, troubleshoot this circuit or ask for assistance from a TA before continuing.
When the audio ampli er works, disconnect the function generator from Vin on the transmitter
circuit and instead connect the audio ampli er output to Vin as shown in the block diagram in
Figure 6.
When you activate the circuit you should see the envelope of your Vrf signal is now amplitude
modulated by your voice. Adjust the volume until you see a modulation index under 100% (about
50% would be a good target). Re-insert the antenna wire and ask for an instructor to bring the
AM radio receiver back. You should now have a fully functional Wireless iPod transmitter.
6 Simulator analysis
In Figure 7 we show the core RF oscillator as it would be entered into QUCS. Enter this circuit
into QUCS. Take note to set the switch turn on time to 0 as shown and to change the transient
simulator values to start at 0 seconds and end at 50 microseconds. Also set the transient simulator
Number property value to 1000. Don't forget that to enter microFarads ( F) you must use uF in
QUCS.
Execute the simulation and plot Vrf.Vt. You should see the same oscillation as you saw in the
real circuit (with a small start-up transient in the rst 10 s. Copy this circuit to a folder on
another machine that you can access in the future. We will be using this circuit to illustrate some
points in a future homework.
Put a print out of this circuit and the resulting simulation waveform in your notebook.
47 Conclusions and Extensions
As mentioned before, your circuit resembles that at the basis for most radio systems, including
radio and TV broadcast, Ham radio, cell-phones, navigational radio (GPS), satellite transmission,
Wi , Bluetooth etc.
Use of the radio spectrum is regulated by national and international law. In the
US, the FCC (Federal Communications Commission) grants licenses to individuals and
companies to use particular carrier frequencies with speci c modulation schemes for
well de ned purposes! Setting up a radio station on you own can run up a high cost
in nes and a possible prison sentence!
There are iPod transmitters available commercially. These usually transmit to the FM radio in
a car. Using the FM broadcast system allows for transmission of the full stereo signal instead of
combining them into a single signal as was done in this lab. An FM transmitter uses a di erent form
of modulation. Instead of modulating the amplitude of the sine wave in accordance with variations
in the input signal, instead, it is the frequency of the sine wave with is varied while its amplitude
remains constant.
we will learn a great deal more about modulation and communications systems in future
courses on the topics of signals and communication
in this project i will construct and test an amplitude modulated radio transmitter operating at frequency in the A.M radio band. this circuit is allow you to transmit the audio of an iPod or any mp3 player.
i will make use of the oscilloscope and function generator to observe the signal(wave form) that are generated through out the circuit
because of the high frequency signal that will appear in the project, circuit layout is very important
Follow the suggestions in this text.
In the following material you will be asked to construct these circuits one at a time. This way,
each can be separately troubleshot! Don't jump ahead and build the remainder of the circuit until
each previous section works or else we may never be able to get it to work in the time.
Performance
for a VFO include frequency stability, phase noise and spectral purity. All of these factors tend to be inversely proportional to the tuning circuit q factor since in general , the tuning range is also inversely proportional to Q, these performance factors generally degrade as the VFO's frequency range is increased.
Stability
Stability is the measure of how far a VFO's output frequency drifts with time and temperature To mitigate this problem, VFOs are generally phase locked to a stable reference oscillator. PLLs use negative feed back to correct for the frequency drift of the VFO allowing for both wide tuning range and good frequency stability.
Repeatability
Ideally, for the same control input to the VFO, the oscillator should generate exactly the same frequency. A change in the calibration of the VFO can change receiver tuning calibration; periodic re-alignment of a receiver may be needed. VFO's used as part of a pace locked loop frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.
Purity
Further information: spurious emmision,
A plot of a VFO's amplitude vs. frequency may show several peaks, probably harmonically related. Each of these peaks can potentially mix with some other incoming signal and produce spurious response. These spurii (sometimes spelled spuriae) can result in increased noise or two signals detected where there should only be one. Additional components can be added to a VFO to suppress high-frequency parasitic oscillations, should these be present.
In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.
Phase noise
When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat noise floor. Slight random jitters'in the signal's timing will mean that the peak is sitting on 'skirts' of phase noise at frequencies either side of the desired one.
These are also troublesome in cor-wadded bands.
. They allow through unwanted signals that are fairly close to the expected one, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the received signal. The effect is that what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of strong signals nearby.
The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.
Crystal control
In all performances cases, crystal controlled oscillators are better behaved than the semiconductor- and lc-based alternatives. They tend to be more stable, more repeatable, have fewer and lower harmonics and lower noise than all the alternatives in their cost-band. This in part explains their huge popularity in low-cost and computer-controlled (i.e. PLL and synthesizer-based) VFOs.
Processes
Radio systems used for communications will have the following elements. With more than 100 years of development, each process is implemented by a wide range of methods, specialized for different communications purposes.
Transmitter and modulation
: radio transmitted division
Each system contains a transmitter. This consists of a source of electrical energy, producing a.c of a desired frequency of oscillation. The transmitter contains a system to modulated (change)some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization.
a.m of a carrier wave works by varying the strength of the transmitted signal in proportion to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. It was the method used for the first audio radio transmissions, and remains in use today. "AM" is often used to refer to the medium wave broadcast band.
f.m varies the frequency of the carrier. The instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency shift keying.
FM is commonly used at vhf radio frequency for high fidelity brad cast of music and speech like f.m brad casting. Normal (analog) TV sound is also broadcast using FM.
angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It is another term for phase modulation.
Antenna
: antenna (radio)
An antenna (or aerial) is an electrical device which converts electric current sin to radio wave, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating r.f electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving.
Propagation
radio propagation
Once generated, electromagnetic waves travel through space either directly, or have their path altered by reflection ,refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion; some energy may also be absorbed by the intervening medium in some cases. noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications.
Resonance
rresponding to the projected baseline z-component given by the coordinate transformation from the previous lecture, u is the usual E, or y-component spatial frequency, and d is the declination of the phase center. The geometry is as shown in Figure 5.
electrical resonance
: LC circuits
electrical resonance of tuned circuits in radios allow individual stations to be selected. A resonant circuit will respond strongly to a particular frequency, and much less so to differing frequencies. This allows the radio receiver to discriminate between multiple signals differing in frequency.
Receiver and demodulation
: radio receiver design, radio receivers, crystal radio, and communication receiver,
The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is tuned to respond preferentially to the desired signals, and reject undesired signals.
Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator.
radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistors, which made it possible to amplify weak signals. Today radio systems are used for applications from walkies talkie children's toys to the control of space vehicles, as well as for broad casting, and many other applications.
A radio receiver receives its input from an antenna uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifier it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.
Radio band
radio frequency
| light | |||||||
| Name | Wavelength | frequency(HZ) | PHOTON ENERGY(ev) | ||||
|---|---|---|---|---|---|---|---|
| gamma ray | less than 0.01 nm | more than 10 EHZ | 100 keV - 300+ GeV | ||||
| x-ray | 0.01 to 10 nm | 30 PHz - 30 EHZ | 120 eV to 120 keV | ||||
| ultraviolet | 10 nm - 400 nm | 30 EHZ - 790 THz | 3 eV to 124 eV | ||||
| visible | 390 nm - 750 nm | 790 THz - 405 THz | 1.7 eV - 3.3 eV | ||||
| infrared | 750 nm - 1 mm | 405 THz - 300 GHz | 1.24 meV - 1.7 eV | ||||
| microwave | 1 mm - 1 meter | 300 GHz - 300 MHz | 1.24 meV - 1.24 V | ||||
| Radio | 1 mm - km | 300GHZ-3HZ | 1.24 meV - 12.4 feV | ||||
Radio frequencies occupy the range from a few hertz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum. Other types of electromagnetic radiation, with frequencies above the RF range, are infrared, visible light,ultraviolet, x- rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from datum, radio waves are classified as non ionizing radiation
Communication systems
.
A radio communication system sends signals by radio. Types of radio communication systems deployed depend on technology regulation, radio spectrum allocation, user preferment, servies positioning, and investment.
The radio equipment involved in communication system includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system.
The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information not the energy, that was transmitted.
Classical radio communications systems use frequency division multiplexing (FDM) as a strategy to split up and share the available radio frequency band width for use by different parties communications concurrently. Modern radio communication systems include those that divide up a radio-frequency band by time division multifaceted (TDM) and code division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony
A radio communication system may send information only one way. For example, in broadcasting a single transmitter sends signals to many receivers. Two stations may take turns sending and receiving, using a single radio frequency; this is called "simplex." By using two radio frequencies, two stations may continuously and concurrently send and receive signals - this is called duplex operation.
Data (digital radio)
Systems that need reliability, or that share their frequency with other services, may use "coded orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM .
Heating
Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interference with other radio signals. microwaveable use intense radio waves to heat food. diathermy equipment is used in surgery for sealing of blood vessels. Induction furriness are used for melting metal for casting, and induction hub for cooking.
Amateur radio service
Unlicensed radio serRadio amateurs use a variety of modes, including nostalgic ones like Morse code and experimental ones like Low-Frequency Experimental Radio. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband (SSB), AM, digital packet radio and satellite repeaters. Some amateur frequencies may be disrupted illegally by power-line internet service.
Unlicensed radio services
Unlicensed, government-authorized personal radio services such as Citizens' band radio in Australia, the US, and Europe, and Family Radio Service andMulti-Use Radio Service in North America exist to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world. These radio services involve the use of handheld units.
Free radio stations, sometimes called pirate radio or "clandestine" stations, are unauthorized, unlicensed, illegal broadcasting stations. These are often low power transmitters operated on sporadic schedules by hobbyists, community activists, or political and cultural dissidents. Some pirate stations operating offshore in parts of Europe and theUnited Kingdom more closely resembled legal stations, maintaining regular schedules, using high power, and selling commercial advertising time.
Radio control (RC)
Radio remote controls use radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and airplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.
In Madison Square Garden, at the Electrical Exhibition of 1898, Nikola Tesla successfully demonstrated a radio-controlled boat. He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles."
vices
Unlicensed, government-authorized personal radio services such as Citizens' band radio in Australia, the US, and Europe, and Family Radio Service andMulti-Use Radio Service in North America exist to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world. These radio services involve the use of handheld units.
Free radio stations, sometimes called pirate radio or "clandestine" stations, are unauthorized, unlicensed, illegal broadcasting stations. These are often low power transmitters operated on sporadic schedules by hobbyists, community activists, or political and cultural dissidents. Some pirate stations operating offshore in parts of Europe and theUnited Kingdom more closely resembled legal stations, maintaining regular schedules, using high power, and selling commercial advertising time.
Radio control (RC)
Radio remote controls use radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and airplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.
In Madison Square Garden, at the Electrical Exhibition of 1898, Nikola Tesla successfully demonstrated a radio-controlled boat.He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles.
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Figure 3: Beginning of oscillation
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Figure 4: time = 1/4T
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Figure 5: time = 1/2T
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Figure 6: time = 3/4T
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