Radio- Transmitters, Final)

8/6/2019 Radio- Transmitters, Final)

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Radio Waves

Radio waves carry music, conversations, pictures,

and data invisibly through the air over millions of

miles.

Radios can transmit and/or receive radio waves.


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Frequency of AM & FM

When you listen to AM broadcast, radio is tuninginto sine waves oscillating at a frequency around1,000,000 cycles per second.

FM signals operate in range of 10,000,000 Hz. So,90.9 on FM dial corresponds to 90,900,000 Hz =90.9 MHz


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Radio Basics

Any radio setup has two parts: TransmitterandReceiver

Transmitter takes some form of message

(someones voice, pictures forTV set, etc.) encodesit into a sine wave and transmits it with radio waves.

Combination of encoded message on a radio waveis commonly referred to as a signal.

Receiver receives radio waves and decodesmessages from the sine waves.

Both transmitter and receiver use antennas to

radiate and capture radio waves.


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TransmitterDescription

Information

(voice message)

Combine

Sine

Wave

Radio Transmitter

Antenna

RadioWaves

Transmitter generates its own sine wave using oscillators.


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ReceiverDescription

Separate

SineWave

Radio Transmitter

Antenna

Information

(voice message)


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Modulation

If you have a sine wave and a transmitter that is

transmitting the sine wave into space using an

antenna (more antennas later), you have a radio

station.

Sine wave has to be modulated in some way so

that it contains information, e.g., voice message.


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Basic Modulation Methods

Pulse Modulation (PM): turn sine wave on and off,

and an easy way to send Morse code.

Amplitude Modulation (AM): Amplitude (peak-to-

peak voltage) of sine wave is changed so as to

contain information.

AM radio stations and picture part ofTV signals use

amplitude modulation to encode information signal.


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Example of AM

carrier = sine wave with a given frequency


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Basic Modulation Methods:

Frequency Modulation (FM): Radio transmitter

changes frequency of sine wave according to

information signal.

Frequency modulation is most popular. Used by FM

radio stations, sound part ofTV signal, cellular

phones, cordless phones, etc.


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Frequency of Signal after

Modulation

Radio wave transmitted after modulating sine wavewith information signal is not just a sine wave withfrequency f.

For example, in FM, the frequency varies aroundthis frequency f. For example, it may increase up tof+(f and be as small as f-(f.

After modulating information signal, the radio wavehas some range of frequency, called the frequencyband, e.g., 2(f.

The bandwidth, width of frequency band, dependson the information signal (voice, data bit rate, etc.)


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Summary of Modulation

By modulating a sine wave at a transmitter,

information can be encoded into the radio wave.

The resulting radio wave occupies a band of

frequency, centered on the frequency of the sine

wave.

Receiver needs to demodulate the radio wave to

extract the information signal.


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How do we receive AM

signals?

Unless you sit right next to the transmitter, you needan antenna to pick out the radio waves from the air.

An AM antenna is just a wire or a metal stick thatincreases the amount of metal the transmitterswaves interact with.

Radio receiver also needs a tuner. Antenna willreceive thousands of sine waves; tuner separatesout the radio wave that the listener desires, e.g., theradio wave transmitted at 880 KHz.


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AM Reception

Tuners operate using a principle called resonance.

That is, tuners resonate at and amplify one

particular frequency and ignore all other frequencies

in the air.

This is done using a demodulator


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AM Reception

One type of a AM detector is something called an

envelope detector. Simply, it determines the

magnitude (amplitude) of the sine wave.

An amplify magnifies this amplitude signal and then

the receiver sends the output to the car radio

speakers.


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Frequency Modulation

In this the instantaneous frequency of the

carrier is caused to vary by an amount

proportional to the amplitude of the modulating

signal. The amplitude is kept constant.

More complex than AM this is because it

involves minute changes in frequency

FM is more immune to effects of noise

FM and PM are similar


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What about FM?

Difference: FM detector outputs changes in the sinewave frequency as opposed to amplitude.

Specifically, FM detector converts changes in sinewave frequency into sound.

Antenna, tuner, amplifier are largely the same in FM

as in AM.

The theoretical bandwidth required for FMtransmission is infinite


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Common b/w AM & FM:

In both systems a carrier wave is modulated by an audio

signal to produce a carrier and sidebands. The technique

can be applied to various communication systems eg

telephony and telegraphy

Special techniques applied to AM can also be applied to

FM

Both systems use receivers based on the super-

heterodyne principle.


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Transmitters

In an AM transmitter, provision must be made for varying

the carrier amplitude whilst for FM the carrier frequency

is varied.

AM and FM modulators are therefore essentially different

in design. FM can be produced by direct frequency

modulation or by indirectly phase modulation.

The FM carrier must be high usually in the VHF band as

it requires large bandwidth which is not available in the

lower bands.


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Receivers

The FM and AM receivers are basically the same,

however the FM receiver uses a limiter and a

discriminator to remove AM variations and to convert

frequency changes to amplitude variations respectively.As a result they (FM) have higher gain than AM.

FM receivers give high fidelity reproduction due to their

large audio bandwidth up to 15 kHz compared with about8 kHz for AM receivers.


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Radio Frequencies

A radio wave is an electromagnetic wave

propagated by an antenna.

Radio waves have different frequencies and by

tuning a radio receiver to a specific frequency, youcan pick up a specific signal.


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Radio Frequencies

Frequency Band

10 kHz to 30 kHz Very Low Frequency (VLF)

30 kHz to 300 kHz Low Frequency (LF)300 kHz to 3 MHz Medium Frequency (MF)

3 MHz to 30 MHz High Frequency (HF)

30 MHz to 328.6 MHz Very High Frequency (VHF)

328.6 MHz to 2.9 GHz Ultra High Frequency (UHF)

2.9 GHz to 30 GHz Super High Frequency (SHF)30 GHz and above Extremely High Frequency (EHF)


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Frequency Bands:

AM radio stations must use frequencies in 535 KHz

to 1.7 MHz band.

FM radio stations transmit in band of frequenciesfrom 88 MHz to 108 MHz


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Why is AM radio at a lowerband

than FM radio?

AM was invented before FM.

Transmitting at higher frequencies means thatelectronic equipment has to be faster.

Fast enough to oscillate and detect highly-changingsignals.

When AM radio invented electronic capabilities werefairly limited (compared to nowadays).

Hence lower frequencies were allocated. Later when FM radio was developed, it was

assigned unused frequencies at a higher band.


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Radio Propagation: Radio Channel

There is another very important player in the wireless

game: the physical environment over which radio

waves travel.

Radio waves can take many different paths to getfrom transmitter to receiver.

Transmitter

Receiver


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Radio Channel

Essentially, the radio waves interact with the

physical environment along each of these paths.

There are typically (unless you are in free-space)

many paths from the transmitter to the receiver.

Each path is called a multipath.

Radio- Transmitters, Final)

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