Advanced Radioand Radar

Part 3AM and FM

IntroductionTransmitters come in all shapes and sizes.

Man-made satellites, Your mobile phone,

The car alarm remote,WiFi home hub

Such devices can have a very small power outputof only about ½ Watt

(not enough to light a single Xmas tree light)up to hundreds of Watts for a satellite.

but a Medium Frequency (MF) radio transmitterwill have a power rating of up to 500,000 Watts

ie ½ Megawatt.

AM Transmitter

Master Oscillator Generates the carrier wave at the required RF frequency.

Buffer Amplifier Isolates the oscillator from the power amplifier,and prevents instability occurring.

Power Amplifier Used to increase the power of the signal to therequired level before radiation to the aerial.

Amplifier Amplifies the microphone signal for output.

Master Oscillator

Y

Buffer Amplifier

Power Amplifier

Amplifier

Block Diagram

Let’s examine a basic transmitterand what it consists of.

Aerial

Microphone

AM Transmitter

Master Oscillator

Y

Buffer Amplifier

Power Amplifier

Amplifier

Modulation takes place in the power amplifier stage.

The modulator adapts

the amplitude or frequency of the carrier wave,

duplicating the modulating signal.

Block Diagram

Modulation

Microphone

AM Transmitter

Master Oscillator

Y

Buffer Amplifier

Power Amplifier

Amplifier

Modulation takes place in the power amplifier stage.

If frequencies to the modulator are RF from the oscillator and AF from the microphone,

output of the power amplifier will consist of 3 frequencies:

a. The carrier (RF).b. The carrier minus tone frequency (speech) (RF – AF).

c. The carrier plus tone frequency (RF + AF).

RF

AF

RF

AF

OUTPUTSRF

RF - AFRF + AF

Block Diagram

Modulation

Microphone

Sidebands

UPPERLOWER

3000Hz

300Hz

3000Hz

300Hz

0.997MHz

0.9997MHz

1.0003MHz

1.003MHz 1 MHz

Carrier Band

If the carrier frequency was 1 MHz, and audio frequency ranged from 300 to 3000 Hz, then frequencies in the output would look like this:

There are two sidebands – upper and lower.

Power is shared between sidebands & carrier,each sideband being the mirror image

of the other.

Sidebands

UPPERLOWER

3000Hz

300Hz

3000Hz

300Hz

0.997MHz

0.9997MHz

1.0003MHz

1.003MHz 1 MHz

Carrier Band

Some operation modes use only one sideband, Single Sideband (SSB) transmission.

But receiving this on an ordinary AM radio; the audio would be utterly garbled !

This mode has somerange advantage over Double Sideband DSB, but fidelity is too poor for entertainment radio.

Although the reduced size and weightis an important factors in aircraft systems.

Single Sideband (SSB) transmissiondoes have its uses.

Sidebands

UPPERLOWER

3000Hz

300Hz

3000Hz

300Hz

Military Tactical Data Link tends to use theLower Side Band(in the HF band)

0.997MHz

0.9997MHz

1.0003MHz

1.003MHz 1 MHz

Carrier Band

Used extensively byMilitary and Merchant Navy

Used by Armies for beyondline of sight commsThe missing sideband is re-synthesised

on reception by a SSB Receiver

Civil & Military long range voice comms tend to use Upper Side Band

(in the HF band)

AM Transmitter

The great drawback with the AM system is the need for such a large bandwidth

(all frequencies with both sidebands - approximately 6KHz)

in a limited frequency spread (30 KHz to 3 MHz).

This means the AM system could only have

148 stations at any one time.

AM Transmitter

With so many AM transmitters crammed into a small band there is a problem of signals breaking into one another –

"interference".

To overcome this, short-range frequency modulated systems

have become popular.

Frequency Modulation (FM)

With radio Frequency Modulation (FM); audio or data information is conveyed

over a carrier wave by varying its instantaneous frequency.

With amplitude modulation (AM); the amplitude of the carrier is varied,while its frequency remains constant.

With frequency modulation, the carrier wave has a constant amplitude,

and a much higher frequency than AM signals.

Frequency Modulation (FM)

Signals recovered Signals modulated AM signal poor

AM

FM

FM signal goodSame Signal

With frequency modulation, the carrier wave has a constant amplitude,

and a much higher frequency than AM signals.

Frequency Modulation (FM)Modulation is achieved by shifting

the carrier frequency up and down slightly in step with the tone frequency.

This shift is small and gives better results because it is less prone to atmospheric or manmade noise.

Amplitude Modulationof the carrier

Frequency Modulationof the carrier

Frequency Modulation (FM)

Amplitude Modulationof the carrier

Frequency Modulationof the carrier

Try listening to an AM signal as you pass an electric pylon or enter a tunnel.

The AM signal is distorted or lost, but an FM signal will be largely unaffected.

FM is used for high quality broadcasting; within a Very High Frequency (VHF) band.

Check of UnderstandingA buffer amplifier is used in a transmitter

to provide isolation from the power amplifier stage.The buffer amplifier also helps prevent:

The signal being modulated

Amplifying the input from the microphone

The need for a quartz crystal to keep the transmitter frequency

Instability occurring

In a diagram of a basic transmitter what does the item ‘S’ represent?

Buffer Amplifier

Power Amplifier

Master Oscillator

Microphone

Check of Understanding

Check of UnderstandingIn a transmitter, the modulator . . .

Amplitude or frequency modulates the radio wave,

duplicating the modulating signal.

Amplifies the carrier wave.

Amplifies the radio wave.

Amplitude or frequency modulates the carrier wave,

duplicating the modulating signal.

In a diagram of a basic transmitter what does the item ‘Q’ represent?

Buffer Amplifier

Power Amplifier

Master Oscillator

Microphone

Check of Understanding

Solo Silicone Band

Solo Side Band

Single Side Band

Single Silicone Band

Check of UnderstandingWhat does the abbreviation SSB stand for?

Aerial

Master Oscillator

Buffer Amplifier

Power Amplifier

Check of UnderstandingIn a diagram of a basic transmitter what does the item ‘P’ represent?

In a basic AM Transmitter,where does modulation take place?

The Buffer Amplifier Stage

The Power Amplifier Stage

The Master Oscillator Stage

The Amplifier Stage

Check of Understanding

Aerial

Power Amplifier

Buffer Amplifier

Master Oscillator

Check of UnderstandingIn a diagram of a basic transmitter what does the item ‘R’ represent?

If only one shaded area is used in a radio system, what is it called?

Single Shift Band

Single Side Broadcast

Single Carrier Band

Check of Understanding

Single Sideband

In a diagram of a basic transmitter what does the item ‘T’ represent?

Microphone

Power Amplifier

Amplifier

Master Oscillator

Check of Understanding

A few more questions.

1. What is meant by the term Modulation?

2. What do the initials AF and RF stand for?

3. True or False ? FM is a higher frequency than AM.

4. What is the great drawback with the AM system?

5. True or False ? AM is more prone to atmospheric andmanmade noise?

Check of Understanding

Advanced Radioand Radar

End of Presentation