Advanced Radio and Radar Part 3 AM and FM

Introduction Transmitters 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 output of 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 transmitter will have a power rating of up to 500,000 Watts ie ½ Megawatt.

Let’s examine a basic transmitter AM Transmitter Block Diagram Y Aerial Master Oscillator Buffer Amplifier Power Amplifier Let’s examine a basic transmitter and what it consists of. Microphone Amplifier 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 the required level before radiation to the aerial. Amplifier Amplifies the microphone signal for output.

AM Transmitter Block Diagram Y The modulator adapts Master Oscillator Buffer Amplifier Power Amplifier Modulation Microphone Amplifier Modulation takes place in the power amplifier stage. The modulator adapts the amplitude or frequency of the carrier wave, duplicating the modulating signal.

AM Transmitter Block Diagram Y OUTPUTS RF RF - AF RF + AF RF RF Master Oscillator Buffer Amplifier Power Amplifier Modulation AF AF Microphone 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).

Sidebands 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. 3000Hz 300Hz LOWER UPPER 1 MHz Carrier Band 0.997 MHz 0.9997 1.0003 1.003

Sidebands 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 some range advantage over Double Sideband DSB, but fidelity is too poor for entertainment radio. Although the reduced size and weight is an important factors in aircraft systems. 3000Hz 300Hz LOWER UPPER 1 MHz Carrier Band 0.997 MHz 0.9997 1.0003 1.003

Sidebands Single Sideband (SSB) transmission does have its uses. 3000Hz 300Hz Military Tactical Data Link tends to use the Lower Side Band (in the HF band) Civil & Military long range voice comms tend to use Upper Side Band (in the HF band) LOWER UPPER 1 MHz Carrier Band 0.997 MHz 0.9997 1.0003 1.003 Used extensively by Military and Merchant Navy Used by Armies for beyond line of sight comms The missing sideband is re-synthesised on reception by a SSB Receiver

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. Try tuning through an AM band radio and see how close the stations are together!

AM Transmitter "interference". 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. Try tuning through an AM band radio and see how close the stations are together!

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) AM Signals recovered Signals modulated AM signal poor FM signal good Same Signal With frequency modulation, the carrier wave has a constant amplitude, and a much higher frequency than AM signals. FM

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. Try listening to an AM signal as you pass by an electric pylon or enter a tunnel. The AM signal is distorted or lost, but an FM signal will be largely unaffected by the same conditions. FM is used in the range 88-108 MHz for high quality broadcasting; this frequency range is within a band known as the Very High Frequency (VHF) band. Amplitude Modulation of the carrier Frequency Modulation of the carrier

Frequency Modulation (FM) 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. Try listening to an AM signal as you pass by an electric pylon or enter a tunnel. The AM signal is distorted or lost, but an FM signal will be largely unaffected by the same conditions. FM is used in the range 88-108 MHz for high quality broadcasting; this frequency range is within a band known as the Very High Frequency (VHF) band. Amplitude Modulation of the carrier Frequency Modulation of the carrier

Check of Understanding A buffer amplifier is used in a transmitter to provide isolation from the power amplifier stage. The buffer amplifier also helps prevent: Amplifying the input from the microphone Instability occurring The need for a quartz crystal to keep the transmitter frequency The signal being modulated

Check of Understanding In a diagram of a basic transmitter what does the item ‘S’ represent? Power Amplifier Buffer Amplifier Microphone Master Oscillator

Check of Understanding In a transmitter, the modulator . . . Amplifies the carrier wave. Amplitude or frequency modulates the carrier wave, duplicating the modulating signal. Amplifies the radio wave. Amplitude or frequency modulates the radio wave, duplicating the modulating signal.

Check of Understanding In a diagram of a basic transmitter what does the item ‘Q’ represent? Power Amplifier Buffer Amplifier Microphone Master Oscillator

Check of Understanding What does the abbreviation SSB stand for? Solo Side Band Single Silicone Band Single Side Band Solo Silicone Band

Check of Understanding In a diagram of a basic transmitter what does the item ‘P’ represent? Master Oscillator Power Amplifier Buffer Amplifier Aerial

Check of Understanding In a basic AM Transmitter, where does modulation take place? The Power Amplifier Stage The Buffer Amplifier Stage The Amplifier Stage The Master Oscillator Stage

Check of Understanding In a diagram of a basic transmitter what does the item ‘R’ represent? Power Amplifier Master Oscillator Buffer Amplifier Aerial

Check of Understanding If only one shaded area is used in a radio system, what is it called? Single Side Broadcast Single Sideband Single Carrier Band Single Shift Band

Check of Understanding In a diagram of a basic transmitter what does the item ‘T’ represent? Power Amplifier Master Oscillator Microphone Amplifier

Check of Understanding A few more questions. What is meant by the term Modulation? What do the initials AF and RF stand for? True or False ? FM is a higher frequency than AM. What is the great drawback with the AM system? True or False ? AM is more prone to atmospheric and manmade noise? ANSWERS The combination of low and high-frequency currents into radio waves Audio Frequency and Radio Frequency True The need for such a large bandwidth in a limited frequency spread

Advanced Radio and Radar End of Presentation