The Oscillator

• The basis of any transmitter is the oscillator
• The oscillator generates radio frequency energy at a specific frequency
• The radio frequency is called the carrier
• Turning the oscillator on and off with a key allows morse code to be sent (this is the very basic form of a transmitter)

Below is a concept diagram of a simple Morse code transmitter (click on the image to find out more about Electronic Oscillators).

• Power supply – provides DC power to the oscillator
• Morse key – switches the power on and off to the oscillator
• Oscillator – generates the radio frequency (RF) carrier
• Antenna – radiates the RF signal into free space

A simple radio transmitter is built from the following components (click on the image below to learn more about Radio Transmitter Design).

• Microphone – converts sound waves into electric currents
• Audio Amplifier – boosts the low voltage and currents produced by the microphone to a level that can be processed
• Oscillator – generates the radio frequency carrier
• Modulator – combines the audio frequency with the radio frequency
• Amplifier – amplifies the combined signal for transmission to the antenna
• Antenna – converts the combined signal into electromagnetic waves for transmission into free space

Types of Amplitude Modulation

• Single Sideband (SSB) is a form of amplitude modulation
• The power transmitted on SSB is dependent of the volume of the operators voice
• The level of the voice signal fed into the transmitter is controlled by the microphone gain control

Foundation licence holders are permitted to transmit no more than 10 watts on SSB

• On SSB the voice peaks (loudest volume) should not cause the transmitter to exceed 10 watts
• This power is 10 watts peak envelope power (PEP)

Concept diagram of a simple radio receiver

A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena.^[1] This component is now called a diode.

• Antenna – converts received electromagnetic waves (RF signal) into very low voltage and currents
• RF Amplifier – boosts the very low signal from the antenna so that it can be processed by the detector
• Detector – separates the audio signal from the RF carrier signal
• Audio Amplifier – boosts the audio signal so it can drive a speaker
• Speaker – converts the audio signal into sound waves that can be heard

Modulation

• Voice over radio is “Radio Telephony”
• Superimposing the voice frequencies on to the carrier wave is called modulation
• Superimposing the voice onto the amplitude of the carrier is called amplitude modulation (AM)
• Superimposing the voice frequency onto the frequency of the carrier is called frequency modulation (FM)

Importance of proper transmitter adjustment

• Improper adjustment of a transmitter can cause harmful interference to other radio communications users, both inside and outside the frequency bands allocated to Amateurs
• It is a requirement that all components of transmitter emissions must be contained within the radio frequency bands allocated to Amateurs

Transmitter output matching

• The final power amplifier stage of a transmitter must be connected to a correctly matched transmission line and antenna to avoid possible damage to the transmitter and/or cause interference to other radio communication services
• More on this topic in Section 5 – Antennas and Transmission Lines

Transceiver controls

• AF (Audio Frequency) also called volume control, adjusts the audio output level
• RF (Radio Frequency) or receiver gain control, adjusts the receiver gain to decrease the noise received from a nearby strong station
• Squelch control enables the receiver to cut out the noise when there is no signal present
• Mode is used to select between transmission modes such as CW, SSB, FM, AM etc.
• VFO (Variable Frequency Oscillator) is used to change the operating frequency
• RIT (Receiver Incremental Tuning) is used to adjust the receiver frequency without changing the transmitter frequency to compensate for differences in frequencies of other stations
• Band switch changes the operating band of the transceiver
• Carrier control adjusts the carrier frequency in CW mode

See if you can find all of the above controls on the radio images at the top of this page…

Effects of over modulation

• With Frequency Modulation (FM) the output power remains constant independent of voice level
• A 10 watt FM transmitter will transmit 10 watts at all times
• The frequency of the carrier changes when voice signals are fed into the transmitter
• The amount of frequency change is called deviation
• The amount the carrier deviates is dependent on the level (volume) of the voice signal
• The more deviation the greater the bandwidth required to transmit the signal
• Either AM or FM carriers that are over modulated may cause distorted output and interference to adjacent frequencies
• The AF gain (microphone gain) should be correctly adjusted to avoid over modulation of AM and FM transmitters

Receiver terms

• The receiver is designed to select the required signal, detect the modulation and amplify the voice signal
• The receiver converts the voice signal back to the way it was when spoken
• A simple receiver is the tuned radio frequency receiver
• The circuit that converts the modulated signal back into audio is called a detector
• FM detectors are called discriminators

Sensitivity is the ability of the receiver to receive weak signals

Selectivity is the ability of the receiver to select between stations that are close together in frequency

Stability is the ability of the receiver to stay on the same frequency over long periods of time where there are significant changes in temperature