South Bristol ARC - Foundation Course
South Bristol Amateur Radio Club
Lesson 3 – Transmitters and Receivers
Syllabus sections 4a.1, 4b.1 – 4b.6, 4c.1, 4d.1 – 4d.2
(Syllabus section 4a.1)
Figure 1, below, illustrates the stages in a simple Transmitter.
Figure 1 - Block Diagram of a Simple Transmitter
The order of these components is important:
Sound waves enter the Audio Stage (1) from the microphone. The Modulator (2) picks up the audio and mixes it with the RF signal generated by the Frequency Generator (3). The RF Power Amplifier (4) then amplifies the signal to be radiated by the Antenna.
Technical Requirements of Radio Transmitters
(Syllabus section 4b.1 – 4b.6)
The Frequency Generator (3) is sometimes referred to as the Oscillator, both terms being equally correct. Irrespective of what it is referred to as, its purpose is to set the frequency on which the transmitter operates (the carrier frequency). Effectively when you turn the tuning knob or channel selection switch on the front of a transceiver it is the Frequency Generator or Oscillator that is being adjusted.
Whilst many modern transmitters and transceivers contain interlocks and logic circuitry that prevent the transmitter being operated when the Frequency Generator is set to frequencies outside the allocated amateur bands it remains the responsibility of the operator to ensure that out of band transmissions do not occur.
With older equipment, home brewed equipment, equipment not designed for the UK market or modified for wide band operation it is possible to set the Frequency Generator to a carrier frequency outside of the amateur bands and therefore transmit out of band. Be careful when using unfamiliar or adapted equipment to ensure that this does not occur.
The audio (or data) signal is modulated onto the RF carrier in the modulation stage of the transmitter. Where the RF carrier is modulated by varying its amplitude then the signal is said to be Amplitude Modulated (AM), as shown below:
Figure 2 - Amplitude Modulation
Where the RF carrier is modulated by varying its frequency the resulting signal is said to be Frequency Modulated (FM) as shown below:
Figure 3 - Frequency Modulation
A variation of AM is a mode called single side band (SSB). Speech can be carried by AM, SSB, or FM and computer data may be transmitted by suitable audio tones generated in a modem or TNC (terminal node controller). This would use frequency shift keying (FSK)
The mode used to send Morse code is called CW (continuous wave) where the unmodulated carrier wave is switched (keyed) on and off.
The transmitter must be connected (loaded) to a correctly matched antenna to ensure that the maximum amount of power is radiated. The wrong kind of antenna can damage the transmitter (more about this in lesson 5).
Excessive amplitude modulation can cause distorted output and interference to adjacent channels. This is often referred to as “over modulation”. Excessive frequency deviation will cause interference to adjacent channels.
Excessive modulation can be prevented by a correctly adjusted microphone gain control (if fitted). This will also apply if a TNC is used. It is also important to remember that settings of the microphone gain control are unique to a single operator and the equipment (microphone, TNC etc.) in use.
Changes to any of the equipment should be accompanied by a careful resetting of the gain controls. Switching from a microphone input to a TNC should be included in this process, although many modern rigs incorporate a “data” jack allowing the tones from a TNC or soundcard to be introduced separately to the microphone allowing the gain controls to be independently set for both modes.
As a Foundation licensee you can only use type approved transmitting equipment. No home brew is allowed in terms of transmitting equipment. You may, however, home brew accessories, adapters, soundcard interfaces, antennas and other receiving kits. The restriction is solely on the construction of transmitting equipment.
Some terms you need to remember:
(Syllabus section 4d.1 - 4d.2)
Figure 4 - Block Diagram of a Simple Receiver
The order of these components is important:
The antenna picks up Radio Frequencies that impinge upon it and convert all of the received signals into electrical signals passed along the feeder to the input to the receiver. The tuning stage (1) tunes to the required frequency, isolating it from the other signals received, and amplifies the incoming signal to a level that can be used in the next stage. The modulated RF waveform is passed to the Detection stage (2) which recovers the original audio (or data) waveform from the modulated RF carrier, a process sometimes referred to as demodulation. The output is passed on to the Audio Amplifier (3), which then sends the audio signal to the Loudspeaker (4).
The tuning stage selects one signal from many on different frequencies by a tuned circuit.
A tuned circuit consists of a coil (inductor) and a capacitor.
It is important that the demodulation method is compatible with the original modulation method otherwise partial or no information may be received if a FM signal is received using AM demodulation for example.
Technical Basics 02
Prefixes and Ohms Law
Lesson 3 – Summary
At the end of this lesson you should be able to: