Transmitters and Receivers Simple block or “concept” diagrams of transmitters see Table 2. 4a.1 Identify the items in a simple transmitter block diagram and recall their order of interconnection: Microphone, audio (microphone) amplifier stage, frequency generation stage, modulator stage, RF power amplifier stage, feeder and antenna.
The drawing shows the basic parts of a transmitter. The microphone converts speech into an audio signal. The audio stage amplifies the speech. The modulator Mixes the transmit frequency form the oscillator with the speech to produce a modulated radio wave. The RF power amplifier amplifies the modulated radio wave to a higher power which is then radiated by the antenna.
Technical requirements of radio transmitters 4b.1 Recall that the frequency generation stage(s) (e.g. oscillator(s)) in a transmitter defines the frequency on which the transmitter operates. Recall that incorrect setting of these stages can result in operation outside the amateur band and interference to other users. In a radio transmitter the
oscillators determine the frequency of transmission. In the block
drawing of a transmitter shown above there is one oscillator which
generates the output frequency of the transmitter. In more advanced
transmitters there may be several oscillators involved. It is essential that the oscillator
is used to produce a signal in an amateur band. If it is not properly
adjusted an out of band signal could be produced which would be illegal
and could result in the operator losing their licence. In addition it
could cause interference to other users.
4b.2 Recall that the audio (or data) signal is modulated on to the radio frequency ’carrier’ in the modulation stage of the transmitter. The
modulator mixes the audio signal from the microphone or data signal
with the RF signal from the oscillator so that the frequency varies in
time with the audio.
Recall that modulation is by varying the amplitude or frequency of the “carrier”, resulting in AM or FM modulation modes.
There are
different types of modulator. In AM (Amplitude Modulation) the
amplitude of the RF (Radio Frequency) varies in time with the
audio. In FM (Frequency Modulation) the frequency of the RF moves in time with the audio.
Recall that speech can be carried by AM/SSB or FM and that data may be transmitted by means of suitable audio tones generated in a radio modem or TNC (terminal node controller).
There are several methods of carrying speech. AM, FM and SSB (Single Side Band) Data is sent by converting the input
(E.g. from a keyboard) into a series of audio tones. This is produced
using a computer sound card or a TNC (Terminal Node Controller).
4b.3 Identify drawings of an RF carrier and amplitude modulated, frequency modulated and CW radio signals. Understand the terms carrier, audio waveform and modulated waveform.
In order to see a radio wave we need
to use an oscilloscope or a spectrum analyser. An oscilloscope shows how
the wave changes with time. A spectrum analyser shows how the carrier
varies with frequency.Some modern radios have a display of the whole of a band or part of a band as you would see them on a spectrum analyser.
The simplest wave is a CARRIER. This is an unmodulated wave. It is switched on and off to send CW (Morse Code). By adding the speech to the audio
using an AM modulator the height (or amplitude) of the carrier changes
in time with the speech audio.
By adding the speech to the audio using an FM modulator, the frequency of the carrier changes in time with the speech audio.
Carrier Wave turned on and off to send Morse Code
4b.4 Recall that the power amplification of the radio signal is carried out in the final stage of the transmitter. (RF power amplifier).
The purpose of the RF Power Amplifier
is to increase the strength of the modulated signal to a level that
will be suitable for communication. The power amplifier is the last active stage in a transmitter before the antenna.
4b.5 Recall that the RF power amplifier output must be connected to a correctly matched antenna to work properly and that use of the wrong antenna can result in damage to the transmitter.
The output of an RF power amplifier
is designed to match a 50 Ohm load. The load is an antenna. The antenna
must also look like a 50 Ohm resistance. If the antenna looks like a
different resistance (e.g. 200 Ohms) then this could result in damage
to the RF amplifier transistors.A dipole cut to the required frequency and fed in the centre with 50 Ohm feeder should be approximately 50 Ohm.
4b.6 Understand that excessive amplitude modulation causes distorted output and interference to adjacent channels. Understand that excessive frequency deviation will cause interference to adjacent channels. Recall the need to ensure that the microphone gain control (where fitted) is correctly adjusted.
When using a transmitter it is
important to have the microphone gain control set at the correct level.
If it is set too high AM and SSB signals will be distorted causing
interference to adjacent channels. If the setting is too high on an FM
transmitter the signal will be wider than normal and could cause
interference to stations on adjacent channels.
