Transmitters amp Receivers Transmitters The diagram is only to show the basic functions within a transmitter not to relate to any particular transmitter 1 Audio Stage 2 Modulator AMFMSSB ID: 425498
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Slide1
Foundation Course
Transmitters & ReceiversSlide2
Transmitters
The diagram is only to show the basic functions within a transmitter not to relate to any particular transmitter.
1= Audio Stage
2= Modulator AM/FM/SSB
3=RF Frequency Generation
4=RF Power AmplifierSlide3
Transmitters (Frequency generation)
The frequency generation stage in a transmitter (stage 3 on the previous page) defines the frequency on which the transmitter operates.
If this stage creates frequencies incorrectly, possibly by being badly built, then the output from the transmitter will also be wrong.
If the margin of error is significant, it could lead to operation outside of the amateur band and interference to other users.
Therefore if operating telephony (speech) close to band edges (even with a transmitter working fully to specification) could result in accidental operation out of band- be careful!Slide4
Modulation
The method of superimposing audio or data information onto an RF carrier frequency is termed “modulation” (or mode).
In our Transmitter diagram on the first slide, stage 2 (the modulator ) mixes together the signal from the Audio stage and the Frequency generator stage in preparation for amplification in the 4th Stage.
The modulation stage determines whether the output signal will be AM or FM depending whether the amplitude is changing hence Amplitude modulation or the Frequency is changing hence Frequency Modulation.Slide5
AM (Amplitude Modulation)
Audio Input (note that excessive audio may cause distortion & interference).
RF Carrier
AM Modulated signalSlide6
FM (Frequency Modulation)
Audio Input (excessive audio will cause excessive frequency deviation/interference)
RF Carrier
FM Modulated signalSlide7
Speech and Data modes
Speech can be carried on AM, FM or SSB (which is AM with the carrier and one sideband removed).
Excessive speech audio can be controlled by correct adjustment of the microphone gain (where fitted).
Data can also be carried on AM, SSB or FM but requires that the data is passed through a radio modem (modem = modulator /demodulator) to change the 0 or 1 of the data code into two different tones.
One particular data mode is “Packet" which requires a “TNC” or(Terminal Node Controller). The TNC converts the typed computer letters into data which is then combined into "packets“ sent at high speed as bursts of data.Slide8
CW and FSK Modulation
FSK Signal
CW Signal
Keyer /Data
CW (Morse code) is the simplest data mode.
FSK Frequency Shift Keying is used for higher speed Packet data.Slide9
RF Power Amplifier
The RF Power Amplifier stage is the final stage, amplifies the modulated signal and is connected to the radio’s antenna connection.
An antenna has to be matched to the transmitter frequency. If not the PA stage of the transmitter could be damaged. An ATU (antenna tuning unit) may be used to properly match the antenna and feeder to the transmitter output impedance which is usually 50 ohms
If you use the wrong antenna or no antenna at all then damage to the transmitter could result.Slide10
Receivers
1)The first stage selects (or tunes) to the signal and amplifies it. Just as with the Transmitter the tuning dial is linked to this section.
2)The Detection / demodulation stage recovers the original modulating signal whether CW, AM, USB, LSB, FM etc.
3)Even when detected, the signal is still to small to hear so the signal is sent to an audio amplifier which amplifies it so that you can hear it from a speaker or headphones (4).Slide11
Revision QuestionsSlide12
The bottom waveform in the diagram shows
the
A [ ] audio wave
B [ ] carrier wave
C [ ] amplitude modulated wave
D [ ] frequency modulated waveSlide13
Answer : D) The Frequency Modulated waveSlide14
Too much audio gain will cause a transmitter to
A [ ] stop working
B [ ] increase the SWR
C [ ] interfere with other bands
D [ ] interfere with adjacent frequencies.Slide15
Correct answer : D) Interfere with adjacent frequencies.Slide16
Thurrock Training Team