Transmitters & Receivers Foundation Course Transmitters & 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 AM/FM/SSB 3=RF Frequency Generation 4=RF Power Amplifier

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!

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.

AM (Amplitude Modulation) Audio Input (note that excessive audio may cause distortion & interference). RF Carrier AM Modulated signal

FM (Frequency Modulation) Audio Input (excessive audio will cause excessive frequency deviation/interference) RF Carrier FM Modulated signal

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.

CW and FSK Modulation CW (Morse code) is the simplest data mode. FSK Frequency Shift Keying is used for higher speed Packet data. FSK Signal CW Signal Keyer /Data

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.

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).

Revision Questions

The bottom waveform in the diagram shows A [ ] audio wave B [ ] carrier wave C [ ] amplitude modulated wave D [ ] frequency modulated wave

Answer : D) The Frequency Modulated wave

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.

Correct answer : D) Interfere with adjacent frequencies.

Thurrock Training Team