Modulation (4b2, 4b3 & 4b6)

 

The audio (sound) or data signal is modulated on to the radio frequency ”Carrier” in the modulation stage of the transmitter. 

 

There are several types of modulation used.  The ones that you need to know for the Foundation Licence are shown below.   In all cases it is the modulated signal tht is transmitted.

 

For speech the main types are a.m. (and a special variation of this called s.s.b) and f.m.

 

For data the main types are c.w. and f.s.k.    

 

Amplitude Modulation (a.m.)

 

 

Audio Signal + Radio Carrier = Amplitude Modulated Signal

 

Note that the height of the Amplitude Modulated Signal changes according to the audio content.  Loud sounds result in a bigger signal.  Quiet sounds give a smaller signal.

 

Frequency Modulation (f.m.)

 

 

Note that the height of the Frequency Modulated Signal does not change.  The signal just changes frequency slightly.  This slight change is called the deviation.

 

c.w. (Morse Code) and f.s.k. (Frequency Shift Keying) Data Modulation

 

 

Keyer Data Note that the keyer data is either on or off.  For example the Morse Key is either pressed down (on) or it is up (off).

 

CW Signal When the Morse Key is pressed down, the CW signal is generated and transmitted.   When the key is not being pressed down no signal is generated or transmitted.  So with CW either a transmitter is on or off.

 

FSK Signal When the Morse Key is pressed down, the signal changes frequency.  When the key is not being pressed down, the signal changes back to the normal frequency.  So with FSK the transmitter either on one frequency or another just slightly different frequency.

 

Excessive Modulation

Excessive amplitude modulation (too loud audio signals) can cause distorted output and interference on adjacent channels.

 

Excessive frequency deviation (too loud audio signals) can cause interference to adjacent channels.  

 

Where a transmitter is fitted with a microphone gain control, it should be adjusted to not over modulate (a.m.) or over deviate (f.m.).  The same also applies when using equipment such as a packet radio controller (TNC).   

 

      

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