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10. Modulation

Definitions

  • In Frequency Modulation, the speech causes the frequency of the carrier to go up and down
  • Narrow Band Frequency Modulation (NBFM) means the bandwidth of an transmission must be no more than the 6kHz permitted on AM
  • The frequency of the modulating frequency controls the rate at which the carrier swings up and down from nominal.
  • Capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency or channel will be demodulated
  • CW is "Carrier Wave" and sometime for "Continuous Wave"
  • BFO Beat Frequency Oscillator
  • Splatter interference is caused by over-modulation of a transmitter
  • Spurious emission is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies.
  • Key Chirp is a short frequency shift in the transmitter as it is being "pulled off" slightly off frequency due to the keying or instability of the oscillator.
  • Key Click are generally undesired "clicks" or "thumps" generated by a CW transmitter as the key is put down or let up due to the quick rise and fall times of the transmitted signal.

Diagrams

  • AM and FM

  • Spectrogram of AM vs SSB

Circuit diagrams

Formula

  • Lower sideband = Fc - Fm where Fc is the carrier frequency, Fm is the modulating frequency
  • Upper sideband = Fc + Fm where Fc is the carrier frequency, Fm is the modulating frequency
  • Modulation index = Deviation from carrier centre frequency / Audio frequency producing this deviation
  • Deviation Ratio = Maximum Deviation of the Carrier / Maximum modulating frequency

Graphs

Notes

Amplitude Modulation (Double Side Band (DSB))

  • Starts with a simple carrier wave.
  • The amplitude of the modulating signal (eg. your voice) varies the amplitude of the carrier wave. (See image above)

Problems with AM

  • Not an efficient method of communication as the "speech content" of the total signal is small.
  • The signal has two side bands, upper and lower, approximately +/- 3 kHz from the carrier respectively.
  • Both side bands carry the same information.
  • Hence the RF power is split between the carrier, upper and lower sidebands.
  • Thus, it is very inefficient power wise and bandwidth wise too!

Single Side Band (SSB)

  • Improved form of AM modulation.
  • Supresses the carrier signal and also one side band too!
  • More efficient as power is only given to one side band.
  • Less bandwidth needed on air.

Over modulation

  • Avoid over modulation
  • Distorting the amplitude modulated signal and cause undue bandwidth and interference
  • This results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal
  • If over-modulation does occur the carrier is chopped up and the modulation no longer resembles its modulating signal
  • Signal will have an increased bandwidth
  • Signal will be rich in unwanted harmonics
  • Waveform of overmodulated signal:

Prevent over-modulation:

  • an indication is given when the depth of modulation exceeds, say, 80%
    • monitoring the output with an oscilloscope or by a modulation meter
    • a lamp or LED lights when a predetermined level is exceeded
  • By automatic means, whereby the modulating signal is prevented from exceeding an amplitude that would result in over-modulation

Frequency modulators

  • Most important component is the variable capacitor diode:
    • connected across the tuned circuit of an oscillator
    • reverse biased to a linear part of the Volts/Capacitance curve
    • A conducting diode would damp the tuned circuit and may stop the oscillator

Interference in AM / FM

  • unwanted spikes of interference modulating the amplitude of the carrier
  • the clipper can remove all of the unwanted amplitude modulation and leave the carrier intact
  • changing frequency of the carrier contains all the necessary information to give an undistorted interference free audio output

Weak FM

  • an FM signal has to be reasonably strong to attain the advantage of "interference free reception"
  • a very weak FM signal, will sound much noisier than an AM signal of the same strength

An FM signal, having constant amplitude, is less likely to cause Interference to nearby television and radio receivers.

Morse transmission

  • Simple transmitters for A1A:

    • Consists of an RF oscillator and a power amplifier stage.
    • When the Morse key is pressed down a carrier (a single frequency) is transmitted
    • When the Morse key is released then no carrier is transmitted, and thus no signal at the receive end.
    • This signal requires a receiver with a BFO (Beat Frequency Oscillator) so that you can hear the "dits" and "dahs" of the Morse Code
    • Oscillator must should not be keyed as this causes "chirping"
    • Chirp will be heard at the receiver end as a slight change in audio frequency too!

  • Complex transmitters for SSB:

    • send a carrier continuously but connect the Morse Key to an audio oscillator
    • When the Morse Key is pressed the carrier is Amplitude modulated with this audio tone
    • When the key is released the tone is switched off but the carrier wave continues

  • A2A

    • leave the audio tone modulating the carrier all the time
    • switch the whole modulated signal on and off using the Morse Key
    • Where a tone is added (often referred to as a "sub carrier") then this is given the code A2A
    • an A2A transmission uses a modulating signal which is keyed by a tone or sub-carrier resulting in a double side band amplitude modulated signal
    • can be received with an ordinary receiver, with out the need for a BFO