Chelmsford Amateur Radio Society Intermediate Course (4) Transmitters

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Chelmsford Amateur Radio Society Intermediate
Course(4) Transmitters
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Transmitters

• Summary
• Block diagrams of transmitters
• Oscillators for generating a carrier
• Operation of mixers
• Modulators
• AM, FM, and SSB modulation
• Harmonics
• Filters

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CW Transmitter

• Block diagram of CW transmitter
• Keying stage switches RF on and off
• Slow rise and fall time of RF envelope will avoid
  excess bandwidth

RF oscillator
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AM Transmitter

• Block diagram
• In Modulator the audio modulates the RF amplitude

RF oscillator
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SSB Transmitter

• Block diagram
• Modulator produces double-sideband
  suppressed-carrier
• Sideband filter suppresses unwanted sideband

RF oscillator
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FM Transmitter

• Block diagram
• Audio used to modulate frequency of RF oscillator

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LC Oscillator

• Circuit of a Colpitts LC oscillator
• L1 and C1 determine frequency

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LC oscillator (VFO)

• Varying L or C changes frequency
• Drifts with temperature, supply voltage, output
  load
• Nearby objects affect L or C
• Modulated by supply noise and vibration
• Requires rigid screened construction
• Requires regulated filtered supply
• Needs to be calibrated
• Either by adjusting the dial
• Or by adjusting L C with trimmers
• Prevent drift causing operation outside Amateur
  bands

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Crystal Oscillator

• Circuit of a crystal oscillator
• XL1 determines frequency

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Crystal Oscillator

• Crystal determines frequency of oscillation
• Made out of piezoelectric quartz rock
• Very stable compared to LC oscillator
• Little drift with temperature, supply etc
• Fixed frequency - cannot be tuned
• Synthesisers are stable frequency sources
• Use crystal oscillator as a reference

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Mixers

• Two frequencies can be combined in a mixer
  circuit
• Result is the creation of sum and difference
  frequencies

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AM Modulation

• AMPLITUDE MODULATION (AM) - The audio signal
  varies the amplitude of the RF Carrier

• Note if Audio is too strong, clipping and
  distortion occurs
• Simple AM gives carrier with lower and upper
  sidebands

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Mixer as a Modulator

• Mixer may be used as an AM modulator
• AM has carrier and two sidebands
• Most power is in the carrier signal
• Excessive audio causes over-modulation

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SSB Modulator

• SSB Single Sideband
• Mixing produces two sidebands
• One sideband may be selected by bandpass filtering

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SSB Modulation

• SSB has a number of advantages
• No carrier, so power is not wasted
• Half the bandwidth of AM
• No RF power without modulating audio
• Smaller PSU
• Less heat

Carrier and Unwanted Sideband is suppressed
compared to normal AM, reducing bandwidth
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FM Modulation

• FREQUENCY MODULATION (FM) - The audio signal
  varies the Frequency of the RF Carrier - its
  Amplitude stays constant

• Actual amount of variation is small
• Signal Amplitude is constant.

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FM Modulator

• FM can be achieved by varying the capacitance in
  a VFO
• Varicap diode varies capacitance with reverse
  voltage
• Apply modulating audio DC bias to diode
• Crystal oscillator?
• FM achieved via phase modulation in following
  stage
• Excessive audio causes over-deviation
• Distorted audio at receiver
• Interference to adjacent channels

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FM Modulator

• Oscillator with Frequency Modulation by Varicap
  Diode
• L1 and C1 set nominal frequency, which is varied
  by CD
• Diode DC Bias must be positive. Audio varies the
  the bias/Capacitance

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Data transmission

• Often achieved by modulating two or more audio
  tones (FSK)
• Audio tones generated in a modem

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Harmonics

• Harmonics are multiples of the wanted frequency -
  oscillators, mixers, and amplifiers generate
  harmonics
• Harmonics can be radiated and interfere with
  other radio users

F1 Fundamental 145MHz F2 Second
Harmonic 290MHz F3 Third Harmonic 435MHz F4
Fourth Harmonic 580MHz
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Lowpass filters

• Pass low frequencies only
• Attenuate high frequencies
• Can be used to suppress harmonics

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Bandpass filters

• Pass only a selected range of frequencies
• Attenuate other frequencies
• Can be used to suppress harmonics

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Highpass filters

• Pass high frequencies only
• Attenuate low frequencies
• Not so useful for suppressing harmonics! other
  uses