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Intermediate Course (4) Transmitters Karl Davies

Intermediate Course (4) Transmitters Karl Davies. East Kent Radio Society EKRS. Transmitters. Summary Block diagrams of transmitters Oscillators for generating a carrier Operation of mixers Modulators AM, FM, and SSB modulation Harmonics Filters. Keying stage. Power amplifier.

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Intermediate Course (4) Transmitters Karl Davies

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  1. Intermediate Course(4) TransmittersKarl Davies East Kent Radio Society EKRS

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

  3. Keying stage Power amplifier Low-pass filter Key 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

  4. Modulator Power amplifier Low-pass filter AF amplifier Microphone AM Transmitter • Block diagram • Modulator - the audio modulates the RF amplitude RF oscillator

  5. Balanced modulator Sideband filter Power amplifier Low-pass filter AF amplifier Mic SSB Transmitter • Block diagram • Modulator produces double-sideband suppressed-carrier • Sideband filter suppresses unwanted sideband RF oscillator

  6. RF oscillator Buffer amplifier Power amplifier Low-pass filter AF amplifier Mic FM Transmitter • Block diagram • Audio used to modulate frequency of RF oscillator

  7. +9V TR1 C2 220pF C3 470pF Output L1 C1 R1 10uH 150pF 100k R2 C4 330 680pF LC Oscillator • Circuit of a Colpitts LC oscillator • L1 and C1 determine frequency

  8. 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

  9. +9V TR1 C3 XL1 3.756MHz 470pF Output R1 100k C1 R2 C4 22pF 330 680pF Crystal Oscillator • Circuit of a crystal oscillator • XL1 determines frequency

  10. 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

  11. ~ Mixer 10MHz+1MHz=11MHz and 10MHz–1MHz=9MHz 10MHz ~ 1MHz Mixers Two frequencies can be combined in a mixer circuit Result is the creation of sum and difference frequencies

  12. Audio Input RF Carrier AM Signal AM Modulation Note if Audio is too strong, clipping and distortion occurs Simple AM gives carrier with lower and upper sidebands • AMPLITUDE MODULATION (AM) - The audio signal varies the amplitude of the RF Carrier

  13. 1.401MHz Upper sideband 1.399MHz Lower sideband 1.400MHz Carrier ~ Mixer 1.4MHz ~ DC offset DC offset unbalances mixer and causes carrier component. 1kHz 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

  14. ~ Mixer Sideband Filter 1.401MHz 1.4MHz ~ Upper sideband only - Lower sideband suppressed. 1kHz SSB Modulator SSB = Single Sideband Mixing produces two sidebands One sideband may be selected by bandpass filtering

  15. Carrier Lower Sideband Upper Sideband -3kHz -300Hz +300Hz +3kHz SSB: 2.7kHz BW AM: 6kHz BW 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

  16. Audio Input RF Carrier FM Signal FM Modulation Actual amount of variation is small Signal Amplitude is constant. • FREQUENCY MODULATION (FM) - The audio signal varies the Frequency of the RF Carrier - its Amplitude stays constant

  17. 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

  18. +9V Diode DC Bias TR1 C2 RF Block DC Block 220pF L2 Audio In C3 C5 22pF 470pF FM Output L1 C1 R1 CD 10uH 150pF 100k Varicap Diode R2 C4 330 680pF 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 bias/Capacitance

  19. SSB or FM transmitter Tx audio Data Modem Rx audio Data transmission Often achieved by modulating two or more audio tones (FSK) Audio tones generated in a modem

  20. F1 F2 Power, dBW F3 F4 Frequency, MHz 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

  21. F1 Amplitude F2 F3 F4 Frequency, MHz Lowpass filters Pass low frequencies only Attenuate high frequencies Can be used to suppress harmonics

  22. F1 Amplitude F2 F3 F4 Frequency, MHz Bandpass filters Pass only a selected range of frequencies Attenuate other frequencies Can be used to suppress harmonics

  23. F1 Amplitude F2 F3 F4 Frequency, MHz Highpass filters Pass high frequencies only Attenuate low frequencies Not so useful for suppressing harmonics! – other uses

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