Chelmsford Amateur Radio Society Intermediate Course (4) Transmitters

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

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 Low-pass filter RF oscillator Keying stage Power amplifier Key

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

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

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

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

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

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

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

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

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

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

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

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 +300Hz Carrier Lower Sideband Upper Sideband -3kHz -300Hz +3kHz Carrier and Unwanted Sideband is suppressed compared to normal AM, reducing bandwidth SSB: 2.7kHz BW AM: 6kHz BW

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

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

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

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

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 F2 F3 F4 Frequency, MHz Power, dBW F1: Fundamental 145MHz F2: Second Harmonic 290MHz F3: Third Harmonic 435MHz F4: Fourth Harmonic 580MHz

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

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

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