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Chelmsford Amateur Radio Society Intermediate Course (4) Transmitters

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Presentation on theme: "Chelmsford Amateur Radio Society Intermediate Course (4) Transmitters"— Presentation transcript:

1 Chelmsford Amateur Radio Society Intermediate Course (4) Transmitters

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

3 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

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

5 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

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

7 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

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 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

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 ~ ~ 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 ~

12 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

13 ~ ~ 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 ~

14 ~ ~ 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 ~

15 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

16 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.

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 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

19 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

20 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

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

22 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

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

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