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

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Presentation on theme: "Intermediate Course (4) Transmitters Karl Davies East Kent Radio Society EKRS 1."— Presentation transcript:

1 Intermediate Course (4) Transmitters Karl Davies East Kent Radio Society EKRS 1

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

3 Low-pass filter 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 Power amplifier Keying stage Key RF oscillator 3

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

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

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

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 7

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 8

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

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 10

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

12 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 RF Carrier Audio Input AM Signal 12

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

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

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 SSB: 2.7kHz BW AM: 6kHz BW Carrier and Unwanted Sideband is suppressed compared to normal AM, reducing bandwidth 15

16 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 FM Signal RF Carrier Audio Input 16

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 17

18 FM Modulator Oscillator with Frequency Modulation by Varicap Diode L1 and C1 set nominal frequency, which is varied by C D Diode DC Bias must be positive. Audio varies the bias/Capacitance 18

19 Data transmission 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 19

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: Fundamental145MHz F2: Second Harmonic290MHz F3: Third Harmonic435MHz F4: Fourth Harmonic 580MHz F1 F2 F3 F4 Frequency, MHz Power, dBW 20

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

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 22

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 23


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