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