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East Kent Radio Society EKRS
1
Intermediate Course
(4) Transmitters
Karl Davies
Transmitters
2
Summary

Block diagrams of transmitters

Oscillators for generating a carrier

Operation of mixers

Modulators

AM, FM, and SSB modulation

Harmonics

Filters
CW Transmitter
3
 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
Keying
stage
Key
Power
amplifier
Low-pass
filter
AM Transmitter
4
 Block diagram
 Modulator - the audio modulates the RF amplitude
RF
oscillator
Microphone
Modulator
AF
amplifier
Power
amplifier
Low-pass
filter
SSB Transmitter
5
 Block diagram
 Modulator produces double-sideband suppressed-carrier
 Sideband filter suppresses unwanted sideband
RF
oscillator
Mic
Balanced
modulator
AF
amplifier
Sideband
filter
Power
amplifier
Low-pass
filter
FM Transmitter
6
 Block diagram
 Audio used to modulate frequency of RF oscillator
RF
oscillator
AF
amplifier
Mic
Buffer
amplifier
Power
amplifier
Low-pass
filter
LC Oscillator
7
 Circuit of a Colpitts LC oscillator
 L1 and C1 determine frequency
+9V
TR1
C2
220pF
C3
470pF
L1
10uH
C1
150pF
Output
R1
100k
C4
680pF
R2
330
LC oscillator (VFO)
8
 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
9
 Circuit of a crystal oscillator
 XL1 determines frequency
+9V
TR1
C3
XL1
3.756MHz
470pF
Output
R1
100k
C1
22pF
C4
680pF
R2
330
Crystal Oscillator
10
 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
11
 Two frequencies can be combined in a mixer circuit
 Result is the creation of sum and difference
frequencies
~
Mixer
10MHz
~
1MHz
10MHz+1MHz=11MHz
and
10MHz–1MHz=9MHz
AM Modulation
12
•
AMPLITUDE MODULATION (AM) - The audio signal varies the
amplitude of the RF Carrier
Note if Audio is
Audio Input
too strong,
clipping and
distortion occurs
RF Carrier
AM Signal
Simple AM gives
carrier with
lower and upper
sidebands
Mixer as a Modulator
13
 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
1.4MHz
~
1kHz
DC
offset
1.401MHz Upper sideband
1.399MHz Lower sideband
1.400MHz Carrier
DC offset unbalances
mixer and causes
carrier component.
SSB Modulator
14
 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.
SSB Modulation
15
 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
Lower
Sideband
Carrier and Unwanted Sideband is
suppressed compared to normal AM,
reducing bandwidth
-3kHz
Upper
Sideband
-300Hz
+300Hz
SSB: 2.7kHz BW
AM: 6kHz BW
+3kHz
FM Modulation
16
• FREQUENCY MODULATION (FM) - The audio signal varies the
Frequency of the RF Carrier - its Amplitude stays constant
 Actual
Audio Input
amount of
variation is
small
RF Carrier
 Signal
FM Signal
Amplitude is
constant.
FM Modulator
17
 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
18

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

+9V
Diode DC Bias
RF
Block
Audio In
L2
DC Block
TR1
C2
220pF
C3
470pF
C5
22pF
CD
Varicap
Diode
L1
10uH
C1
150pF
FM Output
R1
100k
C4
680pF
R2
330
Data transmission
19
 Often achieved by modulating two or more audio
tones (FSK)
 Audio tones generated in a modem
Tx audio
Data
Modem
Rx audio
SSB or FM
transmitter
Harmonics
20
 Harmonics are multiples of the wanted frequency - oscillators,
mixers, and amplifiers generate harmonics
 Harmonics can be radiated and interfere with other radio users
Power, dBW
F1
F2
F3
F4
Frequency, MHz
F1: Fundamental
F2: Second Harmonic
F3: Third Harmonic
F4: Fourth Harmonic
145MHz
290MHz
435MHz
580MHz
Lowpass filters
21
 Pass low frequencies only
 Attenuate high frequencies
 Can be used to suppress harmonics
Amplitude
F1
F2
F3
F4
Frequency, MHz
Bandpass filters
22
 Pass only a selected range of frequencies
 Attenuate other frequencies
 Can be used to suppress harmonics
Amplitude
F1
F2
F3
F4
Frequency, MHz
Highpass filters
23
 Pass high frequencies only
 Attenuate low frequencies
 Not so useful for suppressing harmonics! – other uses
Amplitude
F1
F2
F3
F4
Frequency, MHz