4a. Transmitters
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Transcript 4a. Transmitters
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