Transcript Transmitters

Chelmsford Amateur Radio Society
Intermediate Course
(4) Transmitters
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
1
Transmitters
Summary
– Block diagrams of transmitters
– Oscillators for generating a carrier
– Operation of mixers
– Modulators
– AM, FM, and SSB modulation
– Harmonics
– Filters
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
2
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
RF
oscillator
Keying
stage
Power
amplifier
Low-pass
filter
Key
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
3
AM Transmitter
• Block diagram
– In Modulator the audio modulates the RF amplitude
RF
oscillator
Microphone
Chelmsford Amateur Radio Society
Intermediate Licence Course
Modulator
Power
amplifier
Low-pass
filter
AF
amplifier
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
4
SSB Transmitter
• Block diagram
– Modulator produces double-sideband suppressed-carrier
– Sideband filter suppresses unwanted sideband
RF
oscillator
Mic
Balanced
modulator
Sideband
filter
Power
amplifier
Low-pass
filter
AF
amplifier
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
5
FM Transmitter
• Block diagram
– Audio used to modulate frequency of RF oscillator
RF
oscillator
Buffer
amplifier
Power
amplifier
Low-pass
filter
AF
amplifier
Mic
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
6
LC Oscillator
• 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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
R2
330
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
7
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
8
Crystal Oscillator
• Circuit of a crystal oscillator
– XL1 determines frequency
+9V
TR1
C3
XL1
3.756MHz
470pF
Output
R1
100k
C1
22pF
Chelmsford Amateur Radio Society
Intermediate Licence Course
C4
680pF
Anthony Martin M1FDE
R2
330
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
9
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
10
Mixers
• Two frequencies can be combined in a mixer circuit
• Result is the creation of sum and difference frequencies
~
Mixer
10MHz
10MHz+1MHz=11MHz
and
10MHz–1MHz=9MHz
~
1MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
11
AM Modulation
•
AMPLITUDE MODULATION (AM) - The audio signal varies the
amplitude of the RF Carrier
•Note if Audio is too
strong, clipping
and distortion
occurs
Audio Input
•Simple AM gives
carrier with lower
and upper
sidebands
RF Carrier
AM Signal
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
12
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
1.4MHz
~
1kHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
DC
offset
1.401MHz Upper sideband
1.399MHz Lower sideband
1.400MHz Carrier
DC offset unbalances
mixer and causes
carrier component.
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
13
SSB Modulator
• SSB = Single Sideband
• Mixing produces two sidebands
• One sideband may be selected by bandpass filtering
~
Mixer
1.4MHz
Sideband
Filter
~
Upper sideband only Lower sideband
suppressed.
1kHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
1.401MHz
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
14
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
Lower
– Less heat
Sideband
Carrier and Unwanted Sideband is
suppressed compared to normal AM,
reducing bandwidth
-3kHz
Carrier
Upper
Sideband
-300Hz
+300Hz
+3kHz
SSB: 2.7kHz BW
AM: 6kHz BW
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
15
FM Modulation
• FREQUENCY MODULATION (FM) - The audio signal varies the
Frequency of the RF Carrier - its Amplitude stays constant
• Actual amount of
variation is small
Audio Input
• Signal Amplitude is
constant.
RF Carrier
FM Signal
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
16
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
17
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
+9V
Diode DC Bias
RF
Block
Audio In
L2
DC Block
TR1
C2
220pF
C3
470pF
C5
22pF
CD
Varicap
Diode
Chelmsford Amateur Radio Society
Intermediate Licence Course
L1
10uH
C1
150pF
FM Output
R1
100k
C4
680pF
Anthony Martin M1FDE
R2
330
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
18
Data transmission
• Often achieved by modulating two or more audio tones (FSK)
• Audio tones generated in a modem
Tx audio
Data
Modem
Chelmsford Amateur Radio Society
Intermediate Licence Course
SSB or FM
transmitter
Rx audio
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
19
Harmonics
• 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
F1: Fundamental
F2: Second Harmonic
F3: Third Harmonic
F4: Fourth Harmonic
145MHz
290MHz
435MHz
580MHz
Frequency, MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
20
Lowpass filters
• Pass low frequencies only
• Attenuate high frequencies
• Can be used to suppress harmonics
Amplitude
F1
F2
F3
F4
Frequency, MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
21
Bandpass filters
• Pass only a selected range of frequencies
• Attenuate other frequencies
• Can be used to suppress harmonics
Amplitude
F1
F2
F3
F4
Frequency, MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
22
Highpass filters
• Pass high frequencies only
• Attenuate low frequencies
• Not so useful for suppressing harmonics! – other uses
Amplitude
F1
F2
F3
F4
Frequency, MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 6: v1.2 6-Apr-2008
(4) Transmitters
23