intermediate course (4) transmitters karl davies east kent radio society ekrs 1
TRANSCRIPT
Intermediate Course(4) Transmitters
Karl Davies
East Kent Radio Society EKRS
1
Transmitters
Summary
Block diagrams of transmitters
Oscillators for generating a carrier
Operation of mixers
Modulators
AM, FM, and SSB modulation
Harmonics
Filters
2
Low-passfilter
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
Poweramplifier
Keyingstage
Key
RFoscillator
3
AM Transmitter
Block diagram Modulator - the audio modulates the RF
amplitude
ModulatorLow-pass
filter
AFamplifier
Microphone
RFoscillator
Poweramplifier
4
SSB Transmitter
Block diagram Modulator produces double-sideband
suppressed-carrier Sideband filter suppresses unwanted sideband
RFoscillator
Balancedmodulator
Poweramplifier
Low-passfilter
Sidebandfilter
AFamplifier
Mic
5
FM Transmitter
Block diagram Audio used to modulate frequency of RF
oscillator
Bufferamplifier
Poweramplifier
RFoscillator
Low-passfilter
AFamplifier
Mic
6
LC Oscillator
Circuit of a Colpitts LC oscillator L1 and C1 determine frequency
C3470pF
C4680pF
C1150pF
R2330
R1100k
TR1
+9V
C2220pF
OutputL110uH
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
8
Crystal Oscillator
Circuit of a crystal oscillator XL1 determines frequency
C3
470pF
C4680pF
C122pF
R2
330
R1100k
TR1
+9V
Output
XL13.756MHz
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
10
Mixers
Two frequencies can be combined in a mixer circuitResult is the creation of sum and difference
frequencies
Mixer10MHz+1MHz=11MHz
and10MHz–1MHz=9MHz10MHz
~
1MHz
~
11
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
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
Mixer1.401MHz Upper sideband1.399MHz Lower sideband1.400MHz Carrier
1.4MHz
~
1kHz
~ DC offset unbalances mixer and causes carrier component.
DCoffset
13
SSB Modulator
SSB = Single SidebandMixing produces two sidebandsOne sideband may be selected by bandpass filtering
Mixer
1.4MHz
~
1kHz
~
SidebandFilter
1.401MHz
Upper sideband only - Lower sideband suppressed.
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 Less heat
+300Hz
Carrier
Lower Sideband
Upper Sideband
-3kHz -300Hz +3kHz
SSB: 2.7kHz BWAM: 6kHz BW
Carrier and Unwanted Sideband is suppressed compared to normal AM, reducing bandwidth
15
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
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
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 bias/Capacitance
C3470pF
C4680pF
C1150pF
R2330
R1100k
TR1
+9V
C2220pF
FM Output
VaricapDiode
C522pF
L110uH
L2
RF Block
Audio In
DC Block
CD
Diode DC Bias
18
Data transmission
Often achieved by modulating two or more audio tones (FSK)
Audio tones generated in a modem
SSB or FMtransmitter
Tx audio
Rx audio
DataModem
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
F1: Fundamental 145MHzF2: Second Harmonic 290MHzF3: Third Harmonic 435MHzF4: Fourth Harmonic 580MHz
F1
F2F3
F4
Frequency, MHz
Pow
er,
dBW
20
Lowpass filters
Pass low frequencies onlyAttenuate high frequenciesCan be used to suppress harmonics
Frequency, MHz
Am
plitu
de
F1
F2F3
F4
21
Bandpass filters
Pass only a selected range of frequenciesAttenuate other frequenciesCan be used to suppress harmonics
Frequency, MHz
Am
plitu
de
F1
F2F3
F4
22
Highpass filters
Pass high frequencies onlyAttenuate low frequenciesNot so useful for suppressing harmonics! – other uses
Frequency, MHz
Am
plitu
de
F1
F2F3
F4
23