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

Modulator Low-passfilter

AFamplifierMicrophone

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

C3470pF

C4680pF

C122pF

R2330

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

Mixer 10MHz+1MHz=11MHzand

10MHz–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 Carrier1.4MHz

~

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

Mixer1.4MHz~

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, d

BW

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