Download Transmitters-1 - Chelmsford Amateur Radio Society

Chelmsford Amateur Radio Society
Advanced Course
Transmitters
Part-1 - Principles & Synthesisers
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
1
Transmitters
• Advanced Course requires a detailed knowledge of
Transmitters and Receivers
• This session covers Transmitter Block Diagrams,
Oscillators and Synthesisers
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
2
Multimode Transmitter
•
•
•
Modern radios often have a multimode architecture
The modulator may be switchable for AM, SSB and FM
Mixer changes modulated signal to final output RF frequency
Crystal
Oscillator
Mic
Audio
Amplifier
Modulator
& Filter
Crystal
Oscillator
Chelmsford Amateur Radio Society
Advanced Licence Course
Mixer
Frequency
Synthesiser
Filter &
RF Driver
RF Power
Amplifier
Lowpass
Filter
Recall: A Balanced Mixer is used
to null the carrier for SSB
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
3
Simple FM Transmitter
•
•
•
•
FM or Phase Modulation is common at VHF and above
FM can be achieved by Audio pulling the Oscillator
Alternatively Phase modulation can be applied after the Oscillator
Frequency Multipliers are now more common for microwave
bands where full synthesisers are difficult to produce cheaply
Oscillator
Freq
Mod
Buffer
Amplifier
Frequency
Multiplier
Filter &
Driver
Poweramp
& Filter
Phase
Mod
Audio
Amplifier
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Mic
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
4
Oscillators
•
Recall Intermediate Course: Oscillators can be
– Colpitts oscillator based on simple LC resonator
– Varactor controlled LC
– Quartz crystal based - perhaps a switched bank
•
Important to use stable components/PSUs, sound construction, and
temperature compensation
•
LC VFOs need a method to check their frequency
•
A buffer amplifier is often on used at a VFO oscillator output to to
prevent unwanted changes to its output frequency or purity
•
Can use a crystal oscillator as an accurate reference for a synthesiser
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
5
Frequency Synthesis
Crystal
Reference
Oscillator
•
6MHz
•
Start with a free running Voltage Controlled RF
Oscillator (VCO)
Control it by a ratio of an accurate crystal reference
Feed back control signal
Fixed Divider, A
1kHz
Phase
Comparator
LPF
10MHz RF Out
VCO
Divide by 6000
1kHz
FOUT = FCRYSTAL x N/A
Chelmsford Amateur Radio Society
Advanced Licence Course
Programmable
Divider, N
Sample RF Output
Divide by 10000
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
6
Direct Digital Synthesis
•
•
•
•
Conventional Synthesiser uses an analogue VCO to give sine waves
DDS creates the sine wave using a Digital to Analogue Converter
Frequency is limited by D-to-A speed and the number of samples
Sinewave has steps (quantisation) and is filtered to improve purity
Sinewave
Lookup Table
Clock
Chelmsford Amateur Radio Society
Advanced Licence Course
D-to-A
Converter
Lowpass filter
Sinewave
Output
Frequency
Control
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
7
DDS Waveforms
•
Sinewave purity is dependent on
– D-to-A Resolution
– Number of time samples
•
Similar to CD Audio - need enough bits/samples for low distortion
•
If steps are fine - a simple low pass filter will smooth waveform
3 Bits=8 Levels
Chelmsford Amateur Radio Society
Advanced Licence Course
4 Bits=16 Levels
Murray Niman G6JYB
5 Bits=32 Levels
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
8
Synthesiser Spurii
•
Phase comparator time constant and frequency has a degree of
uncertainty which manifests itself as phase noise
•
Situation is not helped if small frequency step resolution, but rapid
tuning are both desired
•
Synthesisers must detect ‘out of lock’ and inhibit transmission
•
Modern synthesisers use dual loops to get small step sizes
•
DDS steps would also show up as sidebands/jitter unless filtered out
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
9
Multipliers
•
Multipliers use a severely non-linear stage to deliberately generate
harmonics - eg a Class-C amplifier or a diode
•
The desired multiples of the input frequency can be selected by a
bandpass filter.
•
Multipliers are not very efficient, needing up to Watts of input
power for milliwatt outputs
•
Used in simple crystal based PMR VHF radios, before synths.
•
Main role now is in microwave multiplier chains eg. for x2, x3, x5
– 432MHz x 3 = 1296MHz (23cms)
– 3.4GHz x 3=10GHz
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 6: v1.01, 1-Oct-2004
(4) Transmitters - Principles & Synthesisers
10