1. Chelmsford Amateur Radio Society Advanced Course Transmitters Part-1 - Principles & Synthesisers

2. Transmitters
Advanced Course requires a detailed knowledge of Transmitters and Receivers
This session covers Transmitter Block Diagrams, Oscillators and Synthesisers

3. 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
Lowpass
Filter
Mic
Audio
Amplifier
Modulator
& Filter
Mixer
Filter &
RF Driver
RF Power
Amplifier
Frequency
Synthesiser
Crystal
Oscillator
Recall: A Balanced Mixer is used to null the carrier for SSB

4. 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
Poweramp
& Filter
Oscillator
Freq
Mod
Buffer
Amplifier
Phase
Mod
Frequency
Multiplier
Filter &
Driver
Audio
Amplifier
Mic

5. 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

6. Feed back control signal Programmable Divider, N
Frequency Synthesis
Start with a free running Voltage Controlled RF Oscillator (VCO)
Control it by a ratio of an accurate crystal reference
Crystal
Reference
Oscillator
6MHz
Fixed Divider, A
Divide by 6000
Feed back control signal
VCO
10MHz RF Out
1kHz
Phase Comparator
LPF
Sample RF Output
1kHz
Programmable Divider, N
Divide by 10000
FOUT = FCRYSTAL x N/A

7. 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
D-to-A
Converter
Lowpass filter
Sinewave Output
Frequency Control
Clock

8. 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
4 Bits=16 Levels
5 Bits=32 Levels

9. 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

10. 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