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

1. Chelmsford Amateur Radio Society Advanced CourseTransmittersPart-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. Crystal Oscillator Filter & RF Driver RF Power Amplifier Audio Amplifier Modulator & Filter Mixer Lowpass Filter Mic Crystal Oscillator Frequency Synthesiser 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 Recall: A Balanced Mixer is used to null the carrier for SSB

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

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

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

8. 3 Bits=8 Levels 4 Bits=16 Levels 5 Bits=32 Levels 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

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