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

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

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

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

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

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

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

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

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

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