Norfolk ARC Advanced Course

Transmitters

Amplitude modulation

Amplitude modulation Depth of modulation

Amplitude modulation Power in carrier & sidebands

SSB Transmitter Must produce clean modulated sinewave output of the desired frequency with minimal output at any other frequency

Single side band transmission

LC Tuned circuit oscillator Variable Frequency Oscillator

Aware of frequency stability A means of checking the frequency

Frequency Modulation Frequency deviation

Peak deviation Deviation ratio = actual deviation/max deviation

Frequency Modulation Bandwidth = 2 (max audio freq. + peak deviation)

At 70cm = 2 x (3kHz + 5kHz) = 16kHz At VHF = 2 x (2.8kHz + 2.5kHz) = 10.6kHz

Simple FM Transmitter + phase modulation

Crystal oscillator Series/parallel resonance of crystal Single frequency – switched crystals

Synthesised local oscillators

• Two main methods of frequency synthesis

• Phased locked loop – PLL

• Direct Digital Synthesis – DDS

• Both combine the flexibility of a VFO with the stability and accuracy of a crystal

Phased locked loop Phase comparator

Phased locked Loop

Direct digital synthesis

Direct Digital Synthesis

Buffer amplifier

Frequency Multiplier - HF Bands Harmonically related amateur bands

Frequency multiplier – 10GHz

Applying frequency modulation & phase modulation

Applying SSB modulation Diode ring balanced modulator

Sidebands but no carrier

SSB modulation mixed to produce final output frequency

Applying amplitude modulation

• Used to be carried out in the PA stage

• Modern rigs use same modulator as SSB

CW – Morse Bandwidth dependent on keying speed and waveform

Applying CW

Data modes – RTTY Two tones 1275 & 1445 Hz

Data modes

• Audio tones via microphone socket AFSK (modulating sub carrier)

• Direct modulation of the carrier FSK

• F2B (FM) AFSK 2 = modulating sub carrier B = data for automatic reception (carrier transmitted)

• F1B (FM) FSK 1 = no modulating sub carrier

• J2B (SSB) J=SSB 2 = modulating sub carrier

Applying data - RTTY

Mixing up to final frequency

RF Power amplifier stage Class A linear for SSB

FM & CW can be class C

PA for 144MHz FM Class C

Valve PA stage

Transistor PA stage

Peak envelope power (PEP)

• Licence states maximum PEP

• The average RF power over 1 cycle of RF at the crest of the modulation envelope.

• i.e. no one cycle should exceed 400W

• Data modes average power

• Speech processors

Other TX functions

• Automatic level control ALC

• SWR protection

Transceiver overview

Transmitter Interference Frequency errors and drift

Transmitter problems

• Supply voltage change

• Changes in output loading and stray capacitance

• Mechanical shock

• Ageing

• Chirp

Chirp

Unwanted emissions - harmonics

Unwanted mixer products and Intermodulation

Filters

Spurious oscillations

Spurious oscillations

Spurious oscillations from frequency synthesizer

Excessive audio bandwidth

Key clicks

Key click filter

Over modulating – AM & SSB

Over deviating - FM

Overdriving the power amplifier

Over driving the power amplifier