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Chelmsford Amateur Radio Society Advanced CourseTransmittersPart-2 - Mixers Modulation Mixers in Transmitters Need to recall and expand Intermediate course on mixers and modulation in Transmitters. In general VFOs, Crystal Oscillators and Synthesisers do not directly generate the final RF Output frequency. Mixers are used to combine two or more frequency sources as part of the modulation and up-conversion scheme. Need to understand that spurious outputs can also occur, as well as deviation issues on FM. Mixers Two frequencies can be combined in a mixer circuit Result is the creation of sum and difference frequencies Mixer as a Modulator Mixer may be used as an AM modulator AM has carrier and two sidebands Most power is in the carrier signal Excessive audio causes over-modulation Balanced Mixers A Balanced Mixer is used to create SSB as it will nullify the carrier component to leave the two sidebands RF Is applied to centre taps which results in null net carrier AM can be generated by deliberately unbalancing it AM SSB Bandwidth SSB has a number of advantages No carrier, so power is not wasted Half the bandwidth of AM No RF power without modulating audio Smaller PSU Less heat FM Modulator Colpitts Oscillator with Frequency Modulation by Varicap Diode L1 and C1 set nominal frequency, which is varied by CD FM Bandwidth is more complex as sidebands extend wider than AM Transmitter Up Conversion SSB generation and other modulation is performed at low or intermediate frequencies to ease design, filtering etc This must be up-converted to final RF frequency by another mixer Example: SSB for 2m band may be generated at a 6MHz IF It is then added to a 138MHz RF LO to produce a 144MHz output Note that using a multiplier (especially for SSB) would destroy the frequency and bandwidth relationships Modulation Terms FM Deviation refers to the max shift away from the nominal carrier Narrow vs Wideband FM. The FM section of 2m the band has been re-channelised to 12.5kHz spa