3.1 Digital Frequency Synthesis
The electronic and digital techniques involved in this include
- Direct Digital Synthesis (DDS): Enough data points for a mathematical sine function are stored in digital memory. These are recalled at the right speed and fed to a digital to analogue converter where the required sine wave is built up.
- Direct Frequency Synthesis: Early channelised communication radios had multiple crystals - one for each channel on which they could operate. After a while this thinking was combined with the basic ideas of heterodyning and mixing described under #Why do radios need a VFO? above. Multiple crytals can be mixed in various combinations to produce various output frequencies.
- Phase Locked Loop (PLL): Using a varactor-controlled or voltage-controlled oscillator (VCO) (described above in #varactor under #Analogue VFO techniques) and a phase detector, a control-loop could be set up so that the VCO's output would be locked to a crytal controlled reference oscillator. This would not be much use unless the phase detector's comparison were made not between the actual outputs of the two oscillators, but between the output of one after frequency division by a slightly different divisor to that of the other. Then by altering the frequency division divisor(s) under computer control, a varienty of PLL output frequencies can be generated.
It is this last, the PLL techniques, that dominates most radio VFO design thinking today.
4 See Also
Electronics
Radio
