Editing Variable-frequency oscillator (section)

====Digital frequency synthesis====
The electronic and digital techniques involved in this include:
;[[Direct digital synthesizer|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-analog converter]] where the required sine wave is built up.
;[[Direct frequency synthesis]]: Early channelized 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 [[#Purpose|purpose]] above. Multiple crystals 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|varactor]] under [[#Analog_VFOs|analog VFO]] techniques) and a phase detector, a control-loop can be set up so that the VCO's output is frequency-locked to a crystal-controlled reference oscillator. The phase detector's comparison is made between the outputs of the two oscillators after [[frequency division]] by different divisors. Then by altering the frequency-division divisor(s) under computer control, a variety of actual (undivided) VCO output frequencies can be generated. The PLL technique dominates most radio VFO designs today.