Editing Variable-frequency oscillator (section)

==Types==
There are two main types of VFO in use: [[analog circuit|analog]] and [[Digital electronics|digital]].

===Analog VFOs===
An analog VFO is an [[electronic oscillator]] where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency.
The passive component whose value is adjustable is usually a [[capacitor]], but could be a variable [[inductor]].

====Tuning capacitor====
The variable capacitor is a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its [[capacitance]]. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning.<ref name="Rohde88"/>

====Varactor====
{{see also|varactor|voltage controlled oscillator}}

A reversed-biased [[semiconductor]] [[diode]] exhibits capacitance. Since the width of its non-conducting [[depletion region]] depends on the magnitude of the reverse bias voltage, this [[voltage]] can be used to control the junction capacitance. The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design.<ref name="Holt78">{{citation | last=Holt | first=Charles | title=Electronic Circuits | publisher=John Wiley & Sons | year=1978 | isbn=0-471-02313-2 }}</ref>
Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low [[Q factor]] and non-linearity.

===Digital VFOs===
Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal.
The advantages include smaller designs, lack of moving parts, the higher stability of set frequency reference oscillators, and the ease with which preset frequencies can be stored and manipulated in the [[digital computer]] that is usually [[embedded system|embedded]] in the design in any case.

It is also possible for the radio to become extremely [[frequency-agile]] in that the control computer could alter the radio's tuned frequency many tens, thousands or even millions of times a second.
This capability allows communications receivers effectively to monitor many channels at once, perhaps using digital selective calling ([[Global Maritime Distress Safety System|DSC]]) techniques to decide when to open an audio output channel and alert users to incoming communications.
Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques.
Extreme frequency agility lies at the heart of [[spread spectrum]] techniques that have gained mainstream acceptance in computer wireless networking such as [[Wi-Fi]].

There are disadvantages to digital synthesis such as the inability of a digital synthesiser to tune smoothly through all frequencies, but with the channelisation of many radio bands, this can also be seen as an advantage in that it prevents radios from operating in between two recognised channels.

Digital frequency synthesis relies on stable [[crystal oscillator|crystal controlled]] reference frequency sources. Crystal-controlled oscillators are more stable than inductively and capacitively controlled oscillators. Their disadvantage is that changing frequency (more than a small amount) requires changing the crystal, but frequency synthesizer techniques have made this unnecessary in modern designs.

====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.