A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e., varied) over some range. It is a necessary component in any tunable radio transmitter and in receivers that work by the superheterodyne principle. The oscillator controls the frequency to which the apparatus is tuned.
Purpose In a simple superheterodyne receiver, the incoming radio frequency signal (at frequency f I N {\displaystyle f_{IN}} ) from the antenna is mixed with the VFO output signal tuned to f L O {\displaystyle f_{LO}} , producing an intermediate frequency (IF) signal that can be processed downstream to extract the modulated information. Depending on the receiver design, the IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion), f I N + f L O {\displaystyle f_{IN}+f_{LO}} or more commonly, the difference frequency (down-conversion), f I N − f L O {\displaystyle f_{IN}-f_{LO}} . In addition to the desired IF signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies, f I N {\displaystyle f_{IN}} and f L O {\displaystyle f_{LO}} and various harmonic combinations of the input signals. These undesired signals are rejected by the IF filter. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter. The advantage of using a VFO as a heterodyning oscillator is that only a small portion of the radio receiver (the sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency. In a direct-conversion receiver, the VFO is tuned to the same frequency as the incoming radio frequency and f I F = 0 {\displaystyle f_{IF}=0} Hz. Demodulation takes place at baseband using low-pass filters and amplifiers. In a radio frequency (RF) transmitter, VFOs are often used to tune the frequency of the output signal, often indirectly through a heterodyning process similar to that described above. Other uses include chirp generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscopes and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment.
Types There are two main types of VFO in use: analog and 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.
Varactor
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. 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 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 (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 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:
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