VFO Why Do Radios Need A VFO Analogue VFO Tuning Capacitor

Any radio receiver or transmitter that works by the superheterodyne principle, and which can be tuned across various frequencies, will need a VFO. Altering the frequency of this VFO will control the frequency to which the radio is tuned.

1 Why do radios need a VFO?

In a simple superhet radio receiver, incoming radio frequencies from the antenna are made to interfere or beat with an internally generated radio frequency from the VFO in a process called mixing.

If the required incoming radio frequency and the VFO frequency were both rather high ( RF) but quite similar, then by far the lowest frequency produced from the mixer will be their difference. In very simple radios, it is relatively straightforward to separate this from all the other spurious signals using a filter, to amplify it and then further to process it into an audible signal. In more complex situations, many enhancements and complications get added to this simple process, but this mixing or heterodyning principle remains at the heart of it.

2 Analogue VFO

An analogue VFO could be an electronic oscillator where the value of at least one of the active components is adjustable under user control so as to alter its output frequency. The active component whose value is adjustable is usually a capacitor, but there is no reason why it could not be an inductor.

2.1 Tuning Capacitor

The variable capacitor may be a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its capacitanceCapacitance is the ability of a capacitor to store potential difference or voltage for a given amount of stored charge. The SI unit of capacitance is the farad. where C is the capacitance, measured in farads Q is the charge, measured in coulombs V is the. In the case of adusting this via a front-panel knob a mechanical step-down gearbox may be introduced.

2.2 Varactor

A reversed-biased semiconductor diodeA diode functions as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but blocks it in the opposite direction. Applications Radio demodulation T also exhibits capacitance. Since the width of its non-conducting depletion layer depends on the magnitude of the reverse bias voltage, this voltage can be used to control its capacitance. This has the advantage of requiring much smaller and more robust components. The bias voltage may be generated and controlled in a number of ways and there may need to be no significant moving parts in the final design. It has a range of disadvantages including temperature and ageing drift, electronic noise, low Q factorThe Q factor or quality factor is a measure of the "quality" of a resonant system. Resonant systems respond to frequencies close to the natural frequency much more strongly than they respond to other frequencies. On a graph of response versus frequency, t and non-linearity.

3 Digital VFO

Modern radio receivers and transmitters usually use some from of digital frequency synthesis to generate their VFO signal. The advantages of this are manifold, including smaller designs, lack of moving parts, and the ease with which preset frequencies can be stored and manipulated in the digital computer that is usually embeddedIn mathematics, see Embedding. In computer engineering, see Embedded system. Embedded is a play by Tim Robbins about journalists covering military conflict in the US-Iraq war of 2002-2004. in the design for other purposes anyway.

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 capablility allows communications receivers effectively to monitor many channels at once, perhaps using digital selective calling ( DSCDSC might be an acronym or abbreviation for: differential scanning calorimeter digital selective calling document structure convention ( PostScript programming) Distinguished Service Cross (USA) an American military award. Distinguished Service Cross (UK)) 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 are currently gaining 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 almost always relies on crystal controlled frequency sources. Crystal controlled oscillators have enormous advantages over inductive and capacitatively controlled ones in terms of stability and repeatability as well as low noise and high Q factor. The disadvantage comes when you try to alter the resonant frequency to tune the radio, but a wide range of digital techniques have made this unnecessary in modern practice.

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

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