Variable-frequency oscillator

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Variable-frequency oscillator

A variable frequency oscillator (VFO) in electronics is a oscillator with an oscillation frequency that can be electronically changed (hence, variable).cite book |title=The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition |editor=Larry D. Wolfgang |year=1991 |publisher=American Radio Relay League |location=Newington, Connecticut |isbn=0872591689 |pages=Chapter 10 ] It is a necessary component in any radio receiver or transmitter that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned.

Why do radios need a VFO?

In a simple superhet radio receiver, incoming radio frequencies from the antenna are made to mix (or multiply) with an internally generated radio frequency from the VFO in a process called mixing. The mixing process can produce a range of output signals:
* at all the original frequencies,
* at frequencies that are the sum of each two mixed frequencies
* at frequencies that equal the difference between two of the mixed frequencies
* at other, usually higher, frequencies.

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.

There are two main types of VFO in use: analog and digital.

Analog VFO

An analog VFO is 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 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

"See varactor and 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.Varactors have a number of disadvantages including temperature drift and aging , electronic noise, low Q factor and non-linearity.

Digital VFO

Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal. The advantages 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 embedded in the design for other purposes.

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 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 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:
* 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 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 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 under 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 oscilators 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.

Performance

The performance of a radio's VFO strongly influences the performance of the radio itself.

Accuracy

It is useful if the frequency produced by the VFO is both stable and repeatable.

Stability

An unstable VFO's output frequency will drift with time. The root cause of this can often be traced to temperature dependency in some of the voltages and component values involved. Often as radios warm up it is necessary to slightly re-tune them to remain on frequency.

Repeatability

Ideally, for the same control input to the VFO, the oscillator should generate exactly the same frequency. A change in the calibration of the VFO can change receiver tuning calibration; periodic re-alignment of a receiver may be needed. VFO's used as part of a phase-locked loop frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.

Purity

A plot of a VFO's amplitude vs. frequency may show several peaks, probably harmonically related. Each of these peaks can potentially mix with some other incoming signal and produce a "spurious" response. These "spurii" (sometimes spelt "spuriae") can result in increased noise or two signals detected where there should only be one. Additional components can be added to a VFO to suppress high-frequency parasitic oscillations.

In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.

Phase noise

When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat noise-floor. Slight random 'jitters' in the signal's timing will mean that the peak is sitting on 'skirts' of phase-noise at frequencies either side of the desired one.

These are also troublesome in crowded bands. They allow through unwanted signals that are fairly close to the one we expect, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the signal we are after. The effect is that what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of all the strong signals nearby.

The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.

Crystal control

In all performances cases, crystal controlled oscillators are better behaved than the semiconductor- and LC-based alternatives. They tend to be more stable, more repeatable, have fewer and lower harmonics and lower noise than all the alternatives in their cost-band. This in part explains their huge popularity in low-cost and computer-controlled (i.e. PLL and synthesizer-based) VFOs.

References

* Voltage-controlled oscillator
* Oscillator
* Resonance
* Tuner
* Feedback and control
* frequency division
* Microcontroller

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