VFO
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Acronym for: Variable Frequency Oscillator
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.
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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.
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: analogue and digital.
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.
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 capacitance. In the case of adusting this via a front-panel knob a mechanical step-down gearbox may be introduced.
Varactor
See varactor and voltage controlled oscillator.
A reversed-biased semiconductor diode 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 factor and non-linearity.
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 embedded 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 (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 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.
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 can be set up so that the VCO's output is frequency-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 outputs of each after frequency division by two slightly different divisors. Then by altering the frequency-division divisor(s) under computer control, a variety of actual (undivided) VCO output frequencies can be generated.
It is this last, the PLL technique, that dominates most radio VFO design thinking today.
Performance
The performance of a radio's VFO strongly influences the perfomance 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 slightly to re-tune them to remain on frequency.
Repeatability
Ideally, for the same selected radio channel, the VFO in your radio is generating exactly the same frequency today as it was on the day the radio was first assembled and tested. This will mean that any built-in errors seen that day during the manufacture will have been calibrated out, and this calibration will not have changed through to today. If this is not the case, then you will not be able entirely to trust your tuning dial.
This would be a source of irritation on a receiver, where you may have to tune slightly off the known frequency to receive a certain station. The problem can be more serious in a transmitter as you could unwittingly and illegally be transmitting on a frequency for which you are not authorised or licensed. If you do so, it is your responsibility, and trying to blame your badly calibrated circuitry will be no defence.
Purity
You can imagine the shape of the VFO's frequency vs amplitude graph to be the shape of the 'window' through which the radio receives (and in the case of a transmitter, through which it transmits when you ask it to transmit a pure sine-wave tone). In the ideal case, this frequency/amplitude plot is very simple, i.e. there is absolutely no output at any frequency except one, and plenty of pure output at exactly that frequency. In this ideal case, of course, the 'window' is unique and infinitely narrow. The ideal radio will receive and transmit only exactly what is expected.
Spurii
A VFO's frequency vs amplitude graph (or Fourier Analysis) may exhibit not one but several narrow peaks, probably harmonically related. Each of these other peaks can potentially mix with some other incoming signal and produce a spurious response. These spurii (sometimes spelt spuriae) result in you hearing two stations at once, even though the other is nowhere near this one on the band.
The extra peaks may be many hundreds or thousands of times lower in value than the main one, but don't forget that the other, interfering station may be hundreds or thousands of times more powerful at the antenna than the one you are after.
In a transmitter, these spurious signals are actually generated along with the one you expect. If they are not completely filtered out before they are transmitted, then the licence-holder may again be in breach of the terms of his or her licence.
Phase noise
See Intermodulation Distortion
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 at all costs 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. PPL and synthesiser-based) VFOs.