|
VFO is an acronym for Variable Frequency Oscillator. Acronyms and initialisms are abbreviations formed from the initial letter or letters of words, such as NATO and XHTML, and are pronounced in a way that is distinct from the full pronunciation of what the letters stand for. ...
A variable frequency oscillator is needed in any radio receiver or transmitter that works by the superheterodyne principle, and which can be tuned across various frequencies. Altering the frequency of the VFO will control the frequency to which the radio is tuned. The Super Heterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver) was invented by Edwin Armstrong in 1918. ...
...
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 Super Heterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver) was invented by Edwin Armstrong in 1918. ...
In biology, antenna (plural: antennae) refers to the sensing organs of several arthropods. ...
In communications, interference is anything which alters, modifies, or disrupts a message; as it travels along a channel, between a source and a receiver. ...
Beating is striking more than once, in violence, beating a drum, etc. ...
In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the...
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. Rf or RF may stand for: Radio frequency, a term in broadcasting Right field(er), a defensive position in baseball Rutherfordium (Rf), symbol for the chemical element RF, rheumatoid factor RF, a Mazda piston engine The Russian Federation This is a disambiguation page — a navigational aid which lists other pages...
A spurious emission is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies. ...
The term filter may refer to: A device to separate mixtures. ...
Generally, amplification is a basic process sometimes seen in nature, and often used in processes which involve a signal which must be made stronger. ...
In telecommunications, to heterodyne is to generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. ...
There are two main types of VFO in use: analogue and digital. For the Analog Science Fiction and Science Fact publication, see Astounding Magazine. ...
A digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. ...
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. An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. ...
Various types of capacitors A capacitor (occasionally referred to using the older term condenser) is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ...
An inductor is a passive electrical device that stores energy in a magnetic field, typically by combining the effects of many loops of electric current. ...
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 adjusting this via a front-panel knob a mechanical step-down gearbox may be introduced. Capacitance is a measure of the amount of electric charge stored for a given electric potential. ...
Varactor See varactor and voltage controlled oscillator. A varicap diode, varactor diode or tuning diode is a type of diode used in electronic circuits. ...
A voltage-controlled oscillator or VCO is an electronic circuit that uses amplification, feedback and a resonant circuit to generate a repeating voltage waveform. ...
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. A semiconductor is a material with an electrical conductance that is intermediate to those of an insulator and a conductor. ...
Types of diodes A diode functions as the electronic version of a one-way valve. ...
In the physical sciences, potential difference is the difference in potential between two points in a conservative vector field. ...
The Q factor or quality factor is a measure of the quality of a resonant system. ...
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. The tower of a personal computer. ...
In mathematics, see Embedding. ...
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. DSC might be an acronym or abbreviation for: differential scanning calorimeter digital selective calling DSc - A Doctor of Science document structure convention ( PostScript programming) Distinguished Service Cross (USA) - an American military award. ...
Spread-spectrum telecommunications is a technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. ...
Wi-Fi (or Wi-fi, WiFi, Wifi, wifi), short for Wireless Fidelity, is a set of product compatibility standards for wireless local area networks (WLAN) based on the IEEE 802. ...
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 capacitively 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. A crystal oscillator is an electronic circuit that uses the mechanical resonance of a physical crystal of piezoelectric material along with an amplifier and feedback to create an electrical signal with a very precise frequency. ...
The Q factor or quality factor is a measure of the quality of a resonant system. ...
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 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 #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 crystal 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. In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ...
In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analogue signal (current, voltage or charges). ...
Many electronic systems use internal clocks which are required to be phase-aligned to and/or frequency multiples of some external reference clock. ...
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 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 authorized 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. A spurious emission is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies. ...
Fourier (SAMPA: [fVri:eI]) can mean: Jean Baptiste Joseph Fourier, a French mathematician and physicist. ...
sdf f sdf sd ...
In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the...
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 license-holder may again be in breach of the terms of his or her license.
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 synthesizer-based) VFOs. Crystal oscillators have been around for a long time. They can be used as a frequency converter stage in a receiver, and also as the signal source for a transmitter. In the latter application, broadcast stations still use these in their older transmitters. A great many radio amatuers still use them for their old tube type transmitters, both commercial and home-made. A few suppliers still offer the old FT243 and HC6U crystals. However, most modern crystals are supplied in tiny metal cases with small pins or wire leads, and the latest types use SMD, surface mount configuration with no pins or leads at all. An LC circuit consists of an inductor and a capacitor. ...
See also | 
Feeds |
Contact