The electronic and digital techniques involved in this include:ĭirect digital synthesis (DDS) Enough data points for a mathematical sine function are stored in digital memory. 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. Crystal controlled oscillators are more stable than inductively and capacitively controlled oscillators. 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.ĭigital frequency synthesis relies on stable crystal controlled reference frequency sources. Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques.Įxtreme frequency agility lies at the heart of spread spectrum techniques that have gained mainstream acceptance in computer wireless networking such as Wi-Fi. 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. 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. 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. Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal. Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low Q factor and non-linearity. 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. 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. See also: varactor and voltage controlled oscillatorĪ reversed-biased semiconductor diode exhibits capacitance. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning. 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. The passive component whose value is adjustable is usually a capacitor, but could be a variable inductor. There are two main types of VFO in use: analog and digital.Īn 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. 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. 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. Demodulation takes place at baseband using low-pass filters and amplifiers. In a simple superheterodyne receiver, the incoming radio frequency signal (at frequency f I N Hz.
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