Transmitter
From Freeradio
The Transmitter is the equipment which creates the modulated radio waves, for broadcast through the Antenna.
A wide range of transmitters are used by Pirates.
Contents |
VCO FM Transmitters
The simplest form of FM transmitter is a VCO (Voltage Controlled Oscillator), where the frequency is controlled by an inductor-capacitor network. Generally a variable capacitor is used to adjust the frequency. The modulating audio is fed (generally via a buffer amplifier) across a Varicap diode, to create the frequency modulation.
The VCO is generally followed by amplification circuitry, to boost the radio frequency signal to a usable level (generally measured in Watts or tens of Watts) and to prevent antenna loading from causing frequency instability.
A few pirates have used very cheap transmitters, which consist of a high-powered oscillator (up to 3W). However, these are not recommended because they tend to have very poor frequency stability and non-existant interference suppression.
Crystal controlled transmitters
A disadvantage of the VCO is that the frequency may drift slightly, especially if the transmitter is in an area which is prone to fluctuations in temperature (such as a tower block roof!). This is not ideal at the best of times, and can be especially problematic in areas where clear frequencies are in short supply and close to frequencies used by other stations (legal stations included).
Where high frequency stability is needed, a transmitter may use a crystal-controlled oscillator to control the frequency. The disadvantages of crystal controlled transmitters are cost (crystals are relatively expensive and hard to obtain), and the transmitter can only be used on one frequency (may be a problem if the station needs to move frequency, due to another station taking their original frequency).
PLL (Phase Locked Loop) Transmitters
The "best of both worlds", a PLL transmitter has a stable reference oscillator (crystal controlled), a VCO, and circuits which constantly "correct" the frequency of the VCO and keep it stable. The design of PLL transmitters allows the transmit frequency to be set (or "programmed"), generally by the use of small switches on the circuit board.
In the early 1980s, PLL circuitry was relatively complex so PLL transmitters tended to be too expensive to be used by all but the largest Pirates. However, over the years the price of PLL circuitry has fallen, so PLL transmitters are more commonplace. This is just as well, because clear frequencies are in increasingly short supply across much of the UK, especially in the larger towns where Pirates tend to operate.
Transmitter power
The output of the transmitter's oscillator (the circuit which produces the radio waves) is generally low, measured in milliwatts. To raise the power to a usable level, amplification is necessary. This is achieved by amplification circuitry, involving high-power transistors.
Higher powers, in tens or hundreds of Watts, tend to be achieved by having a number of RF amplifier stages coupled together. This becomes more complex, as the transistors must be matched using matching circuitry.
It should also be borne in mind that VHF FM transmitters of more than a few Watts generally contain transistors which contain Beryllium Oxide. This is highly toxic, cancer-causing and pretty horrible stuff! It's safe enough as long as it stays inside the transistors, just take care not to crack open an RF power transistor!
