A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses. A transmitter can be a separate piece of equipment, or an electronic circuit within another device. Most transmitters consist of an electronic oscillator which generates an oscillating carrier wave, a modulator which impresses an information bearing modulation signal on the carrier, and an amplifier which increases the power of the signal. To prevent interference between different users of the radio spectrum, transmitters are strictly regulated by national radio laws, and are restricted to certain frequencies and power levels, depending on use. The design must typically be certificated (formerly type approved) before sale. An important legal requirement is that the circuit does not radiate significant radio wave power outside its assigned frequency band, called spurious emission.
Design issues A radio transmitter design has to meet certain requirements. These include the frequency of operation, the type of modulation, the stability and purity of the resulting signal, the efficiency of power use, and the power level required to meet the system design objectives. High-power transmitters may have additional constraints with respect to radiation safety, generation of X-rays, and protection from high voltages. Typically a transmitter design includes generation of a carrier signal, which is normally sinusoidal, optionally one or more frequency multiplication stages, a modulator, a power amplifier, and a filter and matching network to connect to an antenna. A very simple transmitter might contain only a continuously running oscillator coupled to some antenna system. More elaborate transmitters allow better control over the modulation of the emitted signal and improve the stability of the transmitted frequency. For example, the Master Oscillator-Power Amplifier (MOPA) configuration inserts an amplifier stage between the oscillator and the antenna. This prevents changes in the loading presented by the antenna from altering the frequency of the oscillator.
Determining the frequency
Fixed frequency systems For a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a crystal oscillator to fix the frequency. Where the frequency has to be variable, several options can be used.
Variable frequency systems An array of crystals – used to enable a transmitter to be used on several different frequencies; rather than being a truly variable frequency system, it is a system which is fixed to several different frequencies (a subset of the above). Variable-frequency oscillator (VFO) Phase-locked loop frequency synthesiser Direct digital synthesis
Frequency multiplication
While modern frequency synthesizers can output a clean stable signal up through UHF, for many years, especially at higher frequencies, it was not practical to operate the oscillator at the final output frequency. For better frequency stability, it was common to multiply the frequency of the oscillator up to the final, required frequency. This was accommodated by allocating the short wave amateur and marine bands in harmonically related frequencies such as 3.5, 7, 14 and 28 MHz. Thus one crystal or VFO could cover several bands. In simple equipment this approach is still used occasionally. If the output of an amplifier stage is simply tuned to a multiple of the frequency with which the stage is driven, the stage will give a large harmonic output. Many transmitters have used this simple approach successfully. However these more complex circuits will do a better job. In a push-push stage, the output will only contain even harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit are canceled by the second device. In a push-pull stage, the output will contain only odd harmonics because of the canceling effect.
Adding modulation to the signal The task of a transmitter is to convey some form of information using a radio signal (carrier wave) which has been modulated to carry the information. The RF generator in a microwave oven, electrosurgery, and induction heating are similar in design to transmitters, but usually not considered as such in that they do not intentionally produce a signal that will travel to a distant point. Such RF devices are required by law to operate in an ISM band where interference to radio communications will not occur. Where communications is the object, one or more of the following methods of incorporating the desired signal into the radio wave is used.
AM modes When a radio frequency wave is varied in amplitude in a manner which follows the modulating signal, usually voice, video or data, we have amplitude modulation (AM).
Low level and high level In low level modulation a small audio stage is used to modulate a low power stage. The output of this stage is then amplified using a linear RF amplifier. The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence high efficiency class C amplifiers cannot be employed, unless a Doherty amplifier, EER (Envelope Elimination and Restoration) or other methods of predistortion or negative feedback are used. High level modulation uses class C amplifiers in a broadcast AM transmitter and only the final stage or final two stages are modulated, and all the earlier stages can be driven at a constant level. When modulation is applied to the plate of the final tube, a large audio amplifier is needed for the modulation stage, equal to 1/2 of the DC input power of the modulated stage. Traditionally the modulation is applied using a large audio transformer. However many different circuits have been used for high-level AM modulation.
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