A transmitter is intended to alter a signal so it can pass through the media involved to a receiver. In the case of a radio transmitter, the media is one that can support an electromagnetic wave. Examples are space and the atmosphere. There needs to be rapid variations in voltage and current driving the antenna, which converts these changes to an electromagnetic wave (radio wave). Any change in electrical current in a conductor results in electromagnetic waves being radiated, but generally these need to be suitable for the purpose. This means a controlled frequency so many transmitters can operate simultaneously without interfering between each other at the receiver/s. The frequency is chosen for the bandwidth of the message signal, the propagation mode, and according to national and international regulations to minimize spectrum pollution. The signal (information) is impressed on a carrier wave that can be radiated or passed through the medium, by the process called modulation. The carrier wave uses an AC waveform so it can be continually changing, but of a suitable frequency that the resulting electromagnetic wave will be propagated usefully.
The transmitter consists of a number of stages or blocks or modules that together drive the antenna with appropriate signals. Start with a power supply. This provides appropriate voltages at the required current levels. The power radiated from the antenna is drawn from the power supply.
An oscillator generates the reference frequency, use to produce the carrier frequency. This is often a quartz crystal oscillator for its excellent frequency precision. The oscillator frequency may be used directly, but is more commonly converted to another frequency, in a process called synthesis (of a new frequency). This allows a single crystal oscillator to provide different channels selected by the operator.
The electrical power from an oscillator is generally insufficient, and it needs to be buffered from the load (antenna etc) anyway. That is so variations in load do not affect the oscillator. This means that amplifiers are used to increase the power and provide isolation. The amplifier is called an RF amplifier, meaning radio frequency amplifier, considering it operates at some radio frequency. This can be in the range of 10s of kHz to many GHz, depending on the transmitter purpose. The power level, bandwidth and frequency of amplifiers covers a broad range of types.
The carrier signal alone carries little information at a receiver location, simply it is there or not there, and perhaps the direction it came from. The information or message being transferred is modulated on the carrier. There are many different modulation processes that might be used. The receiver must have an appropriate demodulator to suit the transmitted modulation.
The simplest modulation to understand, and the original radio kind, is to just switch the carrier on and off, using these two states to allow a code like morse code to represent the information or message. Morse code is a digital code with long and short periods of off and on. Groups represent letters. It is a form of amplitude modulation, and can be considered digital as there are just the two on and off states and the two periods of on or off states. The encoding was by the operator using a hand operated key (switch) while decoding was achieved by the human operator listening to the receiver sounds. It directly evolves from the earlier land line telegraph systems. This modulation form is still used for garage door openers, so a code can be added to the carrier to provide security. It is now called amplitude shift keying (ASK).
The next form of modulation was analogue amplitude modulation which we know as AM radio. This can transmit speech and music, and with some adjustment was later used for analogue TV. The modulation process takes place in a mixer, but this mixer does not just add the sound signal to the carrier. It multiplies (mathematically speaking) the instantaneous values of the carrier with the instantaneous value of the sound signal. This is called a product mixer or frequency mixer for this reason. A non-linear device is involved. This can be a modified RF amplifier, where the two signals are applied to different parts of the amplifier so they are multiplied. If the audio signal varies the gain or loss of the modulator for the carrier, the carrier amplitude changes, so AM is achieved. The result is the sum and difference frequencies of the carrier and the modulating signal, and usually some harmonics which are not desired. The harmonics are filtered out by tuned circuits using inductors and capacitors. These sum and difference frequencies in an AM broadcast transmitter are called the upper and lower sidebands, and are mirrored each side of the carrier.
Another form of modulation is FM, where the frequency of the carrier is modulated by the message signal. This typically involves a voltage variable capacitor or inductor in the modulator, associated with a carrier frequency oscillator, that changes the oscillator frequency slightly. The amplitude of the carrier is constant, and the instantaneous frequency represents the instantaneous level of the modulating signal. A simple digital version that could transmit Morse code and other such message signals is to just switch between two different frequencies separated by some fixed amount. This is called frequency shift keying (FSK) and was often used to transmit teleprinter messages using short wave radio. A 12MHz carrier might be shifted only a few hundred Hz. This shows one reason for a precision frequency in radio systems.
Many of these principles are used in other mediums like telephone systems over cables or fibers, so there is quite an overlap in technology. Another kind of transmitter is to transmit sensor information reliably over cables in an industrial environment. Modern radios tend to use digital modulation involving many states from a mixture of AM and FM or PM (phase modulation). Look up QAM and OFDM for more information. Another widely used technique is spread spectrum. One form of this is called frequency hopping (for a simple explanation). This involves transmitter and receiver switching channels simultaneously and rapidly. If some channels are experiencing difficulties, it has little effect on the resulting output.
http://en.wikipedia.org/wiki/Modulation