FM TRANSMITTERS
FM is an abbreviation for Frequency Modulation. Modulation is how information is imparted to a radio frequency signal. In the case of FM the audio signal modulates what is called the carrier frequency (which is the frequency of the broadcast signal) by causing it to shift up and down ever so slightly in response to the level of the audio signal. An FM radio receives this signal and extracts the audio information from the radio frequency carrier by a process called demodulation.
Modulation of the signal takes place within the FM broadcast transmitter. The transmitter consists of several different sections: the oscillator, phase locked loop, and gain stages. Generation of the broadcast carrier frequency is the responsibility of the oscillator section. Tuning (as distinct from modulation) or changing the frequency of the oscillator section is either done electronically or manually. For a practical radio station that will be operated for more than a few minutes, it is almost essential to have the tuning done under electronic control since free running or manually tuned oscillators will drift in frequency due to temperature and inherent design limitations. This is an important consideration in selecting a transmitter. Since one of the goals is to deprive the FCC of technical objections to micropower broadcasting, it is critical to have transmitters that stay on frequency and do not drift. This, of course, rules out using transmitters based on free running oscillators. Frequency control brings us to the next section. Oscillator frequency drift is corrected by a circuit known as a phase lock loop (PLL) controller. In essence, it compares the output frequency of the oscillator to a reference frequency. When the frequency starts to drift it applies a correction voltage to the oscillator which is voltage tuned, keeping it locked to the desired frequency. In a PLL circuit the frequency is selected by setting a series of small switches either on or off according to the frequency setting chart that comes with the transmitter. In some cases, the switch array may be replaced by four dial-up switches that show a number for the FM frequency of transmission, i.e. 100.1 for 100.1 MHz. Even simpler, some units have a display like a digital radio with up and down buttons for changing frequency. One part of the oscillator section, the voltage tuning circuit, serves a dual purpose. As described above, it allows the oscillator to be electronically tuned. In addition, it is the means by which the broadcast carrier frequency is modulated by an audio signal. When the audio signal is applied to this section the variations in the audio signal voltage will cause the frequency of the oscillator to shift up and down. Frequency shifts brought about by audio modulation are ignored by the PLL controller due to the inherent nature of the circuit design. It is important not to over modulate the transmitter by applying an audio signal whose level is too great. Many transmitters are equipped with an input level control which allows one to adjust the degree of modulation. Further control of the audio level is provided by a compressor/limiter which is discussed in the studio section. As the modulation level increases, the amount of space occupied by the FM signal grows as well. It must be kept within a certain boundary or interference with adjacent FM broadcast channels will result. FCC regulations stipulate a maximum spread of plus or minus 75,000 cycles centered about the carrier frequency. Each FM channel is 200,000 cycles wide. Over modulation- the spreading of the broadcast signal beyond these boundaries- is known as splatter and must be avoided by controlling the modulation level. As a result, the signal will be distorted and interference with adjacent channels will take place. Following the oscillator section are a series of gain stages which buffer and amplify the signal, bringing it to a sufficient strength for FM broadcast purposes. In most cases this will be 1/2 to 1 watt of output power. This level is sufficient for a broadcast radius of 1-2 miles depending on circumstances. For increased power, a separate amplifier or series of amplifiers is used to raise the power level even higher. Amplifiers are covered in the next part of this primer. Transmitters are available in kit form from a number of different sources including Free Radio Berkeley, Progressive Concepts, Panaxis and Ramsey. Assembly requires a fair degree of technical skill and knowledge in most cases. Free Radio Berkeley offers an almost fully assembled 1/2 watt PLL transmitter kit requiring a minimal amount of assembly. Kits from Ramsey are debatable in terms of broadcast quality. An English firm, Veronica, makes some nice kits as well.