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.