From the April 2004 Idaho Observer:
Liberty Fix at Six
A forum for discussing the importance and methodology of establishing a national network of Low Power Radio Stations
by Mike Heit
Welcome to the Liberty Fix at Six Low Power FM column, brought to you by The Idaho Observer as a community service for freedom of speech. In this third article, I will expand on the basic operation of a low power FM radio station, also called mircobroadcasting. In previous articles, I covered some of the basic components of a LPFM system. In this one let us take a closer look at the components of a system, what each do, and how they relate to the quality of the broadcast signal as transmitted to the public.
The Exciter Module
As I stated in an earlier column, the component of the LPFM system that converts the sound we can hear as human beings into signals that can be transmitted a long distance and then recovered and listened to by many, is known as the exciter. In Figure 1 is a typical LPFM exciter board that I build for use in my LPFM systems. There are four main sections to the exciter. The power regulator section, the frequency synthesizer section, the audio input section, and the RF Output section.
Now we shall take a deeper look at each section and what it does.
The Power Regulator Section: As the name suggests, the Power Regulator Section as seen in the upper left hand corner of the picture in Figure 1, is the section where the input voltage from the 13.5 volts DC is sectioned off. Part of it, the 13.5 volts DC, is used to supply up to 9 volts DC at a steady current for the RF output section, and part of it is regulated down to 5 volts DC for the microprocessors mounted on the board.
A power regulator integrated circuit is mounted on the board in this section, and if the power supply varies in either its voltage or current, the power regulator IC will keep the voltage and current supplied to the circuitry on the exciter board constant. This will assure a good signal out to the airwaves.
The Frequency Synthesizer Section: The frequency synthesizer section is the section where a frequency signal is generated; it is this signal that will become the carrier signal taking the music and voices we hear on the receiver end.
At the left of the frequency synthesizer section are some red and white switches. These are set to the frequency that the exciter is set to transmit on. This set of switches creates what are called logic commands. By turning the switches in either the HI or LOW position, a digital signal is created; this signal is then applied to the digital logic integrated circuits [those little black squares with silver legs on them] and then the final output waveform is applied to the Audio Input Section and the RF Output Section.
There is a crystal mounted near the final section of the digital logic frequency section, it is used to prevent the tuned section from drifting off frequency. It also serves to create a very narrow logic signal, and this is a very important feature for LPFM operations. I will discuss that issue in detail in a future column.
The Audio Input Section: The audio input section is where the sound is introduced into the exciter circuitry. Human beings can only hear sounds up to the 20,000-hertz [20KHz] range. Beyond that we cannot hear sound waves. Radio signals used to transmit audio energy we can hear are located in the frequency spectrum called light waves. In the case of the LPFM exciter, that spectrum falls between 87,500,000 cycles per second [87.5 MHz] up to 107.9 MHz.
The audio input section is also known as the mixer stage since the radio frequency signal from the synthesizer stage and the audio frequency signal from the CD, or microphone, is mixed together in a process called heterodyning.
Once the audio coming into the exciter is processed, and mixed with the carrier frequency signal from the frequency synthesizer, it is further filtered (those silver coils the red pointer is marking) so that no frequencies other than the one we set the switches to can be passed into the RF output section. The mixed signal is passed to the RF output section where it is amplified and sent to either the antenna, or to another amplifier if the final signal is to be of a much higher power level.
The RF Output Section: This is the section where the mixed signal, set to a specific frequency, is amplified from a few microwatts [.W] up to a level of one to four watts RF energy. This is enough of a power signal to transmit approximately one to two miles using a tuned antenna.
In the RF output section, a low-pass filter is also used to prevent any stray signals that were either created in the circuitry board or induced into the exciter, from being transmitted out the antenna. This could cause, at best, distorted-sounding music, and at worst, interfere with someone else's transmissions -- and that is certain to cause big problems with the neighbors not to mention the FCC.
If one to four watts is too limiting in range, the output signal can be sent to another amplifier section where it can be converted up to well over 100 watts RF. The exciter shown in the photo above can drive up to a 250-watt power amplifier. I will cover high power amplifiers in another column.
That pretty well covers the basic operation of the exciter board. It is the first major component in a LPFM system. In addition to the exciter, there is also a compressor limiter board, a power amplifier can also be added to the system and most often is, and the antenna system. The photo below in Figure 2 shows a typical 100 to 300 watt RF amplifier module.
In next month's column I will cover RF amplifier modules, power supplies that drive those monsters and the compressor limiter boards that keep them from blowing the wax out of our ears.
Thank you very much for all your comments and ideas. You may contact me through my website or through The Idaho Observer. I welcome all comments and suggestions; I will try to answer as many of your questions as I can. Please feel free to send us an email. May God richly bless each and every one of you.
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