English Developing and integrating a computer-based AM / FM radio station

This article presents the development of a personal computerbased AM/FM radio station operating on the AM/FM commercial band. Developing the graphical user interface for the software and the interconnection with an RF function generator by means of a GPIB is presented. Computer users can enter the desired transmission frequency within the commercial band as well as the transmission potency.


A brief history of radio
This communications' medium emerged from a succession of discoveries which began in the late nineteenth century.
A esa primera comunicación le siguió una serie de avances tecnológicos que poco a poco fueron mejorando la calidad de las comunicaciones y también la cantidad.El origen de las estaciones de radio en distintas partes del mundo ha sido muy diverso: estatal, privado, con fines culturales, con fines comerciales, etcétera.

In English
The first wireless telegraph communication happened on May 14 th 1894 between Laverck Point and Fratholm Island in the Bristol Channel which are separated by a distance of about 5 miles.The author of this great discovery was Gugliermo Marconi, an Italian physicist (Tomasi, 2003).This first wireless communication continued a series of technological advances which gradually improved communication quality and quantity.The origin of radio stations in different parts of the world has been very different (i.e. government, private, for cultural, commercial).
They were originally designed for information purposes only and then began to vary their range of action to provide information, music, culture, entertainment, etc.

General purpose interface bus.
Hewlett-Packard designed the Hewlett-Packard interface bus (HP-IB) in 1965 to connect their line of programmable instruments to their computers.This bus quickly gained popularity due to its high transfer rates (nominally 1 Mbytes / s).It was later accepted as IEEE standard 488 in 1975 and was involved in ANSI / IEEE 488.1 in 1987.The name general purpose interface bus (GPIB) is more used today than HP-IB.ANSI / IEEE 488.2 strengthened the original standard defining how controllers and instruments communicate in 1987; standard commands for programmable instruments (SCPI) took command structure defined as IEEE 488.2 and created a single comprehensive programming command which is used with any SCPI instrument.

Fundamentals of amplitude modulation (AM)
This modulation technique varies radio frequency wave amplitude.Amplitude modulation occurs when a low frequency (LF) signal controls the amplitude of a high frequency (HF) wave.Radio and television could not have been possible without such modulation.
Transmission involves two fundamental processes.Low frequency (LF) information is mixed with the radio frequency (RF) carrier; this is called modulation (Jardon, 2002).Then there is the demodulation process (i.e.information retrieval, which is also called detection). Figure 1a shows an example of a 300 Hz information signal; Figure 1b shows the 5 kHz carrier signal.It can be seen in Figure 1c that amplitude modulation produces an envelope signal on the carrier.

Fundamentals of frequency modulation (FM)
The carrier's peak amplitude remains constant in FM, while carrier frequency is changed by the action of the modulating signal (Ibarra et al., 2007).Since information signal amplitude varies, it produces proportional shifts in carrier frequency.Increasing modulating signal amplitude also increases carrier frequency.If the amplitude of the first decreases then carrier frequency also decreases, as shown in Figure 2. Likewise, the inverse relationship can be implemented.El espectro electromagnético de la banda de FM se encuentra entre 88 MHz y 108 MHz y las ondas portadoras para estaciones individuales están separadas por 200 kHz para un máximo de 100 estaciones.Estas estaciones de FM tienen una desviación máxima de 75 kHz de la frecuencia central, lo que les deja 25 kHz arriba y abajo (bandas de guarda) para minimizar interferencias con la banda de la frecuencia adyacente.
In English The FM band's electromagnetic spectrum is between 88 MHz and 108 MHz; carrier waves for individual stations are separated by 200 kHz (up to 100 stations).These FM stations have a maximum 75 kHz deviation of centre frequency, leaving them 25 kHz above and below (guard bands) to minimise interference with adjacent frequency bands.This separation of the stations is much wider than that for AM stations, thereby allowing the transmission of much wider frequency bands for more high-fidelity broadcast music.This also allows the use of sub-carriers to enable FM stereo signal transmission.
Varying modulating signal amplitude changes carrier frequency deviation.The number of sidebands produced, their amplitude and spacing depends on modulating signal frequency deviation.It should be remembered that an FM signal has constant amplitude.
As an FM signal has the sidebands' frequency, then these bands' amplitude should vary with modulating signal frequency deviation and frequency.In theory, the FM process produces an infinite number of upper and lower sidebands and, therefore, a theoretically infinite bandwidth.However, in practice only sidebands having larger amplitudes are those contributing to carrier information.Any side band whose amplitude is less than 1% of that of the unmodulated carrier is usually considered negligible.The FM thus passes through the communication channels or circuits in an infinite bandwidth.A pesar de ello, el ancho de banda de una señal de FM en general es más amplio que el de una señal de AM con la misma señal moduladora.
equivale a la frecuencia de la señal moduladora.p v es el valor pico de la portadora sin modulación.
La onda de FM se expresa como una composición de ondas senoidales de frecuencias y amplitudes diferentes que al sumarse dan una señal de FM en el dominio del tiempo.El primer término es la portadora con la amplitud dada por el coeficiente 0 J .El siguiente término representa un par de frecuencias laterales superior e inferior iguales a la suma y diferencia de la frecuencia portadora y la frecuencia de la señal moduladora.La amplitud de estas frecuencias laterales es 1 J .El término que sigue es otro par de frecuencias laterales igual a la frecuencia portadora ± 2 veces la frecuencia de la señal moduladora.

Desarrollo experimental
En la figura 4 se muestra el diagrama a bloques de la estación de radio AM/FM desarrollada (Luna, 2005), el usuario desde la computadora personal (PC) reproduce la señal de audio con cualquier software comercial, el audio del locutor se introduce a la PC por medio de un micrófono común para tarjeta de sonido y el mezclado de las señales de audio se realiza dentro de la PC.La PC envía la información del audio a través de la tarjeta de sonido al generador de RF (Rohde & Schwarz SML01), así como también la información de la frecuencia y potencia de transmisión a través del bus GPIB, selecciona tipo de modulación, activa/desactiva modulación y enciende/apaga la señal de RF.El generador de funciones efectúa el mezclado de la señal de información con la portadora de RF, generando la señal de AM/FM, la cual es enviada por medio de un cable coaxial a una antena omnidireccional.

Desarrollo del software
Este software se desarrolló utilizando la utilería GUIDE de la plataforma de Matlab.
In English However, FM signal bandwidth is generally larger than that of an AM signal having the same modulating signal.
Bessel functions: Given the modulation index, the number and amplitudes of significant sidebands can be resolved in a basic FM signal equation (Frenzel, 2008).Such equation would thus be: ( Where V FM represents the FM signal's instantaneous value and f m , represents the modulation index.The equation given as ( 1) is solved using a complex mathematical process known as Bessel functions (Frenzel, 2008).This approach does not need to be replicated; however, the result is expressed in the following equation ( 2):  The FM wave is expressed as a composition of sine waves having different frequencies and amplitudes so that when the sine waves are added they create an FM signal in time domain.The first term is the carrier with the amplitude given by the coefficient 0 J .The next term represents a pair of upper and lower side frequencies equal to the sum and difference of the carrier frequency and modulating signal frequency.The amplitude of these sidebands is 1 J .
The term that follows is another pair of side frequencies equal to the carrier frequency ± 2 times modulating signal frequency.

Experimental development
Figure 4 shows the block diagram for the AM / FM radio station developed here (Luna, 2005).The personal computer (PC) user would reproduce the audio signal with any commercial software.Broadcast audio would be fed into the PC via an ordinary microphone for sound card and audio signals mixed inside the PC.
The PC would send the audio information through the sound card to the RF generator (Rohde & Schwarz SML01).Information about power frequency and transmission, modulation type selected, on / off modulation, RF signal on / off would also be sent through the GPIB bus.The function generator would mix the information signal with the RF carrier signal thereby producing the AM / FM signals which would be sent through a coaxial cable to an omnidirectional antenna.6 shows that ca nd its power wa despite the fact t e leaks during pro e all power was r ntenna).

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