Background

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Radio Astronomy Background
Radio astronomy is the study of celestial objects that emit radio waves. With radio astronomy, scientists can study astronomical phenomena that are often invisible in other portions of the electromagnetic spectrum. Using radio astronomy techniques, astronomers can observe the cosmic microwave background radiation, which is considered the remnant signal of the birth of our universe.

The properties of electromagnetic radiation depend strongly on its frequency. Electromagnetic radiation is produced whenever electric charges change either the speed or direction of their movement. In a hot object like the sun, molecules are continuously vibrating or bumping into each other, sending each other off in different directions and at different speeds. Each of these collisions produces electromagnetic radiation at frequencies all across the electromagnetic spectrum.

Electromagnetic radiation is also generated as the result of free electrons or protons spiraling around magnetic field lines. The electrons and protons become energized in the magnetic field and begin to accelerate. As the electron and protons accelerate they release electromagnetic waves. It is observed that the shorter the wavelength (and higher the frequency), the more energy the radiation carries. It is this radiation which can be detected by radio telescopes.

Earth’s atmosphere acts as a barrier to much of the electromagnetic spectrum. The atmosphere absorbs most of the wavelengths shorter than ultraviolet, most of the wavelengths between infrared and microwaves, and most of the longest radio waves. This leaves only visible light, some ultraviolet, infrared, and short wave radio to penetrate the atmosphere and bring information about the universe to the surface of our planet. One of the largest frequency ranges allowed to pass through the atmosphere is referred to as the radio window. The radio window is the range of frequencies from approximately 5 MHz to over 300 GHz (wavelengths of almost 100m down to approximately 1mm). Although these wavelengths have no discernable effect on the human eye, they can be detected with an antenna. Electronic filters in a receiver can be tuned to amplify a small frequency range at a time or, using sophisticated data processing techniques, thousands of separate narrow frequency bands can be detected. With this method it is possible to find out what frequencies are present in the RF radiation and what their relative strengths are.

Radio JOVE Background
 The radio JOVE project is a collaborative effort by NASA and several universities that began in 1998, to get students and amateur radio operators interested in radio astronomy by designing a relatively inexpensive direct conversion radio receiver and antenna kit. The kits come with everything you would need to listen to RF radiation coming from Jupiter and the Sun. The receiver is powered either by battery or wall transformer, has an adjustable gain feature, and an audio jack to plug headphone into or to connect it directly to the microphone jack on a personal computer. Software is used to see waterfall plots, strip charts and record data which can be shared with other amateur radio astronomers and scientists by submitting your data online. The receiver its self is a simple design. A block diagram can be seen in figure 1.2.1. The main limitation of the radio JOVE receiver is the narrow 3.5KHz range limited by the lowpass filter and the small dynamic range of 25dB.

Figure 1.2.1: Radio JOVE receiver block diagram

Software Defined Radio Background
A software-defined radio system, or SDR, is a radio communication system where components that have typically been implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices. The ideal receiver scheme would be to attach an analog-to-digital converter to an antenna. A digital signal processor would read the digitized data from the converter, and then its software would transform the stream of data from the converter to any other form the application requires. Most present day receivers use a variable-frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. In some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the analog-to-digital converter (after amplification). Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt, nanowatt-power radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example, if spurious signals are present (which is typical), these compete with the desired signals within the amplifier's dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility - which some see as the main purpose of a software defined radio. A simple block diagram of an SDR receiver is shown in Figure 1.2. Figure 1.2: Simple Generic SDR Receiver Diagram

The receiver performance of this style of SDR is directly related to the dynamic range of the analog-to-digital converters used. Radio frequency signals are down converted to the audio frequency band, which is sampled by a high performance ADC. The newer software defined radios use embedded high performance ADCs that provide higher dynamic range and are more resistant to noise and RF interference. The SDR software performs all of the demodulation, filtering, and signal enhancement.