Until quite recently spread spectrum techniques were almost exclusively in the military domain. Their use in GPS and the latest cellular phones will be followed by many other civil applications. This article, the first of [two] parts, examines the technology by describing an experimental direct sequence voice transmission system as a worked example.
Click here to download the full size of the above Circuit.
Most Communication Engineers are used to minimising transmission bandwidths. The trend has been to use narrower bandwidths, as with the transition from double sideband to single sideband modulation. It is quite obvious that narrower bandwidths permit more communication channels to be packed into a defined frequency band. However the rationale of using the very wide bandwidths required by Spread Spectrum systems needs explanation. Claude Shannon produced a ground breaking paper on the mathematical theory of communication in 1949. Shannon`s resulting theorem can be expressed as: where C = data rate in bits per second, W = bandwidth (Hz), S = average signal power (W), N = mean white gaussian noise power (W). It can be seen from the equation that the only options available to increase a channel`s capacity are to increase either the bandwidth (W) or the signal to noise ratio (S/N). An increase in the signal to noise ratio requires an increase in transmitter power as the noise within the channel is beyond our control! Thus we can either trade power or bandwidth to achieve a specified channel data rate. Because of the logarithmic relationship, increasing the power output is often unrealistic. However if frequency allocation constraints permit, the bandwidth can be increased. An appreciable increase in data capacity or signal to noise ratio (for a fixed data rate) can then be achieved. From the derived relationship it can be...
This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....
characters left: