Wireless: More Chapter 2

Analog and Digital 

-Analog signal intensity (voltage or amplitude) varies and is broadcast (occurs) continuously.  Examples are audio, video, voice and light.
-You turn it on, ramps up like a lightbulb, when the power is cut, it powers down and the signal ramps down.

-How do you get a negative voltage with a battery?  Turn it around ;)

-LED cannot use a backward battery.  The way they are built, it only conducts in one direction, so if the flow is reversed, it is blocked, sometimes catastrophically.

-The flow of energy in a battery is out the negative side (gtfo!) and in the positive.

-DC (direct current) Resistance causes electrons to lose their energy over time. 

-AC The highest points are called peaks, the lowest is also called a peak.  P-P (peak to peak) voltage is calculated by the distance between the two.   

-Digital signal consists of discrete or separate pulses.  It has numerous starts and stops throughout the signal stream, for example; Morse code.  Computers operate using digital signals.  The analog signal must be converted into a digital format before it can be stored and processed or interpreted by a computer. 

-A Modem (modulator/DEModulator) is a piece of equipment that converts the distinct digital signals from a computer and encodes them into a continuous analog signal for transmission over analog phone lines.  Modulation is the process of encoding the digital signals (bits) onto an analog wave.

More After the Break

Frequency

-Frequency is calculated by how long it takes for a cycle to complete.  If a cycle takes 10 seconds, the frequency is .1Hz. If it takes 1/10^th of a second, its 10Hz. And so on and forth

-A Cycle is composed of one top and one bottom peak. 

-A Carrier signal is sent by radio transmitters in a continuous wave (CW) of constant amplitude and frequency.  The signal is an up and down wave called an oscillating signal or a sine wave. 

-A change in voltage does not create a change in frequency.  The term Hertz (Hz) is used to measure frequency.  The wave measured as 710,000 Hz is referred to as 710 KHz. 

          -Kilohertz (KHz) is 1,000 Hz

          -Megahertz (MHz) is 1,000,000 Hz

          -Gigahertz (GHz) is 1,000,000,000 Hz

This wave is 3Hz.
 

-Antennas are a length of copper wire or similar material with one end free and the other connected to a receiver or transmitter.  Electrical current moves the antenna at the same frequency as the radio waves.  The electric and magnetic fields are always at a 90 degree angle of each other. 

Transmission Speed

-The speed of transmission is usually shown in bits per second (bps).  The baud rate is another term used in measuring the speed of radio transmission.  It is the number of signal units per second that are required to represent the bits transmitted.  The Baud is a change in the carrier signal.  It is possible to have a change in signal (a baud) represent more than 1bit. 

-Analog modems transmit at a rate of 4800 baud, the maximum number of signal changes per second that a phone line can support.  Current modems can transmit up to 33,600 bps using more complex modulation techniques along with data compression. 

-Bandwidth is the range of frequencies that can be transmitted by a particular system or medium.  It also refers to the maximum data transmission capacity and is only accurate when referring to purely digital systems.

-IEEE 802.3 is what covers the transmission of data over cables.

-Baseband is dc on a wire.  Un-modulated pulses on a wire. Broadband is modulated pulses. Broadband is the range of frequencies that must be carried on a line.  The bandwidth is the space between the top and bottom frequencies. 

Analog Modulation

-Analog Modulation is a representation of analog information by an analog signal. 

-Types of Analog Modulation:

          Amplitude

          Frequency

Phase

-Amplitude modulation (AM); the height of a carrier wave is known as the amplitude and can be measured in volts.  The height of the carrier wave is changed in accordance with the height of the modulating signal.  This is used by broadcast radio stations and are very susceptible to interference from outside sources.

-Frequency modulation (FM) is the number of waves that occur in one second change and is based on the amplitude of the modulating signal.  It is often used by broadcast radio stations and is not as susceptible to interference from outside sources.  FM carrier has a wider bandwidth that allows it to carry Hi-Fi as well as stereophonic signals. 

-Phase modulation (PM) changes the starting point of the cycle.  It is not generally used to represent analog signals.  A signal composed of sine waves has a phase associated with it.  Phase is measured in degrees and one complete wave cycle covers 360 degrees.  A phase change is always measured with reference to some other signal.  PM systems almost always use the previous wave cycle as the reference signal. The following phase shifts are considered to be lagging. 

  

Digital Modulation

-A method of encoding a digital signal onto an analog wave for transmission over a medium that does not support digital signals.  In a digital system, the changes are distinct using binary signals which exist in one of two states, a 1 or 0.  For a computer to be able to understand these signals each bit must have a fixed duration to represent 1 or 0.

-Advantages are:

          Makes better use of the bandwidth available

          Requires less power to transmit

          Performs better when the signal experiences interference from other signals

          Its error-correcting techniques are more compatible with other digital systems

-There are three basic types of digital modulations; amplitude, frequency and phase.  As users demand more transmission speed; today there are dozens of different types of modulation.