Wireless: Chapter 3, Radio Communications

Filters removes unwanted F signals.  RF filter either passes or rejects a signal based on frequency.  There are three basic types of filters; low-pass, high-pass and bandpass filters.  Low pass blocks the high, lets the low through, high blocks low and keeps high.  The bandpass sets a range called a passband, which has a min and max threshold.  

Filters are also found in transmitters and are used to eliminate some unwanted frequencies that result from the process of modulating the signal before transmission. 

-Intermediate frequency (IF) signal is the resulting output from the modulation process.  IF signal is filtered through a bandpass filter to remove any undesired high or low frequency signals.  

Mixers combine two radio frequency inputs to create a single output.  Output is in the range of the highest sum and the lowest difference of the two frequencies.  The sum and differences are known as the sidebands of the frequency carrier, and shield transmitted signal from “stray” signals.  It is used to convert an input frequency to a specific desired output frequency.  

 

 

300-3400 Hz would be your voice, 20,000 Hz is the carrier signal

 

542-558 would be the AM bandwidth

Amplifiers increase the amplitude of an RF signal.  RF signals tend to lose intensity (amplitude) as they move through circuits, air or space.  It is an active device that must be supplied with electricity.  They increase a signals’s intensity or strength by using the input signal to control a circuit that “shapes” the output and reflects the shape of the input signal exactly.  

 

 Antennas

 

Design of a Radio System

Designers of radio communications systems need to consider how the systems will be used.  

Multiple Access

Only a limited number of frequencies are available for radio transmission, so conserving the use of frequencies is important.  Conserving a frequency means you share a frequency among several individual users.  Some methods that allow multiple access are:  

CDMA is used primarily for cellular telephone communications.  It uses direct sequence spread spectrum (DSSS) with a unique digital spreading code (PN code).  Before transmission occurs a high rate PN code is combined with the data to be sent and imprints a unique address on the data.  The longer the code is, the more users will be able to share the same channel.  The number of chips in the code determines the amount of spreading or bandwidth.  

 

Transmission Direction

Simplex transmission occurs in only one direction and is rarely used in wireless communication today except for broadcast radio and television.

Switching

Switching involves moving the signal from one wire or frequency to another. 

-Circuit switching is a type of switching used by telephone systems.  A dedicated and direct physical connection is made between the caller and the recipient and lasts until the end of the call.

Signal Strength

Strength of the signal is a radio system must be sufficient for the signal to reach its destination with enough amplitude to be picked up by the antenna and for the information to be extracted from it.  Electromagnetic interference (EMI) is one factor that affects radio signal strength and is also called noise.  Signal-to-noise ratio (SNR) compares signal strength with background noise.   

When signal strength falls near the level of noise interference can take place.  To reduce the interference of noise boost the strength of the signal and use filters when receiving the signal.  Attenuation is a loss of signal strength.  Multipath distortion occurs as a radio signal is transmitted, the electromagnetic waves spread out.