Basic concepts in Radio communications
In this first subject we are going to try to communicate to us by means of waves at a distance (that can be mechanical or of radio frequency). We will follow the book of paraninfo, that once read the different sections, we will make the activities that we are going to expose in each one of the sections.
- We see a ray that sounds past 5 seconds. Compute the range where one has taken place.
- The sonic speed in the air is 340 m/s. To calculate the speed in km/h
- Two points located in the moon are 2000 meters. To calculate the time that takes a receiver if the emitter emits a sound sufficiently hard.
- A train visualizes when we are with the ear stuck to the route of the train. If we took 6 seconds in listening to the sound of the wheel of the train, compute the range of the listener to the machine invented by George Stephenson (British, inventive of the first locomotive between Liverpool-Manchester, 1830).
- Task of investigation. Search in Internet the physical factors that influence in the speed of propagation of the mechanical and electromagnetic waves, as for example, the temperature in the mechanical waves. Also, to add as it varies the speed with the increase or diminution of that physical variable. For example, if an increase of the temperature causes that the speed raises or lowers.
To part of the concepts given in the book, we must remember that the average value of a signal is the average of the values of a signal in a period.
The effective value of a signal is the value that, in cc, generates the same power that the signal in alternating. The relation for a sine one is
Effective value = Value of tip/2
- A sine signal has an effective value of 100 volts. To calculate the value of tip
- To calculate the average value of a sine signal of 50 volts of tip
- A sine signal behaves by equation V (t) = 50 sen w*t, where w = 2 * Î * f radians. To calculate the value that takes at moments 1, 3 and 4.1237 seconds, if the frequency is 1 Khz. Solution
- To calculate the frequency of a signal whose period is 60 milliseconds
- To calculate the wavelength of 5 the WIFI signals of 2.4 and GHz
- To calculate the average value of the signal that is in the image. We have simulated to the right which must leave to create a track of work.
- To calculate the average signal of the following wave.
- A signal has form v (t) = 100 sen (wt) with a frequency of 20 Mhz. To calculate the times where the signal takes the maximum and minimum values.
Linear functions and NONLINEAR
Generally, a function represents, of mathematical form, a variation of something when another thing varies, for example, represents the distance of a mobile when it spends the time. In this first case, that representation, in graphical form, has the aspect of the left image. If a bike moves secondly at a speed of 1 meter/, in the first one secondly (axis of the time) we have a meter of route, in 2 secondly already has advanced two The human ear does not respond to a linear function either. This means that if we increased the sound pressure of a loudspeaker by 2, we do not have the sensation much less of which we have double power, but. Of some way, it is possible to be shelp that the ear is sluggish and we must generate great increases of pressure so that an increase is perceived. In particular, an increase of 10 times the power, is perceived by the ear as an increase of the double.
This imagines by the logarithmic functions, where is an example in the following graph:
The curve is defined by a logarithmic function (the inverse one to the exponential one), so that great increases in x-axis are needed to have small increases in the axis and. we will at greater length see It in the following link
1 the decibel is defined as an acoustic and electrical unit of measurement that is defined as 10 times the logarithm in base 10 of the quotient between the power that we measured and a reference power, that is:
db = 10 lg (Ws/We)
Where Ws is We and the power output the reference power
In the case of measurements of sound, EP is taken as the minimum sound pressure perceived by the human ear (20 micropascales), in which case, take the pressures to the squared one, this is:
What the logarithm in this case does is to adapt the numerical variation to the sensitivity of the ear., for example, an increase of the pressure of 10 times, of 20 to 200 micropascales, is translated in an increase of 20 * lg 200/20 = 20db
If we increased other 10 times but, we have 20 lg (2000/20) = 40 db
That is, for an increase of 10 times in the power, it has only increased to 2 times db.
We have two types of db. When we want to measure powers, the coefficient is 10, but when it is pressures (sound), tension or intensities, the coefficient will be 20.
The Db are of special importance within the communications. Before following with the subject, we are going to deepen a little in the mathematics of the logarithms to have suitable ease and to acquire some interesting tricks. For it, we can unload this document of the ICTP.
Activities with dB
1 Calcular the loss of a system whose power of entrance 5mW and the power output is 1 mW.
a) 3 dB
b) A factor of 2
c) 10 dB
d) 6 dB
a) It raises by a factor of 2
b) dB increases 3
c) dB increases 6
d) dB increases 10
a) 60 dB
b) 40 dB
c) 20 dB
d) 6 dB
6 Entramos a signal of 2mW a 345 Y amplifier we have when coming out of mW.
Which is the gain of power in decibels?
7 Calcular the gain in db of an amplifier if its gain is 400
8. A cascaded amplifier has as first stage a gain of 25,8 and the second stage of 117. To calculatethegainintensionof1and2stage, as well asthetotalgainindb
9. The gains of three 20 amplifying stages are dB, 32 dB and 46 dB. To calculate the total gain of the system in db and tension
10. An equipment enters a signal to him of 2 mW and that it finds dull, which is its power when coming out?
a) -30 dBm
b) 0 dBm
c) - dBm
11. We have a system with entrance of signal of 4 mW made up of a line of communication with 12 dB of attenuation, a power amplifier of equal gain to 35 dB, and finally a line of communication with 10 dBde loss. Which will be the power when coming out?
Relation between the waves and the matter
The entailment between the waves of RF and the influence that this one exerts on the matter was studied mainly by Max Plank and comprises of the exciting quantum mechanics.
Of simplified form and without entering in detail, this theory says us that the energy that has fot³n* is equal to a constant (the one of the author, plank) by the frequency of the wave, that is:
E = h * f
By the work that carried out Einstein on the photoelectric effect, to this equation equation of Plank-Einstein is called to him.
If that energy is the one that needs the electron that turns in the nucleus to leave the nucleus, it will make it and happen be a free electron within the material. The exaggerated image is something. They do not leave the material, which wants to show is the abandonment in the nucleus of the atom.
* photon: It is the elementary particle, of mass zero and that travels at the speed of the light and that she is protagonist of the electromagnetic manifestations,
Development of the https://es.slideshare.net/sergiusz2/propagacin-de-ondas-electromagnticas-66982426 subject
The noise as it says the book well, to the set of signals is defined that, without being own of the signal. they are connected to the same, created a distortion of the signal.
The equipment with small signals of noise will have better quality than what they connect those signals nonwished.
We have 3 types of noises.
- Thermal noise. He is the caused one by the agitation of atoms within the matter. We leave an illustration where this movement imagines, that increases when increasing the temperature and it is annulled when arriving at absolute zero (-273 C)
- Pink noise (or 1/f). The noise diminishes when increasing the frequency, for that reason 1/f
- Impulsive noise, from diverse origin, comes caused by pulses (signals of high value and cuts duration
In order to know if we have something good not in hand or, we considered a denominated factor SNR, that comes defined as the power of the signal and the power of the noise. As the power divisions go to subtractions in decibels, he is but simple to use dBm, so that
Referred SNR are called to DBM as the difference in decibels between the received signal and the basic noise level (noise level). For example, if one radio (device client) receives a -75 signal dBm and the noise level is moderate to -90 dBm, the 15 SNR is of dB.
In the previous relation, we have the SNR is the relation between the signal and the noise, therefore we have
If we replaced the SNR definition, to the signal and the noise and we passed it to decibels, we have:
Propagation of Electromagnetic waves
It is necessary to say that, although a priori, the waves are in force by some formulas of electromagnetic physics, is not the same to be in the desert that in a large city. What we are going to find in the city is going to modify the behavior of the field. We are going to see this video where it is spoken of these aspects
Effect of the matter in the attenuation of the RF signal
The conductive means of the signals influence in the attenuation that this one is going to suffer. It is not just like a signal crosses a concrete wall that another one of plaster. The constitution of the matter is determining. In the book we have the aspects that influence in the attenuation of the RF in the air, detailing 4 cases.
Activity. It studies the 4 cases of signal attenuation that is indicated in page 11 of the book, establishing some relation among them. For this exercise, search in Internet the attenuation of diverse materials in the attenuation.
1.4.5 Frequency bands
Activity. In the table of frequencies, we began by the VLF (indicating VLF, from the 10 KHz to the 30 KHz), soon the LF, MF, etc. To make table with
- Abbreviations of band
- Rank of frequencies
- A great column where it is indicated how they change the characteristics
- Another column where it is indicated how they change the applications
With respect to both last points, in that column one is due to put things as In LF we have low attenuations, increasing as goes of band
Activity. To visit the Web tdt1 and search the information of three contiguous channels. To write the frequency of the channel and to deduce the bandwidth of the channel.
Propagation of signals in the layers of the atmosphere
To part of direct communications between emitter and receiver, the signals can go by the low layers of the atmosphere (troposphere) and the discharges (Ionospheres)
The atmosphere is layer that surrounds to the Earth and that is formed by numerous gases in different proportions, (carbon dioxide or CO2, oxygen, hydrogen, helium, nitrogen and aqueous vapour).
It is possible to emphasize what it happens in the Ionosphere, that starts off from 40 km to the final limits of the atmosphere. In this layer it happens phenomena of ionization of the gaseous atoms that in her exist (hydrogen, helium). These atoms, by the effect indicated above of plank, lose electrons positively, being the loaded atoms, next to an electron cloud. This circumstance makes possible the reflection of the HF signals.
- It explains each one of the elements that take part in a communication fitting each one of the terms to a concrete example.
- It looks for what speed moves the sound in the water, air and the iron. Compute the range of a ray if we listened to it the 5 seconds
- Fundamental difference between the hertzian waves and the mechanics
- Emitter, receiver and station of radio communication. It realises a summary next to a graph to explain the process of transmission by radio frequency waves (apart 1.1.3)
- It investigates what is the bandwidth of audible sounds in the man, the dog and other two animal to choose.
- With the help of a graph, it explains the terms of Amplitude, Frequency, period and wavelength
- To also calculate the wavelength of a wave of 10 kHz when propagating by the air and when propagating by the water.
- Bandwidth. It explains that it is with the help of a drawing and investigates real bandwidths (auditory lamps, phantom, etc)
- With the help of the table of page 7, it passes the following units: a) 345 Gb to kb, b) 560000 microvolts to volts
- It explains that it is the decibel. Differences between decibel for signals and the sound. It realises a graph where we give several values to the X-axis and in the one of ordinates the result in logarithmic form imagines and as quotient between the entrance and exit. The value of entrance reference will be 10.
- It realises a study of how one goes the electrical signals to sound and vice versa
- With the help of a drawing, it explains the sampling, the quantification and codification of a signal
- It investigates the experiments of Maxwell to explain the radio frequency
- It investigates the channels used in the TDT of your zone. It explains that it is the term of Multiplex
- Reach, noise and attenuation. The signals generated by different devices are disturbed by noises (to find out the types that exist). These noises are reconciled to the signal, creating a final signal, as sum of the generated signal and the connected noise. The factor of noise (f) is defined as the quotient when coming out enters the Signal/Noise the entrance of a device and the /Ruido Signal of the same. It offers a information to us of how much the signal when happening through a device is degraded. It completes this study.
- That it is to modulate and to demodulate a signal. It explains with the help of an image the modulation A.M. and FM. That advantages have FM with respect to the A.M.?
- Radioelectric phantom. More remarkable summary and aspects.
- It realises a study on the effects of means on the signals (reflection, diffraction and refraction). Influences of the atmosphere.
- It realises a simple study of because the fading of a signal takes place. It uses a drawing to explain it
a) The Earth is curved and without embardo two radio hams who are a great distance among them, are able to communicate. You do a study on this subject that explains because it happens. How he affects the atmospheric conditions?
Problem of the sine signal.
V (t) = 50 sen w*t, where w = 2 * Î * f radians. Steps:
a) We are going to calculate the first value of tip. Good, already we know the value of tip, that is 50, but are going to calculate the time for which, we have the maximum signal
b ) The frequency is 1kHz, which allows to know us the period, that 1 millisecond is the same to 1/1000 =
c ) If in a millisecond the wave is completed, in a quarter of time we have the maximum point of the wave, therefore in 1/4 of millisecond, or what is the same 1/4 * 10 -3
d) We calculate V = 50 * sen (2 * Î * f * t) = 50 * sen (2 * Î * 10 3 * 1/4 * 10 -3) =
50 * sen (Î /2).
e) Now we must ourselves assure that our calculators are in the correct way. If it is in radians, when doing sen (Î /2) must leave one. If it does not leave one, to try to happen to degrees, thus, 180 Î is and sen (90) = 1
f) Once verified this, we make the rest of the problems. This and anyone, with the security that our calculator is in the correct way
g) For example, for the 3 seconds, the equation is: 50 * sen (2 * Î * 10 3 * 3) = 50*sen (6 * 50 Î * 1000) = sen (Î ) = 0
h) I have eaten 6000. Yes, but because it is possible to be done. As 2* Î is 360 degrees, the wave has completed a complete return and therefore, I can eliminate the complete returns that exist and to remain and so it exceeds, that in this case has agreed with an even number.
i) For the 4.1237 seconds, we have left 50 * sen (2 * Î * 10 3 * 4.1237) = 50*sen (8247.4 Î ). This is 8246 complete returns but 1.4 returns to complete. If we took that rest leaves to us equal: 50 * sen (1.4 Î ) = 50 * (- 0,9511) = -47,55 volts