Let's do it.
We said that we have to built a pass band filter that also act as a tuner. If it's possible we want also that the filter is able to amplificate our signal so we can respect the condition on the volt that have to be more than 0.3 so that the envolope detector can works.
José Maria has shown us a very clever solution based on the series of a resistor an inductor and a capacitor. Formally this is a low pass filter but if we choose parameters properly that filter show a peak in the frequency that we want... and so we have the pass band filter and also the amplificator!
I was very impressed when we saw that a simple antenna (coil and ferrite) in parallel with a capacitor (variable) act perfectly as the scheme that we saw for the passband filter, so in the end we have our antenna that is the same of having an antenna, a passband filter, a tuner and an amplificator. Incredible.
In the second part of the lecture we study how we have to make the coil since we have our variable capacitor (we have to buy it). We found that we have to make 60 rotations around the ferrite.
After that we went to the laboratory and we start to make it, in the first time seemed simple but if you want to make it well you have to do a little effort. Now i' m curios to measure the performance of it.
giovedì 30 settembre 2010
mercoledì 29 settembre 2010
28/09/2010
Simple and inexpensive.
We saw that if we want to recover the original sound signal we have to use a moltiplicator and then a low pass filter to eliminate the high frequency components. But how much difficulty is need to build up a moltiplicator in the receiver that is with the same parameters of that one in the transmitter?
Now think that we want the radio be a mass media? First of all we have to find a way of divide the frequency to different station. This is not a problem beacause at each station we assign a different portant frequency, we only to pay attention to separate enough the different channels. Secondly, we have to produce very cheap receiver because we want to sell a lot, and so people how to pay very few money to buy it.
Our target is to eliminate the moltiplicator at the receiver (is the most difficult, and so expensive, component to make). We find the solution, in doing that we have to complicate the transmitter! Now at the receiver we only need to replace the moltiplicator with an envelope detector: this is the amplitude modulation (AM).
At the end we observed that the envelope detector need more than 0.3 Volts to work and we have to build a pass band filter that works also as a tuner.
After seeing the theory we went to the laboratory to mesure an unknow AM signal, we use PicoScope (a software that works like an oscilloscope). Only seeing at the wave form and mesuring some amplitude and distance (in frequency) we have discover what was the frequency of the portant, what was the frequency of the tone and what was the parameter m (modulation index) that is the ratio between the max amplitude of the envelope signal and the costant volts added to obtain a positive signal.
Here there is two capture from PicoScope
We saw that if we want to recover the original sound signal we have to use a moltiplicator and then a low pass filter to eliminate the high frequency components. But how much difficulty is need to build up a moltiplicator in the receiver that is with the same parameters of that one in the transmitter?
Now think that we want the radio be a mass media? First of all we have to find a way of divide the frequency to different station. This is not a problem beacause at each station we assign a different portant frequency, we only to pay attention to separate enough the different channels. Secondly, we have to produce very cheap receiver because we want to sell a lot, and so people how to pay very few money to buy it.
Our target is to eliminate the moltiplicator at the receiver (is the most difficult, and so expensive, component to make). We find the solution, in doing that we have to complicate the transmitter! Now at the receiver we only need to replace the moltiplicator with an envelope detector: this is the amplitude modulation (AM).
At the end we observed that the envelope detector need more than 0.3 Volts to work and we have to build a pass band filter that works also as a tuner.
After seeing the theory we went to the laboratory to mesure an unknow AM signal, we use PicoScope (a software that works like an oscilloscope). Only seeing at the wave form and mesuring some amplitude and distance (in frequency) we have discover what was the frequency of the portant, what was the frequency of the tone and what was the parameter m (modulation index) that is the ratio between the max amplitude of the envelope signal and the costant volts added to obtain a positive signal.
Here there is two capture from PicoScope
giovedì 23 settembre 2010
23/09/2010
bending the earth!
If the sender and the receiver are too far what happens? We find a problem: the earth is a sphere and the propagation of the electromagnetic wave is straight. Fortunately there are some natural features that gives us an hand: the effect of the surface waves and the refraction by the ionosphere. So we can transmit in HF also at more than 300 kms.
The first thing that we want to transmit is the voice, after understood how the voice is working (at what frequencies) we arrived at the conclusion that we need to "shift" our information (the voice) up in the spectrum of frequencies, to do this we need to build a moltiplicator and to understood how to recover the original voice information.
If the sender and the receiver are too far what happens? We find a problem: the earth is a sphere and the propagation of the electromagnetic wave is straight. Fortunately there are some natural features that gives us an hand: the effect of the surface waves and the refraction by the ionosphere. So we can transmit in HF also at more than 300 kms.
The first thing that we want to transmit is the voice, after understood how the voice is working (at what frequencies) we arrived at the conclusion that we need to "shift" our information (the voice) up in the spectrum of frequencies, to do this we need to build a moltiplicator and to understood how to recover the original voice information.
martedì 21 settembre 2010
21/09/2010
Trying to send and receive.
Now that we have the antenna (dipole) we can try to send and receive some messages. The first thing that we can do is to think at a code (Morse code) where each letter of the alphabet is coded into points and lines. When we want to send a point we close for a short time a switch and we transmit a sinusoid, when we want to send a line we close the switch for a longer time. At the receiver we put an oscilloscope connected to the receving dipole and when we see a short sinusoid we "decode" it like a point and when we see a longer sinusoid we relate it to a line.
There is a problem, in the pauses between the points and lines we don't see the oscilloscope in the zero state, why? Because we see all the electromagnetic noise that belongs from the human industrial activities and from the nature. If we are too far from the transmitter, that noise can cover our desidered signal. We can' t think to amplify it because we amplify also the noise. We observed that there is more noise in the low frequencies due to the fact of real impulse. We defined the ratio between the signal and the noise (S/N) and also the minimum detectable signal (related to the minimum power that i need in reception if i want to obtain some information).
If we are too far from the transmitter we can think to change the antenna with one that is more directive (eg. Yagi-Uda or parabolic).
In the end we saw the definition of dB and dBm and we applied it at the formula of received and trasmitted power. We also observed these definition (dB and dBm) in real datasheets of some commercial antennas.
Now that we have the antenna (dipole) we can try to send and receive some messages. The first thing that we can do is to think at a code (Morse code) where each letter of the alphabet is coded into points and lines. When we want to send a point we close for a short time a switch and we transmit a sinusoid, when we want to send a line we close the switch for a longer time. At the receiver we put an oscilloscope connected to the receving dipole and when we see a short sinusoid we "decode" it like a point and when we see a longer sinusoid we relate it to a line.
There is a problem, in the pauses between the points and lines we don't see the oscilloscope in the zero state, why? Because we see all the electromagnetic noise that belongs from the human industrial activities and from the nature. If we are too far from the transmitter, that noise can cover our desidered signal. We can' t think to amplify it because we amplify also the noise. We observed that there is more noise in the low frequencies due to the fact of real impulse. We defined the ratio between the signal and the noise (S/N) and also the minimum detectable signal (related to the minimum power that i need in reception if i want to obtain some information).
If we are too far from the transmitter we can think to change the antenna with one that is more directive (eg. Yagi-Uda or parabolic).
In the end we saw the definition of dB and dBm and we applied it at the formula of received and trasmitted power. We also observed these definition (dB and dBm) in real datasheets of some commercial antennas.
venerdì 17 settembre 2010
14/09/2010 and 16/09/2010
The first two lectures.
I remember when i was a child that i was always questioning me about things around me . I grew up and i started to disassemble and riassemble everything that was mechanical or electronic like old computers or watch. Then i decided to enter at university. Unfortunately at the end of the most of the courses the questions are more than the answers, i always thought that the cause was the total lack of practical laboratories in my university in Italy.
The course of DISEÑO DE RADIO RECEPTORES is started exactly how i dreamed. Professor José Maria drew in the blackboard a typical block diagram of a communication system (one of those i saw many times in the books and i wrote many times in my notes), i always asked to myself how in the real world (with circuits, resistors, wires, transistors, ecc...) are made each block of those diagrams. This is the same question that José Maria asked to us. Very well, i hope that in this course i will able to answer to old (and new) questions.
The first chapter is REINVENTADO LA RADIO, we have to imagine that we are only experts of circuits theory, and we want to build up a radio (to communicate).
First of all we need to generate electromagnetic waves, so we have to build an antenna (the dipole is the most simple). The lenght of the dipole is very important and it's directly related at the wavelength (or at the frequency of interest). It's important to know that the same antenna used in transmission can be used in reception to capture the desired signal.
Build this dipole antenna can be very difficult in the real world for low frequencies and so we studied others types of antenna: monopole or ferrite coil.
We saw the laws of the radiating and receiving powers: they are not only formulas but each term has a exact physical meaning connected for example to the volume of a sphere, to the gain of an antenna (if is not isotropic) and to the distance from the transmitter to the receiver.
I remember when i was a child that i was always questioning me about things around me . I grew up and i started to disassemble and riassemble everything that was mechanical or electronic like old computers or watch. Then i decided to enter at university. Unfortunately at the end of the most of the courses the questions are more than the answers, i always thought that the cause was the total lack of practical laboratories in my university in Italy.
The course of DISEÑO DE RADIO RECEPTORES is started exactly how i dreamed. Professor José Maria drew in the blackboard a typical block diagram of a communication system (one of those i saw many times in the books and i wrote many times in my notes), i always asked to myself how in the real world (with circuits, resistors, wires, transistors, ecc...) are made each block of those diagrams. This is the same question that José Maria asked to us. Very well, i hope that in this course i will able to answer to old (and new) questions.
The first chapter is REINVENTADO LA RADIO, we have to imagine that we are only experts of circuits theory, and we want to build up a radio (to communicate).
First of all we need to generate electromagnetic waves, so we have to build an antenna (the dipole is the most simple). The lenght of the dipole is very important and it's directly related at the wavelength (or at the frequency of interest). It's important to know that the same antenna used in transmission can be used in reception to capture the desired signal.
Build this dipole antenna can be very difficult in the real world for low frequencies and so we studied others types of antenna: monopole or ferrite coil.
We saw the laws of the radiating and receiving powers: they are not only formulas but each term has a exact physical meaning connected for example to the volume of a sphere, to the gain of an antenna (if is not isotropic) and to the distance from the transmitter to the receiver.
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