The new project: diseño de una radiobaliza - Emisor -.
During the lectures of any electronic or telecom courses we speak of sinusoidal signal, we used source of sinusoidal signal and so on... but where they come from? How in reality we can produce this kind of signal? The basic idea is the sinusoidal oscillator.
If we think how to obtain a sinusoidal signal (a precise "tone" in frequency) we can tought to the parallel of L and C. But it' s important to understand that this is only an ideal circuit. We saw in the practice that we can' t have a real component called L, but we have an inductance with her parasitic resistence R. Hence the circuit is not again an oscillator, because of the presence of the R we obtain a fading sinusoid.
We need a new idea and we start to study a different structure: an amplificator of gain k and a filter H. If in input we have a sinusoid signal from a generator, what we obtain in general at the output? And what are the condiction in the amplifier and the filter to obtain that the output is a replica of the input? If we answer to these question we conclude that the product of k and the amplitude of the filter at the input frequency must be equal to 1 and the change in phased 0.
What happen if we connect the input with the output? Because they are the same (under the condiction writed above) nothing happens in the sense that the results doesn' t change.
Therefore we have removed the generetor and we find that the circuit admits for solution a sinusoidal signal: we found the oscillator sinusiodal!
For example using an OPamp (because we need small amplification factor) we can realize the amplifier and we can realize in practice the filter with a resitance a capacitor and an inductor. We have to pay attention that the conditions are met (1 in total amplification and 0 phase shift for a particular frequency f0 that depends on the value of L and C).
There is two important remarks. First, what happens if the amplify is not perfectly 1? The answer is that the sinusiod can make increasing oscillations or decreasing one. Second, if we don't have the generator (obviusly because we are trying to making it!) how can this circuit start to works? The secret is that at any temperature the amplifier itself make some termal noise, this noise is a constant in all the frequency and therefore the frequency f0 is excited and if we have a passaband filer we obtained the desired sinusoid.
We have to note that the product of the amplify factor k and the value of the amplitude of the filter must be little higher than 1.
Where we have to take our sinusoidal signal producted from our oscillator? There is two possibilities, one is at the output of the filter and the other is at the output of the amplifier. The first chose is positive if we look at the shape of the sinusoid but we have to observe that is an high impedence node and this is not good because anything that we connect modify the oscillator circuit. The other possibility is to take the signal at the output of the amplifier, this is a low impedance node but we find that the shape is not a perfect sinusiod because if the amplitude is more than 1/H(f0) we find a crescent sinusoid that is cutting because of the saturation limit of the amplier.
In conclusion we obtain an oscillator if we have an amplifier and a passaband filter appropriately designed, the output of the filter is connected to the input of the amplifeir. Under certain conditions this circuit start itself to produce sinusoid signal at the frequency of the peak of the passaband filter, we obtain purer sinusoid if the Q quality factor of the peak is higher.
mercoledì 17 novembre 2010
giovedì 11 novembre 2010
11/11/2010
Transformer or not transformer.
In the MW receiver we used a transformer to don't degrade the bandwidth of our antenna-tuner inserting the rest of the circuit. It's obvious that we have always a receiver antenna and we have to connect it to the receiver circuit so this is a general problem, therefore we understand that the transformer could resolve this problem in a very elegant way.
It' s important to observe that there is a price that we have to pay: first of all the high price for the materials (copper and ferrite) and second we lose in amplify factor (i repeat we preserve the bandwidth.)
We also see a "reverse" application of the transformer that is connecting a voltage source in series with a resistance at the secondary stage of a transormer, in the primary we have for example an high value of resistance, we find that we have "adapted the line" that is another very important and common problem building receiver in RF.
There is another clever solution to the problem of not degrade the Q factor of a resonant circuit: the use of two capacitance in a very particular way.
To study this solution we used two particular transformation that sound strange (but that are possible under certain frequency condition): the transformation parallel to series and the vice versa.
We have to understand that now we have the instrument to study complex circuits in an easy way, the observation is that we don' t use the articulated expression of the transfer function (with denominator not of the second grade).
Finally we see how to connect two different stage to obtain a wider passband filter (costant in a wide bandwidth) and further the concept of autotransformer that can simplify the realization of a transformer.
In the MW receiver we used a transformer to don't degrade the bandwidth of our antenna-tuner inserting the rest of the circuit. It's obvious that we have always a receiver antenna and we have to connect it to the receiver circuit so this is a general problem, therefore we understand that the transformer could resolve this problem in a very elegant way.
It' s important to observe that there is a price that we have to pay: first of all the high price for the materials (copper and ferrite) and second we lose in amplify factor (i repeat we preserve the bandwidth.)
We also see a "reverse" application of the transformer that is connecting a voltage source in series with a resistance at the secondary stage of a transormer, in the primary we have for example an high value of resistance, we find that we have "adapted the line" that is another very important and common problem building receiver in RF.
There is another clever solution to the problem of not degrade the Q factor of a resonant circuit: the use of two capacitance in a very particular way.
To study this solution we used two particular transformation that sound strange (but that are possible under certain frequency condition): the transformation parallel to series and the vice versa.
We have to understand that now we have the instrument to study complex circuits in an easy way, the observation is that we don' t use the articulated expression of the transfer function (with denominator not of the second grade).
Finally we see how to connect two different stage to obtain a wider passband filter (costant in a wide bandwidth) and further the concept of autotransformer that can simplify the realization of a transformer.
mercoledì 10 novembre 2010
09/11/2010
HF filter and trasformer.
After seeing that our MW receiver correctly works (was very surprising and exciting!) we start to re-study the circuit that we used in the MW receiver to obtain a more general knowledge about HF filters.
In class we study the basic circuit: inductance, capacitance, low pass, high pass. After that we revise the concept of Q (quality factor) and we try to extend the knoweldge about the antenna-tuner-band pass filter to other circuit: we used the Thevenin equivalent.
Then we study the "dipole RLC" and a very simple but very useful ciruit called "tank", that is a resonant circuit with a resistance Rs in series with the L.
The osservation that we made after a few step of easy math is that for a particular frequency (the resonance one) and for a quality factor Q>5 we can replace the Rs with a resistance in parallel called Rp: this technics is very very useful when we want to understand the trend of different circuits without making a lot of math.
Jose Maria showed us two possible application of the tank, the first is converting a simply inverting amplifier in a band pass filter, and the second is to obtain a tuned amplifier with a transistor.
Changing topic, we started talking about how to make inductances, the first way is around a toroidal ferrite (we understand also why we use a ferromagnetic material and not simply iron!), the second is around a kernel of ferrite (like our antenna in the MW receeiver) and the third is without ferrite component by only rolled in free air (the value of L is lower but we have less parasite resistance and capacitance). N.B: Nagaoka obtained a sperimental equation to describe this kind of coils.
The natural flow of thought moves from inductors to the magnetic coupling and transformer. The professor advised us not to run in error and to understand very well the difference between the ideal-theory concept of transformer and a realistic way of made them: the different is very deep and we have to take it into account.
After seeing that our MW receiver correctly works (was very surprising and exciting!) we start to re-study the circuit that we used in the MW receiver to obtain a more general knowledge about HF filters.
In class we study the basic circuit: inductance, capacitance, low pass, high pass. After that we revise the concept of Q (quality factor) and we try to extend the knoweldge about the antenna-tuner-band pass filter to other circuit: we used the Thevenin equivalent.
Then we study the "dipole RLC" and a very simple but very useful ciruit called "tank", that is a resonant circuit with a resistance Rs in series with the L.
The osservation that we made after a few step of easy math is that for a particular frequency (the resonance one) and for a quality factor Q>5 we can replace the Rs with a resistance in parallel called Rp: this technics is very very useful when we want to understand the trend of different circuits without making a lot of math.
Jose Maria showed us two possible application of the tank, the first is converting a simply inverting amplifier in a band pass filter, and the second is to obtain a tuned amplifier with a transistor.
Changing topic, we started talking about how to make inductances, the first way is around a toroidal ferrite (we understand also why we use a ferromagnetic material and not simply iron!), the second is around a kernel of ferrite (like our antenna in the MW receeiver) and the third is without ferrite component by only rolled in free air (the value of L is lower but we have less parasite resistance and capacitance). N.B: Nagaoka obtained a sperimental equation to describe this kind of coils.
The natural flow of thought moves from inductors to the magnetic coupling and transformer. The professor advised us not to run in error and to understand very well the difference between the ideal-theory concept of transformer and a realistic way of made them: the different is very deep and we have to take it into account.
martedì 2 novembre 2010
28/10/2010 and 02/11/2010
The last stage of our radio receiver.
We had stopped the last time writing about the positive regeneration obtained inserting an another solenoid in the same ferrite kernel of the primary, we obtain an higher amplify and a better selectivity.
I think that to understand better the last necessary parts of our radio receiver i have to make a summary of it:
- first of all we have to build a separating stage;
- second the envelope detector;
- then an audio amplifier;
- finally the transformer fnecessary if we want to connect some speakers.
The need for the separating stage.
We saw in the last lecture how to polarized the transistor to obtain an output voltage in DC of 4,5 V. It's clear that at the output of the transistor we have to insert other circuit for our radio development, but what happen if we directly connect the rest of the circuit in the output terminal of the transisor, obviously we change the value of the resistence and capacitance used for the polaritation.
Therefore we have to build up "something" that separate the rest of the circuit that we will connect to the transistor, in technical language we said that we need to transform our low impedence terminal in an high impedence one.
To make this separating stage we used a common Op-Amp. Correctly Josè Maria spend some time during the lesson to explain to us the carachteristics of these Op-Amp, the most interesting thing is that we have to pay a very high attention reading the specific and understand that we are working in high frequency and therefore the characterist are very very different from what we remember in our last electonic circuit theory.
Finally we understand that we used this Op-Amp in our separating stege for his carachteristic of high input impedance and not for amplify our signal.
Obtaining the original information: the envelope detector.
We have to remember that the original information is in the envelope of the signal that we receive (AM modulation) therefore now that our signal is amplified we need to restore this information. To do it there is a very simple way, we only need a diode a resistence an a capacitance.
First of all we revised the way how to obtained a continuos corrent from a sinusoid, therefore we understand better how this work also in the laboratory (trying different diode -Si or Ge-and different value of resitance).
We have to note that in a sense we need a circuit that follow the trend of the voltage (the envelope of our signal) and therefore we understand that there are some upper (to avoid diagonal distorsion) and lower limits (for the portant) in the value of the resitance.
We went to the laboratory another time and we remain surprise how good is the envelope detector used to recover a voice signal.
(Note that this receiver and this envelope detector works fine with no fast transient music).
The last step: the audio amplifier and the transformer.
In the last part of the today lesson we saw the audio amplifier need to connect the speaker. We have to understand that now we are at audio frequency (20hz - 4khz) therefore we use an active amplify TL081.
We saw the limitation of this amplify in term of current in output and after a few analysis we arrive at the conclusion that we can't connect directly at the output terminal of the amplify the commerical speakers that have a typical value of impedence of 8 Ohm.
Josè Maria showed us a very clever (but commercially expensive) solution to connect the speaker, this idea is based on the transformer, but we have to observed that we need a very high inductance value therefore it can't disturb our circuit, to obtained this high impedance we have to make a lot of coils (with the same number ratio).
We had stopped the last time writing about the positive regeneration obtained inserting an another solenoid in the same ferrite kernel of the primary, we obtain an higher amplify and a better selectivity.
I think that to understand better the last necessary parts of our radio receiver i have to make a summary of it:
- first of all we have to build a separating stage;
- second the envelope detector;
- then an audio amplifier;
- finally the transformer fnecessary if we want to connect some speakers.
The need for the separating stage.
We saw in the last lecture how to polarized the transistor to obtain an output voltage in DC of 4,5 V. It's clear that at the output of the transistor we have to insert other circuit for our radio development, but what happen if we directly connect the rest of the circuit in the output terminal of the transisor, obviously we change the value of the resistence and capacitance used for the polaritation.
Therefore we have to build up "something" that separate the rest of the circuit that we will connect to the transistor, in technical language we said that we need to transform our low impedence terminal in an high impedence one.
To make this separating stage we used a common Op-Amp. Correctly Josè Maria spend some time during the lesson to explain to us the carachteristics of these Op-Amp, the most interesting thing is that we have to pay a very high attention reading the specific and understand that we are working in high frequency and therefore the characterist are very very different from what we remember in our last electonic circuit theory.
Finally we understand that we used this Op-Amp in our separating stege for his carachteristic of high input impedance and not for amplify our signal.
Obtaining the original information: the envelope detector.
We have to remember that the original information is in the envelope of the signal that we receive (AM modulation) therefore now that our signal is amplified we need to restore this information. To do it there is a very simple way, we only need a diode a resistence an a capacitance.
First of all we revised the way how to obtained a continuos corrent from a sinusoid, therefore we understand better how this work also in the laboratory (trying different diode -Si or Ge-and different value of resitance).
We have to note that in a sense we need a circuit that follow the trend of the voltage (the envelope of our signal) and therefore we understand that there are some upper (to avoid diagonal distorsion) and lower limits (for the portant) in the value of the resitance.
We went to the laboratory another time and we remain surprise how good is the envelope detector used to recover a voice signal.
(Note that this receiver and this envelope detector works fine with no fast transient music).
The last step: the audio amplifier and the transformer.
In the last part of the today lesson we saw the audio amplifier need to connect the speaker. We have to understand that now we are at audio frequency (20hz - 4khz) therefore we use an active amplify TL081.
We saw the limitation of this amplify in term of current in output and after a few analysis we arrive at the conclusion that we can't connect directly at the output terminal of the amplify the commerical speakers that have a typical value of impedence of 8 Ohm.
Josè Maria showed us a very clever (but commercially expensive) solution to connect the speaker, this idea is based on the transformer, but we have to observed that we need a very high inductance value therefore it can't disturb our circuit, to obtained this high impedance we have to make a lot of coils (with the same number ratio).
martedì 26 ottobre 2010
26/10/2010
We spent the first our of this morning lesson in the laboratory. We have have to find the curve of amplify of our circuit.
We know from what we study in the theory that we would have bad value in high frequency (more than 1 Mhz) because of the parassite capacitance of the transistor. In the laboratory we found that kind of decrasing curve.
In the previous lecture we saw that we can improve the trend inserting an inductor L to compensate the effect of the parassite capacitance in high frequency, therefore we inserted that inductor and we could find better performance.
In the second part of the lecture Josè Maria explained to us the concept of positive regeneration. The basic idea is that we want to obtain an higher amplify. This idea is patended by Armstrong in 1915 and we have to understand that like in the principle of the electric age the engineers want to obtain the maximum with the minimum, in this case we want the maximum amplify with the minimum number of transistor (at the principle vacuum tube).
The way to obtain this positive regeneration is based on the idea that after we receive the signal we can emitting it with an antenna and then we re-received it.
There are two fundamentals problems: the first one is that we have to re-receive the signal in a constructively way (adding in phase) and the second problem is the saturation.
In practice to build up this "emitting antenna" we have only to add a little solenoid in the same ferrite kernel that we used for the antenna-tuner (paying attention to the phase), and a controlled resistence.
Controlling the resistence we can modify both the sensibily and the selectivity of our circuit (improving both at the same time) and we can avoid the problem of saturation.
We know from what we study in the theory that we would have bad value in high frequency (more than 1 Mhz) because of the parassite capacitance of the transistor. In the laboratory we found that kind of decrasing curve.
In the previous lecture we saw that we can improve the trend inserting an inductor L to compensate the effect of the parassite capacitance in high frequency, therefore we inserted that inductor and we could find better performance.
In the second part of the lecture Josè Maria explained to us the concept of positive regeneration. The basic idea is that we want to obtain an higher amplify. This idea is patended by Armstrong in 1915 and we have to understand that like in the principle of the electric age the engineers want to obtain the maximum with the minimum, in this case we want the maximum amplify with the minimum number of transistor (at the principle vacuum tube).
The way to obtain this positive regeneration is based on the idea that after we receive the signal we can emitting it with an antenna and then we re-received it.
There are two fundamentals problems: the first one is that we have to re-receive the signal in a constructively way (adding in phase) and the second problem is the saturation.
In practice to build up this "emitting antenna" we have only to add a little solenoid in the same ferrite kernel that we used for the antenna-tuner (paying attention to the phase), and a controlled resistence.
Controlling the resistence we can modify both the sensibily and the selectivity of our circuit (improving both at the same time) and we can avoid the problem of saturation.
giovedì 21 ottobre 2010
21/10/2010
Trying the transistor.
Today we start the lesson seeing that if we put our information voltage signal in series with the batteries we don' t obtain the gain that we desidered. Therefore in this montage our transistor "only" act a controlled switch.
Josè Maria said to us that we have to put our signal in parallel with the source power hence from base to ground and we have to add a capacitor that doesn't change the circuit in the DC mode.
Using the incremental model and the fasorial representation we arrived to calcute the amplify in this case: in principle we can choose the parameter to obtain the amplify that we want.
But there is a problem, the high we want the amplify the high result the impedence at the input of our amplifier, and remembering the previous lecture this is exactly what we don't want because we need to preserve the sensitivity of the antenna-tuner stage!
The problem of this high input resistance is well know in literature with the name of Miller effect. Fortunately there is a very clever solution to delete the undesidered Miller effect and hence we to replace the Rb (resistence in the base terminal) with a series of two Rb/2 resistence and in the middle a capacitor.
Jose' Maria showed us two other improvement for our circuit. The first is to insert a inductor L to compensate the effect of the parassite capacitance in high frequency, the second improvement is to insert in the emitter terminal a little resistence Re that doesn't afflict the polaritation circuit but that is seen multiplied for Beta in input (is fine for the sensitivity!).
We spent the last thirthy minutes of the lecture in the laboratory. We built the polaritation circuit, we turn on the power and we measured the real parameters of the circuit. We found a Collector voltage of Voq=4.58 V (theorically is 4.5 V) and a Base voltage of Vbq=0.61 V (teorically 0.6 V), with this mesured value we can calculate the various current and also the amplify factor that is A=169,99 that is in line with the theory.
Today we start the lesson seeing that if we put our information voltage signal in series with the batteries we don' t obtain the gain that we desidered. Therefore in this montage our transistor "only" act a controlled switch.
Josè Maria said to us that we have to put our signal in parallel with the source power hence from base to ground and we have to add a capacitor that doesn't change the circuit in the DC mode.
Using the incremental model and the fasorial representation we arrived to calcute the amplify in this case: in principle we can choose the parameter to obtain the amplify that we want.
But there is a problem, the high we want the amplify the high result the impedence at the input of our amplifier, and remembering the previous lecture this is exactly what we don't want because we need to preserve the sensitivity of the antenna-tuner stage!
The problem of this high input resistance is well know in literature with the name of Miller effect. Fortunately there is a very clever solution to delete the undesidered Miller effect and hence we to replace the Rb (resistence in the base terminal) with a series of two Rb/2 resistence and in the middle a capacitor.
Jose' Maria showed us two other improvement for our circuit. The first is to insert a inductor L to compensate the effect of the parassite capacitance in high frequency, the second improvement is to insert in the emitter terminal a little resistence Re that doesn't afflict the polaritation circuit but that is seen multiplied for Beta in input (is fine for the sensitivity!).
We spent the last thirthy minutes of the lecture in the laboratory. We built the polaritation circuit, we turn on the power and we measured the real parameters of the circuit. We found a Collector voltage of Voq=4.58 V (theorically is 4.5 V) and a Base voltage of Vbq=0.61 V (teorically 0.6 V), with this mesured value we can calculate the various current and also the amplify factor that is A=169,99 that is in line with the theory.
mercoledì 20 ottobre 2010
19/10/2010
Putting transistor in the active region.
After seeing another time how the transistor works we want to stress the fact that the nobel idea in the transistor is that little variations in the input imply big variations in the resistence in the output because there is a strong sensitivity in the voltage Base-Emitter.
Transistor comes from the words transfer + resistor that means that we can transfer the same current from an high resistence to a resistence that we can choose.
If we want to obtain the ransistor effect we have to put the transistor in the active region which means that we need a positive voltage both for the Collector-Base and for Base-Emitter.
If we want to check if the transistor in a circuit is in active region we have to check if at leat one of these current are positive: emitter, collector or base.
In the class we saw some proposals about how put a transistor in the active zone, in the simpliest cases we found some problem about the instability of the circuit due to temperature, the dependence from the value Beta of different transistor or the need for 2 batteries to aliment the circuit.
The last example that we have seen is made with only one batteries, with four resistance and it doesn't depend from the different value Beta of transistor (Note that also if the model of the transistor is the same we can find very different value of the parameter Beta).
In the last part of the lecture we developed an incremental model for study circuit with transistors and we start talking about the possibility of insert our voltage signal in parallel with the voltage Base-Emitter.
After seeing another time how the transistor works we want to stress the fact that the nobel idea in the transistor is that little variations in the input imply big variations in the resistence in the output because there is a strong sensitivity in the voltage Base-Emitter.
Transistor comes from the words transfer + resistor that means that we can transfer the same current from an high resistence to a resistence that we can choose.
If we want to obtain the ransistor effect we have to put the transistor in the active region which means that we need a positive voltage both for the Collector-Base and for Base-Emitter.
If we want to check if the transistor in a circuit is in active region we have to check if at leat one of these current are positive: emitter, collector or base.
In the class we saw some proposals about how put a transistor in the active zone, in the simpliest cases we found some problem about the instability of the circuit due to temperature, the dependence from the value Beta of different transistor or the need for 2 batteries to aliment the circuit.
The last example that we have seen is made with only one batteries, with four resistance and it doesn't depend from the different value Beta of transistor (Note that also if the model of the transistor is the same we can find very different value of the parameter Beta).
In the last part of the lecture we developed an incremental model for study circuit with transistors and we start talking about the possibility of insert our voltage signal in parallel with the voltage Base-Emitter.
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