Oscillator with the transistor bipolar.
In the previous lecture we saw how to use a logical CMOS port to obtain an inversor, this tecnic was is called Pierce oscillator thanks to his inventor.
This morning we understood how to realize the oscillator using a bipolar transistor.
The final scheme is very simple because there is very few component, but is not to so easy to understand in depth it, in fact we had to make a lot of preparatory lessons.
We polarized the transistor in the active region simply using two resistor for the base and one for emitter. We understand that with another resistance Rc in the collector we can take the signal in output of the transistor. But what happen if instead of this resistance Rc we insert a "tank" circuit (a parallel of L and C). In DC mode the voltage at the collector would be Vcc and the transistor remain in active region.
It important to note that the range of the output in AC mode would be from 0 to 2 x Vcc, that is around Vcc.
In what frequency we have to put the tank circuit? It's important to note that is not the frenquency of the total oscillator! We need a frequncy below that, because we remembered that the tank circuit is equivalent to a parallel of a C and a R above the oscillating frequency, that is what we need.
Finally we note that we can use a quarz of the cheap one not tuned only for the oscillator frequency that we need, but cut for low frequency, we are sure that with the tank circuit the quartz crystal works in the tone that we need.
After the theory we moved to the laboratory and we can build up our oscillator, was great! Incredible, so few component but all well working.
An important comment is that the tank circuit can vary with temperature weather umidity hit but this doesn't affect the work of the oscillator: now this circuit can perfectly fit in a boat!!!
In the last part of the lecture we start to understand that in some way we have to add a little information in our signal, the most easy way is an audible tone like On-Off.
martedì 30 novembre 2010
giovedì 25 novembre 2010
25/11/2010
The strength of the stones.
This morning we finished our considerations about the Voltage Controlled Oscillator understanding how we can realize a circuit that can be connected to a PC that is able to controlled the voltage of our varicap diode. This circuit is based on a microcontroller connected to the pc, it generates a digital voltage with a duty cicle tau then there is low pass filter that isolate only the continuos voltage, after that there is an separating stage and at the end the rest of out circuit of the oscillator.
The purpose of our study in the last lecture is to buil an oscillator at 27 Mhz that is used for build a radio beacon for emergency. We have to observe that the frequency of our oscillator have to be very stable and unalterable. However we saw at the laboratory that only a light movement in the inductor is transformed in a change of the oscillating frequency: we have to find a solution.
We want our frequency to be stable and fix like a stone, also to the change of temperature and shot the stone remain unalterated.
Here is the time to introduce the quartz crystals. They have two important properties: the first is the unalterablility and the other is the piezoelectric.
The unalterability is links to the shape and to the cut of the material: a stone.
The piezoelectric is connected to the dimension of the slice of the quartz.
As always when we want to understand a new component the first thing that we made is going in the laboratory (virtually) and make measure on it. If we characterize the quartz crystal we find that for low frequencies it behaves like a condensator, then for a precise fix frequency there is the presence of a short circuit and if we increase a little bit the frequency there is a very strange behavior. In a very small range of frequency from the short is transformed in an open circuit and the behavior is like an inductor that takes in that range all the possible value.
After this discover our question is how to build a circuit with Rs, Ls and Cs that act like this quartz crystal. We analyze quickly a circuit proposed by Jose Maria and we discover that the crystal behavior is like a seris of a little R a little C an high L, everything in parallel of a C (that is the parasitic capacitance of the quartz crystal).
What we can do with this quartz crystal? We understand that its vibrates at only one precise frequency (the one of the short) therefore if we inserted it in our wire of feedback from the output to the input we obtain that our oscillator only works if the frequency is perfectly tune with the one of the crystal (in reality it works in a little range around it).
Our aim is to build from this crystal an oscillator that works always. We don' t want to periodically calibrate it or something similar, therefore we study more in depth this quartz of crystal, a little before we say that is able of be an inductor L of whatever frequency that we want, therefore anywhere in a circuit we have an inductor of value L1 at a frequency we can replace it with a qurt crystal of that frequency.
Later on we study another possibility of make an oscillator that is with an inverter amplifyer and a passaband filter. If the amplifyer is invertent we need to restablish the right phase adding a 180° more of desphase and to do it we need at least three reactive elements.
For the implementation the professor showed us a very clever way that is with the use of a logic port in CMOS tecnology (note that the port is exploited it in the region prohibited for the digital circuit!).
Another way of implementing the inverting amplifyer is by a bipolar transistor; moreover we can substitute the L element of the passband filter with the quartz crystal.
At the end we obtain a very simple realization of an oscillator sinusoidal.
We need to comment that we use a cheap quartz oscillator for the 27 Mhz that is realize expoiting high armonics (the third for precision. High armonic is higher armonic oscillation of the slice of quartz). If we don't want to see the low armonics we have to implement in the circuit also a tank circuit to suppress it.
This morning we finished our considerations about the Voltage Controlled Oscillator understanding how we can realize a circuit that can be connected to a PC that is able to controlled the voltage of our varicap diode. This circuit is based on a microcontroller connected to the pc, it generates a digital voltage with a duty cicle tau then there is low pass filter that isolate only the continuos voltage, after that there is an separating stage and at the end the rest of out circuit of the oscillator.
The purpose of our study in the last lecture is to buil an oscillator at 27 Mhz that is used for build a radio beacon for emergency. We have to observe that the frequency of our oscillator have to be very stable and unalterable. However we saw at the laboratory that only a light movement in the inductor is transformed in a change of the oscillating frequency: we have to find a solution.
We want our frequency to be stable and fix like a stone, also to the change of temperature and shot the stone remain unalterated.
Here is the time to introduce the quartz crystals. They have two important properties: the first is the unalterablility and the other is the piezoelectric.
The unalterability is links to the shape and to the cut of the material: a stone.
The piezoelectric is connected to the dimension of the slice of the quartz.
As always when we want to understand a new component the first thing that we made is going in the laboratory (virtually) and make measure on it. If we characterize the quartz crystal we find that for low frequencies it behaves like a condensator, then for a precise fix frequency there is the presence of a short circuit and if we increase a little bit the frequency there is a very strange behavior. In a very small range of frequency from the short is transformed in an open circuit and the behavior is like an inductor that takes in that range all the possible value.
After this discover our question is how to build a circuit with Rs, Ls and Cs that act like this quartz crystal. We analyze quickly a circuit proposed by Jose Maria and we discover that the crystal behavior is like a seris of a little R a little C an high L, everything in parallel of a C (that is the parasitic capacitance of the quartz crystal).
What we can do with this quartz crystal? We understand that its vibrates at only one precise frequency (the one of the short) therefore if we inserted it in our wire of feedback from the output to the input we obtain that our oscillator only works if the frequency is perfectly tune with the one of the crystal (in reality it works in a little range around it).
Our aim is to build from this crystal an oscillator that works always. We don' t want to periodically calibrate it or something similar, therefore we study more in depth this quartz of crystal, a little before we say that is able of be an inductor L of whatever frequency that we want, therefore anywhere in a circuit we have an inductor of value L1 at a frequency we can replace it with a qurt crystal of that frequency.
Later on we study another possibility of make an oscillator that is with an inverter amplifyer and a passaband filter. If the amplifyer is invertent we need to restablish the right phase adding a 180° more of desphase and to do it we need at least three reactive elements.
For the implementation the professor showed us a very clever way that is with the use of a logic port in CMOS tecnology (note that the port is exploited it in the region prohibited for the digital circuit!).
Another way of implementing the inverting amplifyer is by a bipolar transistor; moreover we can substitute the L element of the passband filter with the quartz crystal.
At the end we obtain a very simple realization of an oscillator sinusoidal.
We need to comment that we use a cheap quartz oscillator for the 27 Mhz that is realize expoiting high armonics (the third for precision. High armonic is higher armonic oscillation of the slice of quartz). If we don't want to see the low armonics we have to implement in the circuit also a tank circuit to suppress it.
martedì 23 novembre 2010
23/11/2010
VCO and spectrum analyzer.
At the end of the last lecture we comment about the measurament of the power of our oscillator that we did at the laboratory.
We understand that our sinusoid is not perfect and therefore we have to mesure the power also of the armonics. A well-build oscillator is an oscillator that have a very low power at the superior armonics.
But what kind of instrument we need to see the spectrum of our oscillator? Obviously we need a spectrum analyzer.
To understand profundly how a spectrum analyzer works Josè Maria had shown us a step-by-step path about how can we build such an instrument from only the basic notion that we knoe about passaband filter voltimeter and envelope detector. We need a passaband tunable filter to window the input signal, we saw that we could have an electronic control to sweep all the frequency.
In reality making a very well shaped filter that can be tunable is very difficult and the BW is very important baceause is directly related to the resolution of the spectrum analyzer. What we can do is a very good filter (in BW and in costant gain) at a fixed precise frequency.
Anyway exist a solution to this problem: we use a fixed frequency very good passaband filter and we use a mixer between the input signal and a signal from an oscillator controlled in voltage. With some passage of math can be demostrated that is the same of having a tunable passaband filter.
We went to the laboratory to test the use of the spectrum analyzer, this spectrum analyzer is made of two stage including an oscilloscope. We test this spectrum analyzer with a sinusoid from a generator of % Mhz and 0,466 Volts, we could saw the spectrum and recognise the armonics and the noise from the FM radio (in the band around 100 Mhz).
At the end we understand how we can control the frequency of our oscillator using a variable resistor and a diode varicap (is s diode polarized in inverse, it works like a variable capacitance) and so varying the voltage. We need it because we want that our oscillator produce a very good sinusoid at a precise frequency and therefore in some way we have to build up a system that can works alone without the need of an human hand that change the capacity of a variable capacitor (like in out old oscillator circuit).
venerdì 19 novembre 2010
18/11/2010
Using transistor for the amplifier of the oscillator.
It' s clear that if we want to create high frequency sinusoids we have to works with a transistor instead of an OP amp.
The structure of the oscillator that we want to study is made of an amplifier made with a transistor and with an amplify of 1, a resistance of input high and a low impedance at the output, in other words this is a voltage buffer (esp: seguidor de tension). The band pass filter have an amplification higher that 1 in the peak of resoncance.
First of all we study that circuit without the connection input-output, so we find the condition of oscillation and the frequency of the peak (that depend also on the value of a variable capacitor).
In a second step we study the design for the voltage buffer, hence we found the equation for the polaritation of the transistor (obviously we used the little signal model for the transistor).
Finally we dimensioned all the value of the rsistance inductance and capacitors to obtain an oscillator fot 27 Mhz.
After the theory part of the lecture we went to the laboratory to build the circuit and to test it. We simulate the load of the antenna with a resistence and we used an autotrasformer that Jose Maria given us. We have to measure the power that is trasmitted to the resistance (the "antenna") we have to observed that we don' t obtain a perfect sin wave and therefore we understand that the power that we measured is not only of the component in the 27Mhz.
The quality of an oscillator stay in how much are attenuated the armonics of the principal frequency f0.
It' s clear that if we want to create high frequency sinusoids we have to works with a transistor instead of an OP amp.
The structure of the oscillator that we want to study is made of an amplifier made with a transistor and with an amplify of 1, a resistance of input high and a low impedance at the output, in other words this is a voltage buffer (esp: seguidor de tension). The band pass filter have an amplification higher that 1 in the peak of resoncance.
First of all we study that circuit without the connection input-output, so we find the condition of oscillation and the frequency of the peak (that depend also on the value of a variable capacitor).
In a second step we study the design for the voltage buffer, hence we found the equation for the polaritation of the transistor (obviously we used the little signal model for the transistor).
Finally we dimensioned all the value of the rsistance inductance and capacitors to obtain an oscillator fot 27 Mhz.
After the theory part of the lecture we went to the laboratory to build the circuit and to test it. We simulate the load of the antenna with a resistence and we used an autotrasformer that Jose Maria given us. We have to measure the power that is trasmitted to the resistance (the "antenna") we have to observed that we don' t obtain a perfect sin wave and therefore we understand that the power that we measured is not only of the component in the 27Mhz.
The quality of an oscillator stay in how much are attenuated the armonics of the principal frequency f0.
mercoledì 17 novembre 2010
16/11/2010
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.
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.
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).
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