The Gilbert cell.
In this lecture we saw how to use an IC to exploit the property of commutant mixer and how to build the input passaband amplifier.
The realisation of our receiver is based on an IC (NE602) that is an implementation of the Gilbert cell. This is an ideal circuit with three input (two signal plus a periodic signal-the shape doesn't matter!) and two output (one the opposite of the other).
Studying the properties of the equation that represent the Gilbert cell we can distinguish two term that depends on the hyperbolic tangent. For the first one we have to ensure that the difference of the input signals is the linear region, on the other hand, for the second term we want to works on the constant region (+1, -1).
After saw the Gilbert cell we study what we need to add to the NE602 to make it works in a circuit (in theory we need an oscillator but in reality it's half implemented in the IC and therefore we only need a few capacitance and the quartz crystal).
We are at the end of this course and we are expert in realising passband filter. I remember that we need the passaband filter here at the input of the receiver because we want a little bit of amplify, we realize the matching condition and we avoid interference.
We want to the laboratory to test the first part of the receiver, the passaband filter and the mixer, we can observe through PICOSCOPE that we have the desired signal at around 7 Mhz (the distance from 10 Khz is because of the little deviation of the oscillator in the emitter and the receiver) .
Now lack only the last part, we have our signal down in frequency (7 Khz) and we want to turn on a led and sound a speaker, the processing in low frequency its very easy and can be both make electronically with circuit (amplifier plus envelope detector) or via the sound card of a PC.
domenica 19 dicembre 2010
mercoledì 15 dicembre 2010
14/12/2010
Receptor de una radiobaliza.
We have started the last part of the programme of DR, after seeing the design of a radio beacon transmitter, we moved to the design of a receiver for the 27 Mhz signal of the radio beacon.
First of all we rewied some aspect of the noise, power of the signal in input at the receiver and signal to noise ratio.
Supposing that we need a amplitude demodulator based on an envelope detector based on a diode we can determine the value K of total gain of our receiver.
We remembered from the past consideration that making a passband filter at 27 Mhz it's very hard and expensive, a very clever (and old) solution it's called "heterodyne receiver".
The fundamental idea is to insert a mixer as a first stage of the receiver, at the output we find the same information signal but shifted around a lower frequency for example 10 Khz. Hence elaborate this signal and build a amplify passaband filter in this frequency is incredible more easier and cheaper. Remark that we have to pay attention to the image frequencies (insert a passband filter before the mixer).
We understand soon that we need new information about how to design a mixer. We always see the schematic block in the book but we don't know how to realize one!
It's important to observe that in reality we don't need a complete mixer but we need a circuit that is able to multiply the incoming signal by a sinusoidal one.
Josè Maria show us an alternative mixer: the switching mixer. The basic idea is to exploit the property of the Fourier series of a square wave (+1, -1) that show as a first term a cosine, a square wave for us is simple switch Vin and -Vin.
In the last part of the lecture we saw how to built such a switching mixer in low frequency with the use of OpAmp and how is it implemented in high frequency commercial device.
We have started the last part of the programme of DR, after seeing the design of a radio beacon transmitter, we moved to the design of a receiver for the 27 Mhz signal of the radio beacon.
First of all we rewied some aspect of the noise, power of the signal in input at the receiver and signal to noise ratio.
Supposing that we need a amplitude demodulator based on an envelope detector based on a diode we can determine the value K of total gain of our receiver.
We remembered from the past consideration that making a passband filter at 27 Mhz it's very hard and expensive, a very clever (and old) solution it's called "heterodyne receiver".
The fundamental idea is to insert a mixer as a first stage of the receiver, at the output we find the same information signal but shifted around a lower frequency for example 10 Khz. Hence elaborate this signal and build a amplify passaband filter in this frequency is incredible more easier and cheaper. Remark that we have to pay attention to the image frequencies (insert a passband filter before the mixer).
We understand soon that we need new information about how to design a mixer. We always see the schematic block in the book but we don't know how to realize one!
It's important to observe that in reality we don't need a complete mixer but we need a circuit that is able to multiply the incoming signal by a sinusoidal one.
Josè Maria show us an alternative mixer: the switching mixer. The basic idea is to exploit the property of the Fourier series of a square wave (+1, -1) that show as a first term a cosine, a square wave for us is simple switch Vin and -Vin.
In the last part of the lecture we saw how to built such a switching mixer in low frequency with the use of OpAmp and how is it implemented in high frequency commercial device.
giovedì 2 dicembre 2010
02/12/2010
On-Off: the last step for the "radiobaliza".
In the first part of the today lecture we made a little brief about what we made up to know: the Pierce oscillator implemented with a bipolar transistor, the tank circuit and the cheap quartz oscillator.
In the last class we stopped talking about how can we interrupt our transmission to implemented a very simple modulation: On-Off.
The first idea that we would have is to switch on and off the voltage source, this is not working fine because of the presence of the condensator electrolytic that have a slow discharge period.
The other possible solution (the one that we decide to implement) is placing the transistor out of the active region, the most easy way is to insert a controlled generator (on-off) with a value of Vcc in series with the R_e, resistance of the emitter. In this way we can alter periodically the polarization network and therefore stop the transmission.
If we want to turn on and off the voltage source we need a component that generate a square wave, because this is a very well-know and common problem there is a IC that makes this works, its called 555. To control the period of on and off we only have to add two resistance and a condesator.
We went to the laboratory, we built the circuit, we added and antenna and we were surprising how our signal is well received by Josè Maria with a commercial radio receiver.
In the last minute of the lesson we talked about how connecting in a boat (and also in a air-plane) the antenna: it's a good engineering problem!
The next step??? We want to build up a receiver for this frequency and look how all the chain tx - rx works fine!
In the first part of the today lecture we made a little brief about what we made up to know: the Pierce oscillator implemented with a bipolar transistor, the tank circuit and the cheap quartz oscillator.
In the last class we stopped talking about how can we interrupt our transmission to implemented a very simple modulation: On-Off.
The first idea that we would have is to switch on and off the voltage source, this is not working fine because of the presence of the condensator electrolytic that have a slow discharge period.
The other possible solution (the one that we decide to implement) is placing the transistor out of the active region, the most easy way is to insert a controlled generator (on-off) with a value of Vcc in series with the R_e, resistance of the emitter. In this way we can alter periodically the polarization network and therefore stop the transmission.
If we want to turn on and off the voltage source we need a component that generate a square wave, because this is a very well-know and common problem there is a IC that makes this works, its called 555. To control the period of on and off we only have to add two resistance and a condesator.
We went to the laboratory, we built the circuit, we added and antenna and we were surprising how our signal is well received by Josè Maria with a commercial radio receiver.
In the last minute of the lesson we talked about how connecting in a boat (and also in a air-plane) the antenna: it's a good engineering problem!
The next step??? We want to build up a receiver for this frequency and look how all the chain tx - rx works fine!
martedì 30 novembre 2010
30/11/2010
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.
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.
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.
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