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!
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