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.