16/12/2010

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.