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).