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.