05/10/2010

Measuring our coil.
Our aim of today is to measure the performance and some values of the coil that we made the last time.
The first things that we observeb is that we can measur the values of inductance L and the parasite resistance Rs from some value that we can see in the oscilloscope, we can calculate L from the value of the frequency of the peak and then with this value and the value of the max amplitude we can calculate Rs.
Like Heisenberg said in his uncertainty principle we can't measure some values without modifing that values, only beecause we are trying to mesure with an instrument. So our target is to minimize the effect of our instrument.
Jorgè Maria showed us a low capacitance probe that is able to "put" only 12 pF o capacity in our mesuring set-up. The price that we have to pay is that the amplitude is scaled by a 10 factor.

Now we were ready to go to the laboratory and test our coil and calculate L and Rs. We made this experiment for two position of the coil respect the kernel of ferrite so we can have an idea of the range of the values. Unfortunately me and my companion can't make the second mesure with the coil in a side of the ferrite because our coil doesn't slide well in the ferrite. The value that we obtain with the coil in the middle is L=310 uH and Rs=22 Ohm.

What happen? Why Rs is so hight? Tthe problem is that at high frequency (in our case around 1 Mhz) the skin effect is high and the copper becomes not a so good conductor.
With an Rs so high the performance of our antenna-filter-amplificator is worse. And so we have less amplification and a more large bandwidth.

Finally we observe that if we want to add an other amplificator in HF we have to minimize the effect of the resistence that is seen at the terminals otherwise we have, another time, a worsening in the performance.