Impedance matching and tuning RF coil

[uzi kiko]

Hi everybody.

I am trying to understand the concept of impedance matching and tuning of RF coil.

As far as I understand there are 2 stages:
At the first stage I should match the coil to the source (Let's say with L network method) and the second stage is to tune coil to the desirable frequency (tuning = tuning the coil so the desirable frequency will be the resonant frequency) using a tuning capacitor.

Now at the first stage I am matching the coil circuit including the tuning capacitor but at the second stage I am changing the capacitor value. But will not this change affect on the impedance matching?

Thanks a lot

 

[tech99]

Hi everybody.

I am trying to understand the concept of impedance matching and tuning of RF coil.

As far as I understand there are 2 stages:
At the first stage I should match the coil to the source (Let's say with L network method) and the second stage is to tune coil to the desirable frequency (tuning = tuning the coil so the desirable frequency will be the resonant frequency) using a tuning capacitor.

Now at the first stage I am matching the coil circuit including the tuning capacitor but at the second stage I am changing the capacitor value. But will not this change affect on the impedance matching?

Thanks a lot

There are several ways of looking at the operation of LC matching circuits, not complicated. With one method, for instance, if you add a reactance in series with the load, there will be an equivalent parallel combination of R and X which looks identical to the outside world. The R and X values of this parallel combination are both different. So you can use the formula to convert from a series to a parallel combination, and aim to get a resistance value which is the one you want. Then you get rid of the reactance by adding an equal and opposite one in parallel.
Another approach is to first decide the ratio of resistances source/load (greater than 1). The Q of the circuit must then be set to be SQRT of the ratio. The L-match circuit has one resistance in a series leg and another in a parallel leg, so for each position, you just choose the reactance of the inductor or capacitor to give the required Q. Simply, Q = X/R for instance.

 

[uzi kiko]

Thanks a lot for the quick and detailed answer!