Tank circuit impedance matching

The impedance at resonance can be changed by changing the L-C combination while satisfying the resonance condition. Therefore, a parallel resonance tank circuit can match the impedance of the coil to that of the rest of the circuit with just one additional component, namely a tuning capacitor. This is simpler and more efficient than schemes having more components. [But with a given coil the tank can have a... 

Fig. 14. A RF tank circuit (as illustrated in (b)) with Ltank= 10 nH and C parasitic - 7 pF was tested to demonstrate how an impedance matching circuit would h ansform various resistance values (as given in (a)).

The probe uses a single-coil for 20 mm spinning samples at room temperature. There are no tuning elements in the probe -- rather the tuning is by a variable inductor at the end of a A./4 cable as described in the discussion of quarter wave cables in V.C.8. The transformer at the end of the second quarter wave cable matches the impedance between the tank circuit and the broadband preamplifier. The output... 

Once the coil has been incorporated into a tank circuit, a dip meter can be used to measure the resonance frequency. For a parallel resonant circuit, the dip meter can be used in the usual way, i.e., look for a "dip" indication as a function of frequency. For a series tank circuit, it is best to use the dip meter as an rf source acting like an artificial NMR signal and maximize the receiver output when the receiving system is fully hooked up. This will work also for the parallel tank. (See section V.C.9. on impedance matching.)... 

As far as the receiver is concerned, the operation of the circuit is also straightforward. The low impedance of the tank circuit plus the resistance of the series resistor is transformed by the quarter wave transformer. The transformed value is matched to the impedance of the receiver by a conventional... 

Now at frequencies below a few hundred MHz, the typical capacitor used in NMR is virtually loss free and r is equal to the resistance of the wire in the coil and possibly of a cable between L and C. Since this is a small number, the series tank circuit requires additional circuitry to raise the impedance to match, for example, to a preamplifier with a 50 ohm input. [The need to present a certain impedance arises because the S/N of the preamplifier depends on this impedance Thus, this is an important matching condition.]... 

As already described, a magic-tee is a four port device in which two of the ports are isolated, i.e., have infinite impedance between them if the impedances at the other two ports are matched. Thus, the device can be used to match a resonant circuit to 50 Q or, for that matter, to measure the impedance at resonance of the tank circuit. The tank circuit under question will have impedance R at resonance when the output of the magic-tee, easily monitored on an oscilloscope, is an absolute minimum. (See the figure under magic-tee on p. 393.) Unfortunately, this is easier said than done because there are relative minima when the capacitors are adjusted to give an impedance which is, for example, equal to R in magnitude but with an imaginary component. When you play around with a scheme like this for the first time, it will be useful to let R be variable so that it can be adjusted for a... 

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