Tank circuit parallel resonant

For building probes, making tank circuits, and as an rf source which acts as a artificial NMR signal without any electrical connections to the circuit, a dip meter is nearly essential. In addition, it is useful in its intended role to passively measure the parallel resonant tank circuit s resonant frequency... 

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.)... 

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... 

A coil in a series tank circuit has the advantage that it is easy to rig in a remote location (as in a dewar tail). Unlike the parallel resonance circuit discussed above, the series tank circuit presents a low impedance to the outside world, i.e., much current with little voltage. [This low impedance could also be an advantage over the high impedance of the parallel tank in dissipating the rf energy after the transmitter shuts off. There are other ways to improve the receiver recovery (see V.A.4.), however, so this in itself is not sufficient reason to choose a series tank circuit.]... 

Now we return to the discussion of tank circuits and, in particular, to that of changing the impedance of resonant circuits to some desired value. First consider the parallel tank... 

Aside from its canonical use as a device for finding the resonance frequency of a parallel tank circuit, the dip meter can be used as an rf source to mimic an NMR signal. Because of the high sensitivity of NMR receivers, the dip meter need only be close to the NMR apparatus. But remember the output level comment in the last paragraph. An old grid dip meter can be put, say, one meter away from the receiver system and tuned close to the phase detector reference frequency to produce an observable signal. A dip meter usually has to be closer. 

Transformers are often used to couple the intermediate-frequency (IF) stages of a superheterodyne radio. These IF transformers, or IF-cans as they are often called, are parallel by capacitors, as shown in Fig. 2.40. Each coil has a moveable ferrite core that may be screwed into or out of the coil changing the inductance of the coil and, hence, the resonant frequency of the tank circuit. The IF transformer wiU thus pass signals near the resonant frequency. The IF amplifier thus have a very narrow bandwidth. 

In the first chapter, a very primitive radio transmitter was described, and it was mentioned that it was similar to the one on the famous ship Titanic. The transmitter shown in Fig. 19.2 is closer to the Titanic s radio, because it has a capacitor in parallel with the inductor, making a tuned resonant tank circuit (see index if necessary). This concentrates the transmissions at one particular frequency range. Also, the oscillating relay repeats those transmissions automatically. If an antenna were connected, as on page 181, the radio waves could be heard on an AM radio farther away. 

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