Tuning knob

This circuit is tuned to the desired frequency by a variable capacitor operated by the TUNE knob to the far left of the meter. 

The TUNE knob is rotated for a maximum meter deflection. Measiirements are then made as before with increased meter sensitivity. 

With more links in the chain, brains have more opportunities to make adjustments—there are more knobs and controls with which to fine-tune the activities of neurons. Current theories of learning and memory identify the plasticity—adjustability—of synapses as the mechanism by which brains acquire and store information. Intelligence evolves from the continual tweaking of chemical transmission in the brain. 

The probe tuning rods are long extensions of the variable capacitors located at the top of the probe, near the probe coil. The capacitors are delicate and there are two ends of the travel of the knob If any force at all is applied at the end of the travel, the capacitor will break. This will usually require that the probe be sent back to the manufacturer for repair, a process requiring a week or two and costing many thousands of dollars. For this reason many NMR labs do not allow users to tune the probe ... 

Sometimes the operator s proximity to the adjustment knobs will affect the tuning condition. This condition should have been avoided in the design of the apparatus. An unsightly but quick-and-sure fix is to extend the adjustment shaft with an Insulator. Standard diameter shafts, e.g., 0.125" and 0.250" diameter, can be extended with plastic shafts of the same diameter using commercial shaft couplers. Insulated shafts can also be adapted for various screwdrivers and similar tuning tools, either commercial or homemade. We have seen insulated shaft extensions as long as one meter, which attests to the practicality of some NMR people. 

In order to check whether proportional control is satisfactory, it is necessary to consider how values of K (which is called the proportional gain) affect the response. Purchased controllers have a knob or screw which enables engineers to physically change within limits the value of K and hence the response of a system. While some fine tuning can be done by trial and error, some prior predictions are essential. 

---