Ringing in coils

[Anonymous]

What are the main factors that contribute to ringing in coils, inductance,wire size,?
Regards Frank Wallis

 

[Anonymous]

Hi Frank,
Ringing in a coil for PI is caused by the TX pulse switch-off exciting the natural resonant frequency of the coil circuit. This frequency is determined by the coil inductance, the winding self capacitance plus the capacitance of the cable to the detector. Also you could add any capacitances associated with the switching transistor circuit. For high sensitivity i.e. using a short pulse delay, the coil and associated capacitances should be kept as low as possible so as to get the resonant frequency up. When properly damped by the parallel damping resistor, this will give the fastest switch off of the TX field. Coil resistance only has a small effect as, at switch off, is is effectively in series with the coil damping resistor. Most PI coils are low resistance (1 - 4ohms) and typically the damping resistor is 300 - 400ohms, so the coil resistance makes little difference. Coil resistance mainly determines the maximum coil current that can flow, although particularly in the SD's it never reaches its maximum as it is still growing due to the coil inductance when it is cut off. In other words it is inductance limited. A properly damped coil should not exhibit any ringing, except maybe one small overshoot.
Eric.

 

[Anonymous]

Hi Eric
Many thanks for the reply to the ringing question.Therefore would I be right in saying that with large coils you have to be careful not to use too large wire on the RX of DD,because that will increase capicitance,and require lower value resistor,this could lower sensitivity
Regards Frank Wallis

 

[Anonymous]

Hi Frank,
You don't need large diameter wire on the RX winding as there is no current to speak of. The capacitance of the winding has a lot to do with the type of insulation and the insulation thickness. I've found that Kynar insulated 0.25mm wire is good.
Eric.