Shielded inductors

Note that in all topologies, the inductor is not in the critical path. So we need not worry much about its layout, at least not from the point of view of noise. However, we have to be wary of the electromagnetic field the inductor creates, because that can impinge on nearby circuitry and sensitive traces, and cause similar (though usually not so acute) problems. So generally, it is a good idea to try and use shielded inductors for that reason, if cost permits. If not, it should be positioned a little further from the IC, in particular keeping clear of the feedback trace. 

This result was predicted mathematically, using basic circuit and field theory, and circuits have been designed using shield discontinuities as inductors. It has also been proved that the discontinuity—or hole—in the shield can assume various shapes without affecting the result obtained in each case. 

Transients generated on the antenna side because of a lightning discharge are no less troublesome. The tower is a conductor, and any conductor is also an inductor. A typical 150-ft self-supporting tower may exhibit as much as 40 /xH inductance. During a fast-rise-time lightning strike, an instantaneous voltage drop of 360 kV between the top of the tower and the base is not unlikely. If the coax shield is bonded to... 

To reduce this problem, most manufacturers add small series resistors and capacitors to ground at the input terminals. Inductors also may be added, but they are susceptible to external magnetic fields. If package shielding is inadequate, the inductors may, in fact, pick up as much noise as they are supposed to filter out. Another RFI defense consists of placing donut-shaped ferrite beads on the input leads of affected stages. 

---