PCB Common Mode Chokes?

[jstarr]

I came across a stash of three types of PCB jumpers going through what looks like a ferrite bead.  At first glance I thought the top component in the attached picture was a jumper but on close inspection it has 4 wires with >4 Mohms resistance between pairs on the same side and 0 ohms between opposite side pairs. I measure the inductance and Q at 15 kHz across the single pairs and found them to be about 5 uH, Q ~20.  Then I measured them after twisting the pairs on each together and still about 5 uH with Q about 26.  Is this a common mode choke?  How would you keep the adjacent wires from touching/crossing on a PCB as the wires insert into the ferrite bead at the same point on each side?  Are they other two types common mode chokes also?  I thought all common mode chokes were 4 wire?


tia john

[T3sl4co1l]

Yes, but even more specifically, that type of core is most often used in RF matching, amplifying and mixing applications, where the wide bandwidth of the core (high inductivity means fewer turns and lower capacitance) allows easy design (it can be treated as a transformer, without having to cancel out the reactance or deal with poor coupling), and wide band performance (one of those transformers might handle the entire cable TV spectrum at once).

The enamel should be colored differently, so that hand insertion would be able to get the correct phase.

The rest are just ferrite beads.  As you say, unless there are four (or more) wires, there's nothing to be common against.

FBs are good for filtering signal level currents.  They are often found in power supply paths, but this can be tricky; they saturate easily at modest DC current, defeating the purpose (for those applications, a proper inductor should be used instead, which can handle the required current).  They are of particular value when it's desirable to isolate internal and external signals: preventing external RFI from entering a circuit (via connector), and filtering internal noise from leaving (same thing, opposite direction).

Tim