Hi All,
I have another riddle for you, this time a video is not needed.
We all know that cable shields need to be connected on both ends to work at
high frequencies, but do they?
Imagine the case of a source in a metal box driving a shielded cable, let's
assume it is shielded twisted pair and we are worried about the common mode
noise on the pair radiating or ESD noise getting in and saturating the receiver
in the box, exceeding its common mode range. We can also assume that the
shielded connector on the first metal box is ra eally good one with a 360
degree connection to the box. The other end of the shielded cable goes to
another metal box with a passive circuit inside, say a temperature sensor
connected to the twisted pair wires. There is one special, but not uncommon,
case where it does not matter if the shield is connected to the second box or
not (or even if there is a second box at all), it will work in either case! Do
you know what that case is?
This is another experiment I do during my classes. I prove the conditions
needed for the situation above and what the limits are.
I am pretty much a lab rat and I have devised experiments to test the limits of
most all EE concepts I have learned. I find there is more than meets the eye on
most every principle. I started doing experiments like this around age 11 when
I duplicated one of Marconi's experiments to see if it really worked, it did,
although I suspect the neighbors may have had a rough time watching TV that day
in 1958. A few years later another experiment brought the FCC to my house, but
that is another story.
Doug
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