23
votes
Accepted
Is twisting a single ended signal with its own GND really useful?
Yes, it is better.
Model the cable as two transmission lines: one representing the differential mode (signal to GND wire), stacked on top of one representing the common mode (GND wire over GND plane)....
- 45.7k
19
votes
Is using the shield in a CAT cable for return current a bad idea?
Yes, it is a bad idea.
The cable shield should not be used as a current conductor.
And as Ethernet is supposed to be a transformer isolated interface, including the PoE part, there should not be any ...
- 172k
18
votes
Cable shielding (best practices)
Since this is an engineering site, I feel obliged to call out that this design is fundamentally flawed: you cannot have plain 3.3V UART signals across a wire which is exposed to heavy EMI. It is an ...
- 22.3k
17
votes
What is a more effective shield for magnetic fields between 300 and 500kHz Solid copper or copper mesh?
My only worry is the high frequency AC magnetic fields
It's really all about a thing called skin depth: -
Graph taken from this wiki page
So, for example, at 100 kHz, copper has a skin depth of ...
- 473k
16
votes
Accepted
Does Shielding "Electrically" shield "Magnetically" too?
You would not be alone in this one. This is an often misunderstood phenomenon.
Static magnetic fields can not be shielded. They can be re-directed using ferrous materials but even those will not ...
- 47k
15
votes
Cable shielding adequate for video, but not audio frequencies
It is conceptually possible to induce magnetic interference at audio frequency in an electrically shielded cable, which is not so much possible at higher frequencies due to the shields becoming opaque ...
- 23.4k
13
votes
Accepted
What is a more effective shield for magnetic fields between 300 and 500kHz Solid copper or copper mesh?
Solid would perform better, all other things being equal, but perhaps not significantly better.
Since the 'holes' in your mesh will be a tiny fraction of a wavelength, the mesh should behave ...
13
votes
Active guard shielding for instrumentation amplifier
[This is a more theoretical addendum to the answer by @VoltageSpike. ]
Active driven guard is used when the signal source (the sensor) has a high output impedance. The amplifier input has a (very) ...
- 38.6k
13
votes
Is twisting a single ended signal with its own GND really useful?
It strongly helps when done right. Allow me to make a small story arc to explain my answer.
The problem
Ground planes are good dump-and-forget nodes for return currents at both high and low frequency ...
- 23.4k
12
votes
Accepted
Failed radiated emissions test on USB cable - USB module hardware and firmware improvements
Two glaring errors:
Unshielded cable.
It might not be fully unshielded, but a few cm of unshielded length is enough to be a problem.
USB over plain pin headers is generally a bad idea, and only ...
- 45.7k
11
votes
USB shielding, device or host side?
How to connect cable shields
a.k.a: A short summary of Henry Ott's approach in Electromagnetic Compatibility Engineering (2009).
Foreword
Does it matter if the shield gets shorted to ground on the ...
- 7,467
11
votes
Cable shielding (best practices)
This thread is kind of case-in-point why I would like to discourage questions on EMC here; the format of this site is not well suited to topics of such complexity.
But as these questions inevitably ...
- 45.7k
10
votes
Accepted
Purpose of bare copper perimiters on HF PCBs
Think of your PCB as parallel plate waveguide, with the top and bottom ground planes acting as parallel plates.
Now, since you want waves neither exiting nor entering this waveguide, you'd try to ...
- 100k
10
votes
Accepted
Cable shielding (best practices)
Ed note: bahaha, I discovered the 30,000 character limit for a single answer. It seems I need to split this off. Well, just as well; it's a more direct answer to the question. Please see my other ...
- 45.7k
8
votes
Accepted
Does an ungrounded shielded cable shield at all?
Acting as a faraday cage, an ungrounded shield covering a twisted-pair will be somewhat effective. When in the presense of an interfering electric field, both wires in the twisted-pair are influenced ...
- 473k
8
votes
Accepted
Long wires causing Raspberry Pi to die?
The wire has inductance and, the longer the wire is the more inductance it has. An inductance likes to maintain the current flowing through itself so, when your switch opens, the small stored energy ...
- 473k
8
votes
75 Ohm Coaxial Cable Used On a 600 Ohm Output
If I searched right, the unit has a maximum output frequency of 1MHz. A 1MHz wave has 300m wavelength in free space. It will be slightly longer in a cable. So the cable you will use, will be ...
- 1,407
8
votes
Active guard shielding for instrumentation amplifier
If the objective is to shield from RF, then its only necessary to connect the shield on one end to the ground of the PCB or chassis ground (and not on the sensor end which would make a ground loop). ...
- 88.8k
7
votes
Accepted
Shielded Twisted Pair noise immunity
A shield usually makes a pretty good protector against electric field disturbances (half the story) but, unfortunately, makes a pretty poor protection against magnetically coupled noise (the other ...
- 473k
7
votes
Cable shielding adequate for video, but not audio frequencies
There's no reason to think the shielding has a monotonic impedance. There are parasitic inductances and capacitances lurking in the cable so it would be expected that there are impedance peaks and ...
- 57.3k
7
votes
Accepted
Does a shielded inductor store less energy than an unshielded one?
Does a shielded inductor store less energy than an unshielded one?
A shielded inductor usually has a construction where the ferrite core that is surrounding the coils is much more complete (more ...
- 473k
7
votes
Why is foil shield more efficient than a braid for high frequency noise?
Usually the difference simply is that the foil covers 100% of the shielded inner wires while a braid only covers 70% to 95% of the inner wires.
So as an analogue, foil shield is an unbroken metal tube ...
- 172k
6
votes
EMI shielding without shield?
I worked for a company that put ground layers at top and bottom and stitched them together all round the board perimeter, trying to make a Faraday cage. The product still radiated a lot.
Turned out ...
- 14.6k
6
votes
Accepted
How should I use a static-shielding bag prior to opening, and why?
Antistatic precautions mean you should never move a device through a large potential difference.
Once you and the ESD mat are connected to ground, they are at the same 0v potential. Take hold of the ...
- 174k
6
votes
Accepted
How does electromagnetic shielding work?
I don't understand why magnetic fields and electric fields cannot
penetrate the cable when there is grounded shield covering the cable.
A magnetic field can certainly penetrate the shield but, we ...
- 473k
6
votes
Is twisting a single ended signal with its own GND really useful?
We can distinguish two kinds of coupling, inductive and capacitative. Using balanced twisted pairs helps reduce inductive coupling. But the question is about unbalanced twisted pairs.
It turns out ...
- 27.9k
6
votes
Cable shielding (best practices)
Depends. A shield introduces a capacitance across the cable and itself. This extra capacitance may or may not cause a problem on bit rise and fall times. In your case, the baud rate is low, 9.6 kBaud ...
- 38.5k
6
votes
Accepted
EMI shields: When to use 2-piece cap+frame, just a frame, or just a shield (or just a cap)?
if you are going to use a cap anyway, what is the point of the circle
thing?
Probably a point for the SMT machine to pick up, seeing how it comes in a tray and is SMD-able.
Given that the holes are ...
- 88.8k
5
votes
Purpose of bare copper perimiters on HF PCBs
Probably every free space of your PCB in grounded to prevent electrical interference as Marcus Müller explain in his answer.
However the ESD logo on the PCB
suggest that the components are ...
- 83
5
votes
Should I connect a cable's shield to the ground layer of board when it's isolated from earth?
You could use a 1M resistor and 0.1uF capacitor in parallel to connect the shield ground and board ground together. Our board designs at my work do this. It essentially grounds the shield while ...
- 63
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