I'm trying to understand a system with rf circuit.
I found that there is a concept called "groundloop"
I searched it at the Wikipedia, but I don't think I now know it.
How can I explain the concept?
What is floating potential?
below is the original statement I'm trying to understand
" Ground loops can add excess noise and therefore efforts were made to avoid these throughout the whole system. As a consequence, many signals are transmitted differentially, the cable shields are typically only connected to ground at one end, many of the power supplies use floating potential, and isolated inputs with optocouplers are used for most digital signals. These precautions were only taken in the parts where the DC level is important, and not in the pure radio frequency (RF) circuitry.
Floating potential just means that the voltage has no direct connection to earth. A transformer secondary is a common source of floating potential. Of course, the static charge on a balloon that holds it to the ceiling is another.
A ground loop is just a conductive loop that is made up, in part, by two separated connections to Earth. This is what you get if you ground both ends of a wire. The Earth completes a loop that connects the two ends through another path than through the wire. Any changing magnetic field that passes through the area enclosed by this loop will generate circulating current around the loop. The resistance, inductance and propagation delay of that current will produce voltage differences between the ends of the wire. So it you are using that wire as a zero volt reference for another signal at both ends of the wire, the measure of that signal with respect to this reference will be different at the two ends of the wire. The loop will have coupled energy from the magnetic field that passed through the loop into the signal, corrupting it to some extent.
If that reference wire were earthed at only one end, there would be no loop, but at the other end, that "ground" reference may not have exactly the same voltage as the local Earth potential. So an isolated receiver that subtracts the reference voltage of the remote ground from the signal, and re references the difference to the local ground potential will break the loop and eliminate the corruption.
Circa Wed, 08 Aug 2007 03:43:40 -0000 recorded as looks like snipped-for-privacy@hanmail.net sounds like:
Differential transmission is another way of describing a balanced line, such as a twisted pair cable, where neither polarity of the signal is referenced to ground. Google "balanced line."
This is now describing an unbalanced line, such as a coaxial cable, where the shield is referenced to ground at only one end of the line, in order to reduce the possibility of current finding a path through the shield. This statement could also refer to a technique of grounding chassis in a "star" pattern, meaning that there is a central point where connection to ground is made, and all chassis are isolated from each other except at the point where ground cables from those chassis are tied to the central point.
Describes not grounding either end of a power supply. This way, current that is produced by the power supply does not return through a ground path, thus does not add to current through ground connections. The return path is separate from ground.
An opto-coupler will isolate return paths for small signals, again keeping the paths for ground current to a minimum.
I'm not sure what this means. It seems specific to the application. In my experience, these methods of isolation are used to minimize the effect of
Sorry, I haven't yet read nearly enough questions to force me to face all the stuff I still don't know.
But there is nothing like trying to stuff more than you know about something into a single paragraph, to make you think really hard. That is what I get out of being here. I'm just getting close to understanding some other people's books, at this point.
Ground loop: You need to draw a picture and think about it, like this: Your (ancient) hi-fi amp is plugged into the wall socket. It has an earth in the mains plug.
Your (ancient) tuner is also plugged into the wall socket. It also has an earth in the mains plug.
You connect them together using a screened co-axial cable because... it is screened and you don't want any mains hum... but you get... lots of mains hum!?
When you listen to the tuner with headphones (only) it is perfect.
When you plug a microphone (only) into the amp it is perfect.
You only get the hum when they are joined together by the coax.
The reason is that both the (ancient) amp and the (ancient) tuner have an continuous earth between their coax screens, to their wall sockets so when they are plugged together a loop is created, an earth or ground loop via the coax, though the amp, through the mains loop, though the tuner back to the coax screen.
Here is the hard-to-understand-bit: both the amp and the tuner have huge, bulky, mains transformers in them. Each transformer is radiating magnetic fields at mains frequency. These changing fields are cutting the ground loop. The ground loop now has changing currents induced in it.
Any conductor carrying a current *must* have a voltage across it driving that current - otherwise there would be no current :)
Therefore the coax screen *must* have a changing voltage (at mains frequency) between each end.
So even with no signal in the coax's center conductor, there is a signal (mains hum) in the screen. And it gets amplified and comes out the speakers.
The problem ceases by breaking the "gound loop" e.g. disconnecting the earth in the plug of the tuner.
That's were the name "ground loop" came from but it applies to any loop grounded or not. But evidently it is a common error particularly in screens and earthing plans.
******************************************* The twisted pair: suppose you plug a mike into the amp but you *still* get mains hum?!
How can that be? The coax screen of the mike should prevent mains radiation getting to the centre conductor! The answer is that is
*does* prevent radiation getting to it and *that* in itself, is the problem. Like this:
Imagine the coax screen at the amp's imput as being zero (or put it another way, that you are standing on it so as far as you are concerned, you and it are always the same voltage i.e. zero) then mains radiation (and any other nearby source) will be inducing currents in the coax screen. These currents flow back and forth between you, and via the mike to the signal wire and thence to the amp's input - hence the mains pickup.
The screen *is* the problem.
An answer could be to use two plain wires instead of the coax - now this is the clever bit, any radiation is now picked up on *both* wires the same and currents flow back and forth in each IN THE SAME DIRECTION and the result is that they *cancel* each other out (you might have to draw a picture here).
Evidently if both wires are laying straight then one wire might pick up a little more than the other if it is closer to the source of radiation so you avoid that by twisting them together to even out the likely pickup from all directions (as well as keeping them close).
This twisted pair thing is usually done inside a screened cable. Your mike cable will have three conductors, i.e. two signal wires, and one screen.
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