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.
"
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.
Robin