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Balanced and unbalanced lines...

by fisico30
Tags: balanced, lines, unbalanced
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fisico30
#1
Mar5-11, 08:56 AM
P: 374
hello forum,

I am trying to read and learn about balun. Some transmission lines are said balanced while other unbalanced....

In a balanced transmission line the voltage on one wire is always as big as the voltage on the other wire but 180 out of phase.
For example wire 1 is at 50 V while wire 2 is at -50 V....Are these two voltage readings measured in reference to a 3rd reference wire/metal?

I am not sure I fully understand the idea of unbalanced transmission line. Voltage is always measured between two points. Would wire 1 in an unbalanced transmission line show +50 V with respect to the 3rd reference while wire 2 always 0 volts with respect to the 3rd reference metal?

A coaxial cable is an example of a unbalanced line. One of its conductor is always connect to earth ground. But voltage is a relative concept at the end...
Can someone clarify these ideas please?

thanks,
fisico30
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vk6kro
#2
Mar5-11, 10:42 AM
Sci Advisor
P: 4,032
In balanced and unbalanced lines, you can take either conductor as the reference and measure the voltage on the other wire relative to this one.

In coaxial cables, it is usual to take the shield as the reference.

In either case, you can also take a third reference point as your reference, as long as you state what you are doing.
One example would be if you were driving the balanced line from a center tapped transformer. In this case, the center tap would be the 3rd reference point.

All that really matters is the difference in voltage between the conductors.
Studiot
#3
Mar5-11, 11:29 AM
P: 5,462
Hello fisico,

This may help sort the confusion.

Firstly 'a balun' is the term given to a device (usually a transformer) which converts between balanced and unbalanced signalling.

You are correct in realising that balanced signalling requires three wires and unbalanced signalling, only two.

Sometimes one of these two or three wires are connected to earth, sometimes not.

Now the main purpose of the introducing the extra cost and complication of the third wire is to reduce or eliminate noise and other interference.

The idea is the same as using an op amp in differential mode to reject common mode signals.

On a balanced line, each of the two signal wires carries the desired signal (Vs)but in opposite phase.
However the noise (Vv) is picked up equally by both wires in the same phase.

So if we take the difference at the other end will have the following result

Vout = (Vs + Vn) - (-Vs + Vn) = 2Vs

That is the noise is cancelled out.

Averagesupernova
#4
Mar5-11, 12:50 PM
P: 2,530
Balanced and unbalanced lines...

Quote Quote by Studiot View Post
Hello fisico,


You are correct in realising that balanced signalling requires three wires and unbalanced signalling, only two.
I can't agree with that.
mattbrrtt
#5
Mar5-11, 02:46 PM
P: 11
A balanced line propagates a differential signal, where both conductors are in close enough proximity to each other that, when exposed to an interference, will have negligible effect to the signal that is transmitted. This is because both conductors of a balanced line are isolated on both sides, using a balun. The signal that propagates from source to destination is only relative to itself when placed across the balun. Since any interference in the transmission line is exposed to both conductors, there is no current flow to represent the interference signal at either end of the transmission line.

In an unbalanced transmission line the shield is used as the reference, and is usually attached to a ground. The shield prevents interference by receiving it and dumping it to a large capacitive sink, or ground. This system is more resistant to outside noise, but more susceptible to noise that is injected to the system through ground.

If your transmission line is less than 1/4 wavelength long, then you only need to match the antenna and transceiver impedances.

If the impedance on the transmission line, transceiver, and antenna are matched, and the transmission line is cut to length based on the electrical wavelength, then a balun is not necessary and would be an efficient transmission of power from your transceiver to antenna. Baluns are used because of poor matching, or sometimes necessary for isolation and noise rejection.
Studiot
#6
Mar5-11, 04:37 PM
P: 5,462
I can't agree with that.
Perhaps you can explain why a balanced jack plug or an XLR connector need three terminals for a single balanced signal?

Note the difference in the extract from these instructions for a professional mixer.
Attached Thumbnails
balun1.jpg  
Averagesupernova
#7
Mar5-11, 08:00 PM
P: 2,530
Quote Quote by Studiot View Post
Perhaps you can explain why a balanced jack plug or an XLR connector need three terminals for a single balanced signal?

Note the difference in the extract from these instructions for a professional mixer.
It doesn't need the third wire which I believe is connected to chassis ground at the mixer. It is called a drain and is used to reduce noise. It doesn't need to be there to convey the information from the mic. It is only there to reduce noise usually in the form of 60 Hz hum.
-
Feeding a dipole antenna with balanced transmission line requires no third wire. A speaker driven with an output transformer is a balanced system with no third wire.
Studiot
#8
Mar6-11, 07:56 AM
P: 5,462
If both balanced and unbalanced signalling was simply the (voltage) difference between two wires or two points, A and B the distinction would be superfluous.

(Electroncis) Dictionary Definition

A balanced line is a transmission line that has conductors of the same length, equal resistances per unit length and equal impedances to earth and to any other electrical circuits.

Can you draw a diagram showing how this last emboldened condition is met without a third wire or connection?

As I understand the term it stems from the idea that the system is a bit like a see-saw. Two opposing (electrical) forces are set in opposition or balanced, but they have to be both referenced to a common third point. Just as a see-saw has to have a fulcrum.
TurtleMeister
#9
Mar6-11, 08:34 AM
TurtleMeister's Avatar
P: 757
Quote Quote by Studiot View Post
(Electroncis) Dictionary Definition

A balanced line is a transmission line that has conductors of the same length, equal resistances per unit length and equal impedances to earth and to any other electrical circuits.

Can you draw a diagram showing how this last emboldened condition is met without a third wire or connection?
That definition in no way implies that a third wire is required. I've used transmission lines in my amateur radio hobby for many years and I've never seen or heard of ladder line with three wires. That last condition you are referring to does not have to be perfectly met in order to qualify the transmission line as balanced. If one conductor comes closer to a metal object than the other, then it is an installation problem.
Studiot
#10
Mar6-11, 11:11 AM
P: 5,462
OK, Thank you for the discussion, gents.

I think I have it now.

Balanced, when talking about a transmission line refers to equality of impedance. It does not refer directly to the signal impressed on the conductors, although it obviously has implications for the signal.

A balanced signal does need a reference to arbitrate the balance however. So for instance a push-pull drive to an output transformer is balanced.
Bob S
#11
Mar6-11, 06:22 PM
P: 4,663
Studiot in post #3 pointed out that signals in general are composed of both a differential (balanced) signal and a common mode signal. The balanced signal has equal and opposite currents and voltages in the two signal wires. There is a type of coax made specifically for balanced signals. It is a "twinax" cable comprised of two conductors, surrounded by an outside braid shield. See picture and specs in

http://www.awcwire.com/Part.aspx?code=740F23F27J44

It has two signal wires, and a third braided shield. Each signal wire has the same capacitance to the braided shield. The nominal characteristic impedance of the two balanced conductors is about 78 ohms. RG-108 requires special connectors.

Bob S
mattbrrtt
#12
Mar7-11, 04:44 PM
P: 11
Balanced lines use a differential signal, that consists of two wires. The notion of differential is that any noise that occurs on one conductor will appear on the other, and since one signal is referenced to the other a common signal nulls out, leaving only the intended signal.
Unbalanced lines generally use an outer conductor as a shield, that is attached to a reference voltage. This shield helps to eliminate outside noise, but does nothing to help if there is a difference in ground potential at each end of the line.
A third conductor will generally be an external shield, with a balanced line internal to it. This uses the idea of both a shield, and keeps the advantage of having a differential signal. However, the grounded shield has nothing to do with the signal that it surrounds other than acting as a shield.
To answer the OP:
If the voltage in a differential line is to be 50V, assuming RMS, then if you used a voltmeter that is accurate for the frequency that is in the transmission line, you should measure 50VAC. The signals are 180 degrees out of phase but since each signal is used for the reference as the other, a voltage specified is from point A to point B.
I think that the third wire in a transmission line has been discussed.
The statement regarding voltage at the end has to do with the losses in the cable, and differing potentials of ground. This is one reason that some choose to use a balun. It can correct a complex impedance mis-match at the end of the line and provide a differential signal to the antenna. This comes at a cost of inefficiencies.
Studiot
#13
Mar8-11, 06:03 AM
P: 5,462
In reality the situation is more complicated than you gentlemen are making out.
Perhaps that is why confusion arises.
In particular a twin wire system may or may not be a balanced line, but it must always carry an unbalanced signal. It is impossible to have a balanced signal on a balanced line without three wires.

I have uploaded some sketches to illustrate this.

Fig 1
Shows the already mentioned loudspeaker fed from a transformer via twin wire.
Since there is no zero reference for the signal, the signal is unbalanced. But the line is balanced.

Fig2
Replaces the loudspeaker with an single power supply amplifier. The transformer supplies an unbalanced signal into the twin wires.

Fig3
The amplifier is now fed from split power supplies. The transformer supplies an unbalanced signal into the twin wires. This is seen by the amplifier as a balanced signal and the amp signal output is also balanced.

Fig4
Returns to the loudspeaker but adds a second one in series. Can anyone claim the line is now balanced as the impedences of each section are now widely different?

Fig5
A variation with the loudspeaker fed from two very different lengths of separate wires.

Fig6
Most would agree with the statement that the centre tapped LHS, of the transformer, is balanced but the RHS is unbalanced. The transformer is a Balanced to Unbalanced device (balun), viewed from the point of view of the signal.

Fig7
The transformer is reversed and now converts Unbalanced to Balanced. To complete this system is is necessary to provide the centre connection to the other end of the line by some means.
Attached Thumbnails
balun2.jpg  
Averagesupernova
#14
Mar8-11, 10:10 AM
P: 2,530
Sorry Studiot, I can't agree.
-
As far as transmission lines go:
Coax can't really EVER be considered balanced transmission line.
Twin-lead, ladder line, zip cord, etc. is considered balanced line but it can be connected in such a way to become unbalanced.
-
I've never heard of a a signal being balanced or unbalanced. ALL signals have to be differential.
-
Your example in number 7 means nothing. Where are you going to connect the wire coming from the center tap at the load?
-
The summary of unbalanced versus balanced is that balanced is usually picked over unbalanced because most noise introduced onto a balanced transmission line is common mode. This means that when the noise signal voltage on one wire goes up it also goes up on the other wire. This is because the impedance to ground on both wires is the same whether it is infinite (case of no third wire) or some finite value (a center tapped transformer). When both inputs to a differential load such as a loudspeaker, balanced mic input (an actual true differential amplifier), etc. have common mode noise as described above the noise is highly attenuated.
Studiot
#15
Mar8-11, 12:14 PM
P: 5,462
Coax can't really EVER be considered balanced transmission line.
I don't recall saying it could or couldn't. In fact I don't recall mentioning Coax.

Twin-lead, ladder line, zip cord, etc. is considered balanced line but it can be connected in such a way to become unbalanced.
So you do agree with me.

I've never heard of a a signal being balanced or unbalanced. ALL signals have to be differential.
Just because you have never heard of it doesn't mean it does not exist, I have already described one situation in a previous post. Perhaps you should reread properly rather than making such sweeping statements.

Your example in number 7 means nothing. Where are you going to connect the wire coming from the center tap at the load?
Is it your contention that such transformers do not exist, or did you fail to read Fig3 properly? The wire should be connected to the input zero or earth of the amplifier. Without this connection the system will not work.
Averagesupernova
#16
Mar8-11, 12:52 PM
P: 2,530
Quote Quote by Studiot View Post
I don't recall saying it could or couldn't. In fact I don't recall mentioning Coax.
Just because you didn't mention it doesn't mean I am not permitted to point it out.

So you do agree with me.
I don't agree with you. I am saying that if you connect a zip cord which is considered to be balanced transmission line to an unbalanced output such as a 1/8 inch phone jack for running an external speaker that you are using balanced transmission line but the conductors do not see the same impedance to ground. One is already grounded since typically the sleeve part of said jack is connected to ground.

Just because you have never heard of it doesn't mean it does not exist, I have already described one situation in a previous post. Perhaps you should reread properly rather than making such sweeping statements.
Not sure what to say to this. You can reference any point in a circuit. The closest I can imagine is to say differential vs. single ended. My main point is that a signal has to consist of a voltage between two points. Sorry for any misunderstanding.

Is it your contention that such transformers do not exist, or did you fail to read Fig3 properly? The wire should be connected to the input zero or earth of the amplifier. Without this connection the system will not work.
On figure three:
The amplifier is now fed from split power supplies. The transformer supplies an unbalanced signal into the twin wires. This is seen by the amplifier as a balanced signal and the amp signal output is also balanced.
I am not saying such transformers do not exist. The transformer is supplying a signal on a balanced transmission line into a balanced load. The fact that the amp has a split supply means nothing. The output is single ended and is unbalanced. Contrary to what you are saying this circuit will work. Of course no negative feedback is shown, I assume it is implied.
Averagesupernova
#17
Mar8-11, 01:01 PM
P: 2,530
Just did a bit of rereading. Figure 4 doesn't mean much since you don't spec how far away each speaker is from the other. Assume that the speakers are right next to one another (read in the same box) even if the speakers are not at all identical and you can say that it is a balanced system.
-
Figure 5 doesn't mean much since by definition I believe a balanced transmission line requires the conductors to be the same length, at least relative to the wavelengths involved.
-
Figure 6 makes no sense. You have to connect the transformer to something to define it as unbalanced. Alone it certainly is balanced, both the primary and secondary.
-
Figure 7 pretty much falls into the same category as figure 6.
-
I'd say you have some misconceptions in general about all of this.
Studiot
#18
Mar8-11, 01:14 PM
P: 5,462
There are some glimmerings of a discussion in your last two posts, but your style is still rather abrasive and dismissive of the other party. For example.

Figure 5 doesn't mean much since by definition I believe a balanced transmission line requires the conductors to be the same length, at least relative to the wavelengths involved.
Of course fig5 means something. I even stated explicitly that both the line and the signal are unbalanced.

You actually seem to agree, so why the snooty response?


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