Differential signaling
Balanced signal transmission is a method to transmit signals even with longer transmission paths as trouble tolerant can. Transmission is done instead of using a single wire with a pair of these. The influence of the useful signal by signal interference on the transmission path is almost identical with symmetrical signal transmission along both lines, so that when the difference of the two circuit - potential disturbance ( almost ) cancels. This is achieved in that each conductor of line pair having the same source impedance, line impedance and the same same load impedance.
Often Balanced signal transmission is also simultaneously associated a differential transmission and a voltage supply symmetrical with respect to a reference potential (ground) with the term, however, is to distinguish impedance characteristics, voltage profile and the reference potential between the three aspects. With symmetrical transmission, Eng. balanced line are, strictly speaking, only meant impedance characteristics. Depending on the application next to the line pair for balanced signal transmission is still the reference potential is transmitted as the third conductor.
- 3.1 Differential transmission
- 3.2 Pseudo- differential transmission
Principle
If an electrical signal is transmitted by wire, so this is basically a closed circuit necessary. Between the signal source and signal sink thus always exist two conductors. With a wire alone, no signal can be transmitted, it is in this case always the earth as a return conductor, or rather, the opposite pole, necessary.
If you want an electrical signal on a line transfer, so all inductive and capacitive environmental influences act after a short distance (this can be up kilometers depending on the signal properties centimeters) to a significant extent interfere with this signal, is this a microphone cable, a phone line or a LAN cable. Experiments in the 19th century, telephone calls unbalanced ( eg with only one line and with the earth as a counterpoint ) to transfer were unsuccessful. After just a few kilometers disorders are stronger than the desired signal, since the disturbances differently to affect a conductor wire and the other conductor earth.
An alternating electric field can be screened in accordance with the principle of the Faraday cage by a conducting surface (such as a metal foil ). A ( low frequency ) alternating magnetic field, however, is virtually impossible to shield, it penetrates even metallic conductors and thus induces in each line yet so shielded. Electromagnetic interference from motors, transformers, and alternating current lines produce frequency proportional to a rectified induced voltage in each wire. Capacitive effects add a proportional frequency ( displacement ) current on each wire. Shielding foil or braid covers only reduce capacitive interference. There is thus no means to transmit a single signal undisturbed. Even a distant receiver is on a completely different may change the potential field. Even if a signal depended undisturbed, the reference would generate a different mass - exchange potential there an interfering signal.
A solution of the problem is not only to transmit a signal, but to transmit an identical reference signal of opposite polarity or a zero signal. Applying the signal and a reference signal, and we construct the cable so that mint identical on both wires all inductive and capacitive interference, then a difference between the two signals eliminate the interference signal exactly. We call this differential or balanced signal transmission.
To impose disturbances on both wires are identical, one uses symmetric cables with twisted wires ( stranding) and mostly still an electrically shielding shell. If many pairs of wires in a cable ( phone technology ), then different drill the different pairs proves to be the most appropriate means to symmetrize reciprocal signal coupling and minimize it. Since it is induced voltages and currents impressed with the interference, identical in-coupling and termination impedances of both wires a precise, symmetrical coupling of disturbances are needed.
Symmetrical routing is the professional sound engineer today used almost exclusively ( XLR connector ); also people are now working in digital technology with balanced cable guides. So are USB and Ethernet modern differential. Tauchspulmikrofone with their symmetrical connection were a century in audio engineering popular, although they had a frequency response which could in no way be referred to as linear. In the electrical measurement technology and hi-fi applications unbalanced signal guides ( RCA, BNC) are primarily for manufacturing reasons, cost reasons and because of their mechanical robustness also preferred.
Theory
Differential transmission
A balanced signal to be transmitted may bear the function of time at the location of the source. The reference signal may have negated time function ( differential transmission). On the way between source and destination may both signals identical for both signals interfering time function act. The original signal would be transferred to line A and the reference signal on line B.
Line A then transmits at the receiver, the time function, while line B is replaced by the time function at the receiver. If we form the difference between the two signals at the receiving
It can be seen that the fault has disappeared. We get for twice the signal amplitude at the receiver. The latter leads - as a side effect - also in an increase in Signal-/Rauschabstands ( by 6 dB ).
Pseudo Differential transmission
If in place of the reference signal, a zero signal transmitted as a function of time (ie, ), then one speaks of a pseudo- differential transmission. When the difference of the two signals at the destination results in this case:
Here, then, is created at the receiver, only the simple signal amplitude, however, the fault is in this case completely disappeared.
Of course, no real existing line impedances were considered in this derivation that lead to an attenuation and distortion of phase and frequency response of both wires.
Application
The differential signal transmission is used for example as follows:
Differential transmission
Signal wire and the reference signal wire transfer the useful signal with opposite polarity. Same source and terminating impedances of both wires and high symmetry of the line allow an almost ideal interference-free signal transmission. Around 1970 she handed in the form of the analog telephone network around the world.
One finds the principle among others, telephone lines, microphone and USB cables, as well as for twisted- pair cables for Ethernet connections. To obtain a symmetrical as possible noise coupling, each pair is also twisted.
Pseudo - differential transmission
In the pseudo-differential transmission saves the effort to make a negated signal, and instead transfers the reference potential as a zero signal ( see above). The reference signal wire on the receiver delivers only the " collected " from interference, but transmits a matter of no useful signal. We follow the procedures in areas where a form of the negated signal would be too costly. This is carried within the shroud with the ground as a reference signal.
When you remove identical source and terminating impedances of both wires, and plug, BNC or RCA connectors can be used for transmission. The main advantage of the balanced transmission, namely the supply of the receiver signal and reference wire and the considerable associated improvement of the signal transmission quality over conventional management remains.
Components
A subtraction of time-dependent signals can be achieved in principle by various means. Will transfer about an isolating transformer ( transformer ), only the difference of the voltage applied to the primary winding voltage signal. By means of modern microelectronics you can also use a subtracting amplifier for generating the difference, if isolation is not required ( home automation, sound engineering, computer technology in the house).
In the sound to be analyzed, depending on the application between the use of a transformer, which allows passive circuits, but a non-ideal frequency response shows, and active circuits with a differential amplifier from.
- Transformers: transformers or transformers are used where insulation is required. They can be found for example as a so-called LAN Magnetics when Ethernet.
- Subtracting amplifier: operational amplifier are preferred when linear frequency response is required ( analog technology, sound engineering ). Although better results than with a balanced pseudo- differential transmission can be achieved with insulation, but the effort is high. Thus, the subtracting amplifier special applications shall be reserved, for example, bring the protective earth. The principle of the subtracting is simple: the reference signal is negated by the operational amplifier and the following added to the signal. A single, negating the operational amplifier and two resistors for adding the signals are required in the simplest form.
Difference principle in digital technology
In the high-frequency technology (USB, Ethernet, TV broadcasting ) comes to the problem of differential transmission is still a problem added. Each cable has a defined characteristic impedance, which is in the order between 50 and 300 Ω. Turnout source or load impedance of the wave impedance from, it comes at this point to signal reflections ( known example of ghost images in the television).
So impedances are required in the field of wave impedance for extremely high data rates or transmission frequencies. These result in the usual signal voltages to high power losses of the signal driving the gate.
Thus, techniques have been established to make do with minimal signal swing ( equal to the lowest power dissipation). This inevitably differential transmission method.
The line is terminated with the characteristic impedance; that is, the differential transmitting wires are connected to a resistor, the value of twice the characteristic impedance equivalent (see Theory ).
Because of the low impedance termination resistor, the transmission gate must be able to provide a high output current. Such gates are called line drivers or buffers. As a recipient Schmitt trigger gates are usually used to increase noise immunity.
Since unbalanced structure, however, is relatively sensitive to external disturbances ( on the ground line ), the symmetrical structure is preferred. The second line is supplied with the complementary signal and uses a differential amplifier comparator as a receiver. An external disturbance affects both lines equally and causes a Gleichtaktaussteuerung, which is filtered by the difference in the comparator.
The complementary signals must have no time lag, which is why (ICs ) are used in practice, on the one hand with complementary output devices and on the other dimensions the length of the two lines exactly the same.
Complementary outputs are provided in circuits in ECL technology, a priori, and therefore no special blocks must be used. ECL blocks therefore particularly suitable for balanced data transmission. Comparators are used at the receiver with the ECL -compatible output, will be referred to as a line receiver.
In the modern digital technology (USB, Ethernet, RS485) ECL blocks were displaced by differentially working CMOS logic gate bipolar output stage ( BiCMOS ).