Balanced / Differential Transmission Line : Why and How It Works.

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In modern electronics, many classic technologies remain relevant and are still widely used. One of them is the signal transmission topology known as a Balanced or Differential Transmission Line (DTL).

In essence, DTL is a method of signal transmission through a cable consisting of two separate signals. These two signals have opposite polarities: one is non-inverted and the other is inverted . On the receiving end, these two differential signals are received by a differential receiver which has two inputs. This receiver then produces an output that is the result of subtracting the inverted input from the non-inverted input.


Why Do We Use Differential Transmission?

The primary purpose of DTL is to eliminate or minimize noise that may be induced into the transmission line along the way. Without balanced transmission, noise interference will appear at the signal's receiving end, degrading the quality of the data or audio.

In addition to the transmitter and receiver needing to be of a differential topology, another crucial requirement for achieving maximum benefit is that the transmission line MUST be constructed as a TWISTED PAIR (the wires are twisted together) as shown in Figure 1.12.

There are two main advantages to using twisted wires:

  1. External Noise Rejection: If an external interference is induced onto the two wires, both will acquire an induced signal with the same or very similar level and phase. Because the receiver subtracts the two signals, the incoming noise is canceled out, resulting in a clean output signal.

  2. Radiation and Eavesdropping Prevention: Since the two wires carry currents with opposite phases, the electromagnetic fields leaking from each wire also have opposite phases. As a result, these fields cancel each other out. This is essential for meeting noise level standards and, in military technology, can minimize the probability of data eavesdropping.

  3. For optimal performance, a twisted pair cable must also meet other technical requirements, such as the number of twists (around 5 to 10 twists per meter). We will discuss this aspect another time.


Fig.1 - Balanced / Differential Transmission Line

Figure 1 provides a simple illustration of how DTL works.

  • The input signal (1) is transmitted in two polarities by drivers (2) and (3) through the differential line (6).
  • In the middle of the line, we assume there is an induced noise (7).
  • Because the two wire cores are very close, the level and polarity of the induction on both are the same.
  • The noise-contaminated signals (8) and (9) are then processed by the differential receiver (10).
  • The result is a clean replica of the input signal, Vout​, free from the noise picked up along the way.

Fig 2. Results of LTSpice Simulation


Figure 2 shows the results of a simulation using LTSpice.

  • VIN : The signal being transmitted.

  • VNS : A random noise signal that successfully induces both differential lines.

  • VLP : The contaminated non-inverted signal.

  • VLN : The contaminated inverted signal.

  • VOU : The form of the output signal if the differential cable is not twisted, but just a parallel pair. You can see there is still some noise riding on the signal, though it's relatively small.

  • VOB : The form of the output signal when a good twisted pair cable is used. The received signal is truly clean from noise, even with a strong disturbance along the way.


Applications of Differential Transmission

In the industry, DTL is widely used in:

  • Professional microphone cables.

  • Interconnection cables between audio gear, both in sound reinforcement and for audiophiles.

  • Various sensors in factory machines.

  • Data communication devices like servers and gate devices.

  • Weapon control systems and fly-by-wire controls in airplanes.

Differential Transmission Lines can be used for both analog and digital signals. While modern technology offers wireless or fiber optic transmission, DTL is often the preferred choice because it provides a more reliable, affordable, and straightforward solution for specific requirements.

I hope these notes are helpful for practitioners and especially for beginners.

TABIK !



(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)

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