--- title: "Noise and two-wire to four-wire converter" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:noise-two-four-wire-converter" source: https://www.bctengineer.com/q/semester_8/telecommunications/noise-two-four-wire-converter site: BCT Engineering --- # Noise and two-wire to four-wire converter Telecommunications · Chapter 2: Transmission Media - Asked in: 2079 Chaitra, 1+3+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/noise-two-four-wire-converter - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question What is noise? Explain its types in brief. Describe the need and operating mechanism of the two-wire to four-wire converter with a neat diagram. ## Short answer Noise is any unwanted signal degrading quality; types include thermal, intermodulation, crosstalk, impulse, and shot. A two-wire to four-wire converter (hybrid) is needed because amplifiers and multiplexers are unidirectional; it separates directions using a centre-tapped transformer and balance network, preventing echo and singing. ## Answer #### Noise **Noise** is any unwanted electrical signal that adds to the wanted signal in a communication system and reduces its quality (measured by signal-to-noise ratio). #### Types of noise - **Thermal (Johnson) noise:** due to random motion of electrons in conductors; present in all equipment; power $N = kTB$. - **Intermodulation noise:** produced when signals of different frequencies pass through non-linear devices, creating sum and difference frequencies. - **Crosstalk:** coupling of signals from one pair/channel into another (near-end and far-end crosstalk). - **Impulse noise:** short, high-amplitude spikes from lightning, switching contacts, power lines; harmful to data. - **Shot noise** in semiconductor devices; also **quantisation noise** in PCM. #### Need for a two-wire to four-wire converter The subscriber loop is two-wire (both directions on one pair), but amplifiers, repeaters, multiplexers and digital transmission equipment work in one direction only, so long-distance circuits are four-wire. A **hybrid (2W/4W converter)** at each end separates the two directions and keeps the received signal out of the transmit path, preventing echo and singing. ```text West East 2-wire ┌──────┐ Tx pair ──> (amp) ┌──────┐ 2-wire ────────┤Hybrid├──────────────────────> ┤Hybrid├──────── sub A │ H1 │ <──────────────────────┤ H2 │ sub B └──┬───┘ Rx pair <── (amp) └──┬───┘ Z_B Z_B ``` #### Operating mechanism ```text 4-wire receive (from far end) │ (amplifier) v ┌──────────────────────┐ 2-wire ────┤ T1 T2 │ line ─────┤ (hybrid transformer)│ (Z_L) │ centre-tapped │ │ windings │ Balance ────┤ ├───> 4-wire transmit network Z_B └──────────────────────┘ (to far end) ``` **Operation of the hybrid (bridge principle):** 1. **Receive → 2-wire:** a signal arriving on the 4-wire receive pair is fed to the centre-tapped winding. It divides equally between the **2-wire line ($Z_L$)** and the **balance network ($Z_B$)**. Half the power reaches the line (3 dB loss). 2. **No leakage to transmit pair:** the two halves flow in opposite senses through the windings coupled to the transmit pair, so their magnetic effects cancel when $Z_B = Z_L$. Ideally no receive signal appears on the transmit side. 3. **2-wire → transmit:** speech from the 2-wire line divides between the transmit and receive ports; the transmit port gets half (3 dB loss), and the part going to the receive port is absorbed by the amplifier output and does no harm. 4. **Balance:** in practice $Z_B$ only approximates $Z_L$ over the band, so a small signal leaks across. This is measured by the **balance return loss** $$ B = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB} $$ Poor balance causes **echo**, and if the loop gain round the four-wire circuit exceeds unity, **singing** (oscillation). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is noise in a communication system? A. Any distortion of signal amplitude with frequency B. The delay between transmission and reception C. Any unwanted electrical signal added to the wanted signal that lowers the SNR D. The loss of power along a cable Answer: C. Noise is unwanted signal added to the wanted one, reducing quality as measured by signal-to-noise ratio. (Key point: Definition of noise) 2. Which noise is caused by signals of different frequencies passing through a non-linear device? A. Thermal noise B. Intermodulation noise C. Shot noise D. Impulse noise Answer: B. Non-linearity creates sum and difference frequencies, which is intermodulation noise. (Key point: Noise types) 3. Why are long-distance circuits made four-wire although the subscriber loop is two-wire? A. Four-wire cable is cheaper than two-wire B. Amplifiers, repeaters and multiplexers work in one direction only, so each direction needs its own path C. Two-wire lines cannot carry speech D. Four-wire reduces thermal noise to zero Answer: B. One-way equipment requires separate go and return paths; the hybrid joins them to the two-wire loop. (Key point: Need for 2W/4W) 4. Why does a signal from the 4-wire receive pair not appear on the transmit pair of a balanced hybrid? A. The amplifier on the receive pair blocks it B. Its two halves flow in opposite senses through the windings coupled to the transmit pair, so their fluxes cancel when Z_B = Z_L C. It is filtered out because it uses a different frequency band D. The transmit pair is shorted by the balance network Answer: B. The signal splits equally into Z_L and Z_B; with Z_B = Z_L the opposing magnetic effects cancel in the transmit winding. 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