--- title: "Crosstalk causes and hybrid transformer" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:crosstalk-hybrid-transformer" source: https://www.bctengineer.com/q/semester_8/telecommunications/crosstalk-hybrid-transformer site: BCT Engineering --- # Crosstalk causes and hybrid transformer Telecommunications · Chapter 2: Transmission Media - Asked in: 2075 Asoj, 3+5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/crosstalk-hybrid-transformer - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question What are the causes of cross talk? Explain the operation of two wires to four wire hybrid transformer. ## Short answer Crosstalk arises from capacitive and inductive coupling between adjacent conductors, unbalanced pairs, long parallel runs, and poor filtering. A hybrid transformer uses a centre-tapped winding and balance network. Incoming signal splits to line and balance; when balanced, no signal appears at the transmit port. Outgoing speech divides to line and transmit, providing 2W/4W conversion. ## Answer #### Causes of crosstalk **Crosstalk** is the unwanted coupling of a signal from one circuit (disturbing) into another (disturbed), so that a conversation on one line is heard on another. - **Capacitive (electric) coupling** between adjacent conductors in the same cable. - **Inductive (magnetic) coupling** between pairs due to mutual inductance. - **Unbalance of pairs** – unequal resistance, capacitance to earth, or poor twisting, so coupled currents do not cancel. - **Long parallel runs** of pairs and close spacing; higher frequencies increase coupling. - **Non-linearity and poor filtering** in FDM/carrier systems (inter-channel crosstalk). - Faulty joints, moisture in cables and poor shielding. Types: **near-end crosstalk (NEXT)** – measured at the same end as the disturbing transmitter; **far-end crosstalk (FEXT)** – at the far end. Reduced by twisting, shielding, balanced lines and separation of go/return pairs. #### Operation of the two-wire to four-wire hybrid transformer ```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). **Use:** at the junction of the two-wire subscriber loop and the four-wire trunk at both ends of a long-distance call, so that one-way amplifiers and digital equipment can be used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are the main coupling causes of crosstalk? A. Capacitive and inductive coupling, aggravated by pair unbalance and long parallel runs B. Skin effect and radiation only C. Thermal motion of electrons D. Lightning only Answer: A. Electric and magnetic coupling between pairs, worsened by unbalance, parallel runs and high frequency. (Key point: Crosstalk causes) 2. Near-end crosstalk (NEXT) is measured: A. At the far end from the disturbing transmitter B. In the middle of the cable C. Only on fibre D. At the same end as the disturbing transmitter Answer: D. NEXT is at the transmitter end; FEXT is at the far end. (Key point: NEXT vs FEXT) 3. 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. (Key point: Why no leakage) 4. Where is the 2-wire to 4-wire hybrid used? A. Inside the telephone handset only B. At the junction of the subscriber loop and the four-wire trunk at both ends of a long call C. Between two fibre cables D. At every repeater of a coaxial cable Answer: B. It lets one-way amplifiers and digital equipment be used on the trunk. 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