Crosstalk causes and hybrid transformer

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

                    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.