Based on trace routing and location of the disturbance on the aggressor and victim lines, crosstalk can be classified as:
- Capacitive crosstalk: It arises due to the traces that run on top or near to each other, producing a capacitive effect.
- Inductive crosstalk: It generates due to magnetic field interaction between traces running parallelly over a long distance.
Inductive crosstalk is of two types: forward and backward. Forward is the noise/disturbance observed at the farthest end from the driver on the driven line, while backward crosstalk is the disturbance observed at the nearest end on the victim line.
Near-end crosstalk (NEXT): It is measured at the transmitter end of the transmission line or a cable.
Far-end crosstalk (FEXT): It is measured at the receiver end of the transmission line or a cable.
NEXT and FEXT are measured with respect to the port to which the stimulus is applied. It can occur anywhere along a line, whether it is a dual conductor or single-ended.
Note: The NEXT value is expressed in decibels (dB) and varies with the frequency of transmission. A higher dB of NEXT means less interference.
3. Power sum near-end crosstalk (PSNEXT): It is the sum of the NEXT of three aggressor pairs as it impacts the fourth victim pair. PSNEXT gives total crosstalks from all the adjacent pairs and involves measuring all pair-to-pair groupings relative to power.
4. Equal level far-end crosstalk (ELFEXT): It is the measurement of the FEXT that involves attenuation compensation.
5. Alien crosstalk: It gives the measurement of crosstalk in PCBs for telecom systems. Above mentioned types are the ways of measuring or quantifying crosstalk in a system. Crosstalk can also be measured using a TDR. For more details, read our post on how TDR impedance measurements work.
How to measure crosstalk?
Crosstalk is generally specified as a percentage of the signal that appears on the victim line, relative to the aggressor line. It can also be expressed in terms of dB below the driven line level. NEXT varies with the frequency of the transmission since higher frequencies create more interference. The higher the dB value, the less crosstalk is received by the disturbed link/channel. FEXT is calculated from the crosstalk elements of the system S-parameters.
The formula for crosstalk is given by:
K = A constant whose value always remains less than 1 and depends upon the rise time of the circuit and the length of the traces experiencing crosstalk.
H2 = It is the product of the height of the parallel traces.
D2 = It is the product of the direct distance between the centerline of the traces.
The above equation clearly shows that crosstalk can be minimized by reducing H and maximizing D.
Crosstalk in dB is given by: (1)
Where, V victim is the voltage on the victim line and Vaggressor is the voltage on the aggressor line.
Factors affecting the magnitude of crosstalk
1. Degree of coupling between aggressor and victim lines
2. The distance up to which coupling occurs
3. Effectiveness of the type of termination used