The venerable RJ45 connector
The 8-position modular connector has evolved significantly since the early days of Category 5.
January 1, 2011
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I have been involved in cabling standards development for over 20 years. In that timeframe, it never ceases to amaze me how much progress has been achieved in the evolution of the eight-position modular connector.
The 8P8C (8 position 8 contact) modular connector is the ubiquitous outlet/connector interface for telecommunications services worldwide with performance specifications extending up to 500 MHz. That is pretty amazing for a connector that was originally specified for low frequency applications below 3 MHz.
What are the performance specifications for a modular plug/jack connection? The performance specifications are shown in Table 1.
I remember the early days of cabling standards development when it was discovered that the crosstalk of a single uncompensated plug/jack connection could be as much as the crosstalk in a 100 meter length of Category 5 cable.
The culprit was the inherent crosstalk between the blades of an 8-position modular plug, especially for the split pair combination between positions 3-6 and 4-5. This started the ball rolling for the connector industry to develop different techniques to compensate (cancel out) the inherent crosstalk in a modular plug. If we look at the NEXT performance of a mated plug/jack connection at 100 MHz from Table 1, a Category 5e connector achieves about 10 dB of crosstalk compensation and a Category 6 connector achieves about 20 dB of crosstalk compensation in the worst case for the full range of specified plugs.
Not all compensation techniques are created equal. The methods of compensation for a modular jack are accomplished by the judicious routing of traces on a printed circuit board (PCB) or by the judicious layout of conductors on a lead frame design.
The designs can get pretty complex and the best designs provide a high Return Loss (minimum signal reflections) and a gradual roll off in crosstalk performance at high frequencies.
The distance between where the crosstalk occurs (in the plug) and where the compensation occurs on the PCB in the jack has a significant effect on the high frequency performance.
For single stage compensation, a physical distance of less than 5 mm between the crosstalk source (in the plug) and where the crosstalk signal is compensated on the circuit board (in the jack) can degrade the crosstalk performance by 3 to 6 dB at 500 MHz. Finally, I am only touching on some of the well known parameters that are specified in the standard.
A good design also ensures that a balanced (differential mode) signal that travels through a connector does not become unbalanced and generate common mode signals on the same pair or on other pairs that share the same physical space within a connector. This phenomenon is called mode conversion. Mode conversion can result in additional alien crosstalk between channels in close proximity and can also affect the electromagnetic compatibility (EMC) performance, along with some other factors.
As you can see the 8-position modular connector has evolved significantly since the early days of Category 5.
The connecting hardware experts in TIA TR 42.7 subcommittee are developing the test methods to measure these new “mode conversion” parameters at frequencies up to 1 GHz using specialized balunless test fixtures. It is the beginning of a new era in connector development in support of future applications beyond 10 Gb/s. CNS