Balance

To maximise the benefits of differential signalling for both signal integrity and EMC, it is vital to maintain its "balance" along the entire signal path. A perfectly balanced signal would have the following characteristics:

equal impedances from each side of the differential signal pair to ground

equal but opposite + and – amplitude waveforms

rising and falling edges would also be equal and opposite at the same time.

For external signal cable transmission the use of twisted pair is pretty much mandatory, to maintain the balanced impedances when the cable passes through an uncontrolled environment.

Any "skew" in the timing converts to a common mode signal over and above that which might be due to other imbalances or other sources: a differential timing skew of X% of a signal's rise or fall time converts to a peak common mode voltage that is (X/2)% of the differential signal voltage. Because this skew-generated CM voltage occurs at the transitions, its spectrum is concentrated in the higher multiples of the fundamental signal clock rate.

Skew can be due to a variety of sources, some related to the driver and some to the lines on a PCB or in a cable:

unequal stray coupling to the tracks (C1/C2): when a differential pair passes across a PCB, one side of the pair will pass closer to or further away from various features on the PCB - other tracks, components, metalwork, ground plane slots and so forth. The sum of these variations between the two sides is very unlikely to be zero, but can be reduced by segregating the pair from such features, or arranging that each side passes by equivalent features by the same distance.

differences in trace widths (w₁/w₂), process-related, accidental or even deliberate.

propagation time differences from end to end, caused by line length (L₁/L₂) or propagation velocity differences.

Driver-related sources include timing asymmetry (t₁/t₂) and different output impedances (Z1/Z2) both between the halves of the pair and between pull-up and pull-down.

Close-coupled differential transmission lines are the best solution if a signal has to leave its reference plane, for example when exiting to the outside world via a connector, or when connecting to a different board in the same product, or when transiting to a different reference plane in the same PCB. The purpose is to restrict the common mode current that would otherwise attempt to flow back to the originating reference plane, via a potentially uncontrolled path. Close-coupled differential pair transmission lines reduce the common mode currents on a ground plane, but a common mode choke is beneficial when leaving a ground plane.

Longitudinal Conversion Loss

Unshielded data interfaces must have good common mode rejection, which will mean ensuring that the physical layout is balanced and, usually, incorporating a wideband common mode choke. It will also be necessary to specify the Longitudinal Conversion Loss (LCL) of allowable connected cables – typically by restricting them to one or other of the IEC 11801 categories (Cat 3, 5 or 6).

The LCL of a balanced cable system – or indeed any one- or two-port network – is a measure of the mode conversion exhibited by the system, that is the degree to which an inadequately balanced termination will develop an unwanted transverse (differential) signal when excited by a longitudinal (common mode) signal. It is measured as shown here. Although this diagram shows a differential mode signal generated by a common mode input, the principle is reciprocal and can be used to describe unwanted common mode signals developed by intended differential signals.

Ethernet

The most widespread example of a balanced data interface is Ethernet.

The performance of unscreened wideband data systems is very much determined by the balance of both the cable and the interface. The better the balance, the higher the LCL and so the less the conversion from differential to common mode current. Various features at the interface will help to maintain this balance in the transition to the external cable. These features are:

DC and LF isolation using a transformer: the active interface circuitry is galvanically isolated and doesn’t need substantial direct protection from overvoltages and other disturbances, but equally importantly, the transformer allows cable balance to be achieved without reference to the internal circuits, provided that the transformer itself is designed for this

enhancement of signal balance using a common mode choke: a choke which is effective in the VHF range and above complements the low frequency balance offered by the transformer and maintains the balance to much higher frequencies, without affecting the wanted signal bandwidth

control of the interface common mode impedance: a resistive common mode termination matches the cable and prevents it from resonating along its length

The Ethernet specification is a good example of how this is achieved. The magnetic components are normally integrated together in one package, and may also be integrated into the RJ45 connector. This removes the risk of poor PCB layout on the outside of the transformer compromising the inherently good balance of the basic circuit.

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