Crosstalk: Systems with multiple analog signal channels, such as broadcast video, have strict requirements for crosstalk, or the presence of an undesired signal from one channel to another channel. This is a major design challenge, because all of the inputs and outputs in a crosspoint system must be brought in close proximity to make the interconnections. In virtually all crosspoint systems, keeping crosstalk to a minimum is critical.
Crosstalk comes from three major mechanisms: an electric field that is coupled by capacitance, a magnetic field coupled by mutual inductance, and current through a common impedance that is shared by two channels. In all cases, crosstalk becomes more difficult to control as the signal frequency increases. Common techniques used to prevent crosstalk consist of circuit shielding and separation, component bypassing, and careful signal routing.
Capacitively coupled crosstalk can occur between two or more conductors. From a circuit standpoint, the crosstalk mechanism looks like capacitive coupling to a resistive load at the input of a crosspoint IC. For low frequencies, the magnitude of the crosstalk is given by:
Crosstalk = 20 log10 [(RSCM) * s]
where RS is the source resistance, CM is the mutual capacitance between the signal circuits, and s is the Laplace transform variable.
Take, for example, two input channels into a crosspoint-switch IC. From this equation, it can be observed that this crosstalk mechanism has a high-pass nature. It also can be minimized by reducing the coupling capacitance of the input circuits. This can be done by using crosspoint devices with a ground or power supply pin between the signal pins to provide shielding against crosstalk. This is a standard pinout in Analog Devices' crosspoint ICs. A pc-board designed for low crosstalk should have the signals routed, as much as possible, on inner layers with ground planes above, below, and in between traces on the signal routing layer.
Crosstalk also can be induced between two conductors, when large drive currents are required in loads such as 150-Ω video applications. From a circuit standpoint, this crosstalk mechanism looks like a transformer with a mutual inductance between the windings that drives a load resistor. For low frequencies, the magnitude of the crosstalk is given by:
Crosstalk = 20 log10 (MXY * s/RL)
where MXY is the mutual inductance between two conductors and RL is the load resistance on the conductors.
By increasing the spacing of the conductors and reducing their parallel length, the effects of inductively coupled crosstalk are minimized. When this isn't possible, or more crosstalk reduction is required, copper shielding of the field lines generated by current in the conductors can be employed. This will block the penetration of these magnetic flux lines by generating localized eddy currents in the shield material, minimizing their coupling to other conductors.
Pc-Board Layout: Extreme care must be exercised to minimize additional crosstalk generated by the system circuit board(s). Areas that must be carefully considered include grounding, shielding, signal routing, and supply bypassing. In component selection, as stated earlier, it's important to choose a crosspoint IC with a ground or power-supply pin between signal pins to provide shielding right up to the IC package. These ground and power-supply pins should be tied to a large plane with as low an impedance return path as possible.
One technique to mitigate the effects of capacitively and inductively induced crosstalk is to use multilayer pc boards for circuit layout. In this case, circuit separation can be accomplished in a vertical direction, as well as horizontally. Furthermore, space is available to route ground planes between signal circuits to lower the effective impedance of these traces, as well as to provide shielding. An example of a four-layer, pc-board layout for signal circuits is shown in Figure 5.
This illustration shows the important dimensions to consider in designing a pc-board signal trace to a characteristic impedance. For example, video applications require a characteristic impedance of 75 Ω. To calculate the dimensions, use this equation:
where ER is the dielectric constant of the pc-board material.
Note that unused regions in the four layers should be filled with ground planes. Then, in addition to having controlled impedances, the input and output traces also will be well shielded.
Signal paths in most systems generally share a common return path or ground. Signal current into this finite impedance path can interact with other signals and cause crosstalk. To avoid this, take care in the layout to minimize the signal-path ground impedance. Doing so will ensure that return-path currents minimally interfere with the signals. Use commonly connect ground planes with the lowest impedance possible to accomplish this.
The power supply also can be a source of crosstalk. When an active device drives a signal channel, the consumed current can create interference reflected back into the power supply. This interference results from current in the impedance of the power-supply circuit. Any channel that shares this power supply will be subjected to this interference signal, which will induce crosstalk. Bypassing the power supplies—as close to the crosspoint devices as possible—with good high-frequency capacitors will help reduce this effect. Typically, a 0.01-µF capacitor is a good choice for high-frequency applications.
Design/Evaluation Tools: Nearly all suppliers of crosspoint ICs provide components mounted on evaluation boards. A good board will minimize the layout and signal routing difficulties discussed above. This allows the designer to fully evaluate the performance of the device itself, with minimal errors due to layout.
Lastly, the vendors' web sites can be a source of additional design information. For example, http://www.analog.com/high-speed-switches contains data sheets, white papers, and applications notes. Such information aids the designer in selecting crosspoint IC components, while helping them avoid deep design pitfalls.