If I am using a Samtec ECUE-12-XXX-T1-FF-XX-1-XX FireFly™ cable assembly, can I run auxiliary power through the twinax?
Auxiliary power is usually run through the PCB. The objective of a Flyover® cable assembly (such as the FireFly™ products) is to take the data off of the PCB to lessen the losses associated with the PCB and run the low-speed signals and power through the PCB. It is best practice to avoid running power through the twinax cables. If power must be run though the twinax cables, the power circuits must be very well bypassed and filtered at both ends. For more assistance, contact PowerIntegrity@samtec.com.
Even if the power circuit is well bypassed and the current does not exceed the current capacity of the chosen conductor(s), the voltage drop has to be carefully checked. The TTF-36100 micro twinax cable specification says that the center conductor has approximately half an Ohm resistance for each foot length. The shield resistance is approximately three times lower. So for each foot of length and for each ampere of current, the voltage drop is 500 mV along the center conductor and 150 mV along the shield.
The relatively high resistance of the shield that comes from the very small size means that the twinax cables lose shielding effectiveness up to higher frequencies (as opposed to bigger coax cables with heavy braid). As such, there is strong interaction among the twinax conductors below about one MHz, so if one carries power, any residual power noise at low frequencies will induce common-mode noise on the high-speed signals.
I am considering using a Samtec UDX6 connector. It has a block of high-speed pins and power blades in the same housing. How should I assign the power blades to different nets, what are the considerations?
In addition to the usual signal-integrity considerations, there are two potential interactions to consider related to the power blades: (1) how the pin and blade assignment impacts the power delivery and (2) signal-to-power crosstalk. For power delivery, the DC resistance (which determines the current carrying capacity) and the loop inductance matter most. The resistance also varies slightly with pin assignment. However, the inductance, which influences the transient response of the power path, will change significantly with blade assignment. When the inductance of the power delivery path matters for the application, we usually want to minimize it. The loop inductance formed by the blades is directly proportional to the size of the loop.
If the goal is to minimize loop inductance, we need to assign the power and power-return (or 'GND') blades in a pattern that forms the smallest possible loop size. This can be achieved by selecting adjacent blades for power and ground.
The crosstalk between power blades and high-speed signal pins also depends on the geometry of interacting power and signal loops. The interaction can be minimized by making the loops smaller, by placing them farther apart, or by forming orthogonal loops so that the magnetic coupling is minimized. If, in addition, we can also create the two loops such that the capacitive coupling also cancels, it will minimize the full electromagnetic coupling.
With multiple signal pins and power blades in the connector, there is no universal way to make use of crosstalk cancellation by orthogonal current loops, so we need to focus on minimizing loop size and placing them as far apart as we can.
In the high-speed block of pins, assigning signal and ground pins adjacent to each other is the accepted norm. In the power-blade section, assigning power and ground to adjacent blades will provide two benefits: as we saw above, it minimizes the power delivery inductance and it minimizes the crosstalk between power blades and signal pins.
And one last detail: the above considerations looked at circuit loops, when the signal, whether it is the high-speed signal or power noise, is considered differentially between the pins or blades. Sometimes common-mode noise is also important to minimize. It can be achieved by placing the pins and blades that are 'quieter' with respect to the ambient ground structure next to each other.
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