GLENAIR
IMPEDANCE MATCHING: Electrical impedance, or simply “impedance,” describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle. How Impedance is Measured: A Time Domain Reflectometer (TDR) transmits a short rise time pulse along the conductor. If the conductor is of a uniform impedance and is properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. Increases in the impedance create a reflection that reinforces the original pulse, while decreases in the impedance create a reflection that opposes the original pulse. The resulting reflected pulse is displayed or plotted as a function of time, and because the speed of signal propagation is almost constant for a given transmission medium, it can be read as a function of cable length. Impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source in order to get maximum power transfer from source to load. When we maximize the contact density in a connector, the conductors invariably come into closer proximity. This decreases the impedance between them. The relative Impedance can be thought of as the amount of electrical coupling there is between the conductors. Most protocols covered in digital signal transmission, require an impedance in the range of 75 to 120 Ohm. In a micro-D connector, the impedance between two adjacent contacts is on the order of 30 Ohm. In order to increase this value, we must either reduce the dielectric constant of the inserts (the best we can typically achieve is with Teflon which has a dielectric
constant of 2.2. Air has a dielectric constant of 1, (but of course using air as the dielectric in a connector is not mechanically possible). Alternatively, we can increase the spacing between contacts (sacrificing the connector’s high density). A third approach would be to maintain the same contact-to-contact spacing, but use smaller contacts overall (resulting in a lower- amperage connector). In all three cases the goal remains the same: that of maintaining a high contact density count while maximizing power transfer from source to load.
Impedance testing of differential contact pairs in multipin connectors makes allowances for ground pin separation of signal pairs. This test board has been wired for a Ground-Signal-Ground-Signal-Ground (GSGSG) method of impedance testing.
TDR testing is used to find the best balance between connector density and return loss. Depending on the application link budget, decisions can be made on which arrangements of grounds to signal contacts is optimal for a given dB return loss and frequency.
QwikConnect • July 2020
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