What Determines Connector Return Loss and Insertion Loss?

October 28, 2019 Cadence PCB Solutions

Green PCB connectors

These connectors will create some return loss and insertion loss when used with high speed or high frequency signals.

 

When introducing friends from unique friend groups to each other, I love to try and play the degrees of separation game with them. Whether it’s finding a coffee shop they both go to, or knowing that they’re both friends with another individual, it’s fun to try and find the connectors between new relationships. I find it fortunate that not every friend belongs to the same hobby or interest. 

Electronics in complicated systems, too, are not always confined to a single board and will require some connectors. Conceptually, this allows an interconnect to span between multiple boards, but improperly designed multi-board interconnects incur some connector return loss and insertion loss. 

Connector Return Loss vs. Insertion Loss

A connector is one of many transmission line elements in a signal path between a transmitter and receiver in a mutli-board system. When discussing interconnects, most designers tend to focus on vias as an inductive impedance discontinuity. In other words, a via in an interconnect acts like an inductor at high frequencies and can create some reflection if not designed properly. However, connectors also behave very similar to their own short transmission lines. They also have their own impedance that determines how signals interact with the connector.

As much as we would like connectors to be perfect, the fact remains that they can create some return and insertion loss when placed in an interconnect. The primary mechanism that leads to return loss and insertion loss at a connector is an impedance mismatch caused by surface mount pads on a connector. Through hole connectors also incur some insertion loss as the pins act like inductive/capacitive impedance discontinuities (more on this below).

Once a signal reaches an SMD connector pad with wider copper, the per length capacitance in this segment of the trace is larger, which then decreases the characteristic impedance of the trace seen at the connector pad. This capacitive impedance discontinuity causes signal reflection and leads to connector return loss and insertion loss.

One way to eliminate this problem is to use a second small ground layer below the connector (i.e., in the third layer). Placing a cutout in the ground plane below the signal layer that is the same size as the connector pad forces the trace to reference to the ground plane in the third layer. This increases the capacitance and helps reduce the capacitive impedance discontinuity, which decreases the connector return loss.

 

Reducing connector return loss and insertion loss

Cutout region in a PCB connector to reduce connector return loss and insertion loss

 

Using just the right cutout size will minimize the impedance mismatch between the trace and the connector. Aside from this simple design choice, you may need to design an impedance matching network for your connector. One example is the use of magnetics for terminating RJ45 connectors in MII/RMII routing.

At high frequencies/data rates (i.e., 14 GHz clock rates used in high speed backplanes), there are other sources of losses in interconnects, including skin effect and weave effect losses. Losses due to roughness in such high speed interconnects is one reason why connectors and cables are often used in high speed networking equipment.

Other Signal Integrity Problems with Through Hole Connectors

Through hole connectors can behave like via stubs and can resonate at certain frequencies. The pin on a through hole connector will have a set of resonance frequencies at which the electromagnetic field in the signal can form a standing wave, which will radiate strongly. The equation below shows the approximate resonance frequencies along the axis of the pin:

 

Through hole connector pin resonance frequency equation

Resonance frequencies of the pin on a through hole connector

 

Note that resonance will occur at these frequencies even if the connector is impedance matched to its interconnect. If the connector is not impedance matched, resonance will still occur and these frequencies will be slightly different; the frequencies will be shifted to lower values and will arise at even multiples of the lowest order harmonic.

There are also resonances that occur along the radial direction at even higher frequencies, although transcendental equation for determining these frequencies is more difficult to evaluate. These radial resonance frequencies are not related to each other by integer or rational number multiples; instead, they are determined by solving a transcendental equation involving cylindrical Bessel and Hankel functions along the wall of the pin. To get a rough estimate of the lowest order radial resonance frequency, just multiply the lowest order axial frequency (found from the above equation) by the aspect ratio of the connector pin.

Note that multiple pins in a through hole connector also act like a set of capacitors in series. Because of the short spacing between pins, as well as their cylindrical geometry, there is strong capacitive and inductive coupling between the pins. The capacitive nature of the pins will dominate at higher frequencies, leading to strong crosstalk between pins.

This issue with resonance and coupling, even in an impedance matched connector, is one reason why surface mount connectors are preferable at high frequencies. Coupling between pins is one reason why the pin arrangement on a high speed or high frequency connector should be arranged with return paths between signal pins. A high speed serial signal pin arrangement is also useful for preventing coupling as this mimics differential signalling, where each inverting/non-inverting signal pair is separated by a ground pin.

 

Pin arrangement on through hole connector

Example through hole pin arrangement for high speed serial signal pairs

 

Adding connectors to a circuit takes the right mechanical CAD tools, but you can also analyze connector return loss and insertion loss with the right PCB design and analysis software package. Allegro PCB Designer and Cadence’s full suite of analysis tools make it easy to perform important signal integrity simulations with connectors in your circuits, offering a comprehensive view of their behavior.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

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