Last month, I commented on one of Bill Schweber's blogs. My remarks were about the (lack of) models for RF connectors.
At that time I remarked:
It is interesting that in the world of passive and active components, it is expected to provide SPICE models and other tools so circuit designers can include them in a higher level simulation. On the other hand, for RF connectors this is not the case. In general, RF connectors are assumed to be 'perfect,' but they frequently are not.
Here I want to expand on the point of what design information you can derive from modeling connectors.
In my past roles in engineering leadership at a few companies, I was a big proponent of simulation in RF systems. There have been steady and ongoing advances in RF simulation software. Companies and products like AWR -- Microwave Office, Comsol -- RF Module, and Ansys -- HFSS, among others, have opened up simulation to many more engineers.
With that, I'm seeing many more problems today than at the start of my career. Over those 30 odd years, the frequency of all systems has increased, both for analog and digital. Today, simulating a 25 Gitb/s digital signal on a backplane requires treating the signal path as a transmission line to understand losses, crosstalk, immunity, and other design factors. On top of that, practically all systems have interconnect, and it isn't remotely possible to treat the connectors as anything but extensions of those transmission lines. That means that connectors can introduce insertion loss from mismatch, and, more worrisome, reflect significant power back up the line into the system.
Most of my work with simulation tools has been in the antenna design domain. Use of RF simulation tools is becoming fairly common in the antenna industry, but even there connectors tend to be overlooked. Extending simulation to include the connectors requires very precise mechanical models, and very accurate material properties for the dielectrics. Many antenna designers ignore connectors in their designs, assuming they are "perfect" in terms of impedance match and discontinuities. At higher frequencies, those assumptions can break down.
I contacted Randy Bancroft of Randwulf Technologies to get some examples for this blog. Bancroft is an IEEE member and professional engineer, as well as past colleague of mine (the latter not being much of a resume point). Bancroft said, "In the past generally connectors were not simulated. The connector was omitted because modeling of physical connectors takes up memory. This often worked well if a connector and its transition to a transmission line had very small impedance discontinuities."
Of course, the interesting cases for me were the ones that didn't work. Bancroft described one design which used an SMA connector (see an example here) to inject the RF signal to a circuit board which also had an antenna implemented in the PCB metal traces. Later, the customer had an application requiring an N connector (see an example here), and the larger impedance discontinuities caused enough mismatch to reduce the bandwidth of the antenna/connector system. A panic of trial and error ensued to find a board design that would work. Accurate simulation could have identified the problem in advance, as well as been used to find a solution.