This blog from Samtec discusses why 50 Ohms coax transmission path isn’t really 50 Ohms and why return loss or VSWR may be higher than expected.
This blog is a summary of a seminar by Samtec’s Michael Griesi, presented at IMS 2023. For those of you who weren’t able to attend, a video of the full presentation is above. In the meantime, here’s an overview of what Michael discussed:
Standardized system impedance allows engineers to democratize the design of a full transmission line channel and integrate components from multiple vendors. Perhaps the most common standard impedance, particularly for single-ended transmission lines, is 50 Ohms.
The reasonable assumption is therefore, provided each section of the transmission path shares the same 50 Ohm characteristic impedance, electrical reflections could be minimized, supporting maximum transmission of the propagating signals. Yet, RF connector selection and integration into a system can have a devastating impact on system performance, leaving engineers scratching their heads with this deceptive question:
If every section of my transmission path is 50 Ohms, why is the Return Loss or VSWR so much higher than expected?
In practice, maintaining 50 Ohms throughout every section of the channel is more nuanced than it may appear, particularly as frequency increases and with close attention to the interfaces between these sections. One of the fundamental tenets of both designing and integrating RF interconnects is minimizing discontinuities in an effort to maximize performance, but this is not the only factor that can impact performance and limit operational bandwidth.
The previously mentioned nuance generally falls into two different responsibility buckets:
- Each individual component will have its own maximum bandwidth and performance, which might not be the expected bandwidth based solely on the series type or size. This simply means it’s the responsibility of the component design engineers, which is to be expected when purchasing a component or interconnect from a vendor.
- Unfortunately, simply connecting 50 Ohms components into a system as shown in Figure 1 does not guarantee a reflection-free channel. This implies that component or interconnect integration is the responsibility of the engineer integrating the component into their system, which may be unexpected and even challenging.
Samtec utilizes as many simulation methods as possible to design, optimize, characterize, and correlate models with real-world measurements. As you might suspect, this can sometimes require us to use different simulation types within a single physics, such as HFSS Driven Terminal, Driven Modal, Transient, Eigenmode, and others. This can also require us to use different simulation tools that span different physics, in order to both predict and explain observed electrical behavior that may have been caused by something non-electrical, such as mechanical deformation or solder flow.
As great as that may be, keep in mind that is only one of the two different responsibility buckets. To address both responsibilities, Samtec also strives to help customers integrate connectors into their system design by sharing models like encrypted HFSS or Icepak 3D Components, reference characterization PCB designs, as well as other models and support for custom PCB launch or breakout region designs.
