Structural Return Loss in High Frequency and High Speed Systems
Structural return loss is used to quantify reflections in a non-uniform structure.
No manufacturing process is perfect, although with the quality controls put into place by many electronics manufacturers, a process can often come pretty close to perfection. With electronics manufacturing, defects in components can cause a number of signal integrity problems in certain situations. As many devices start to require higher data rates and higher frequencies, some manufacturing imperfections create interesting signal integrity problems that are typically ignored at lower frequencies.
In cable assemblies, connectors, and PCB traces, structural return loss can occur due to manufacturing imperfections and design mistakes. If you can spot design flaws that might produce structural return loss, you can ensure your component or interconnect will work as designed and that signals will not be degraded as they traverse an interconnect.
What is Structural Return Loss?
When a conductor has some non-uniformity, it has some impedance variations. This creates reflections along the length of the conductor, and the magnitude of these reflections can be described in terms of return loss. This type of return loss is called structural return loss. The impedance variations along the length of the conductor are compared to the conductor’s rated or desired impedance when calculating structural return loss.
Structural return loss is common in poorly manufactured coaxial cables, where variations in impedance along the cable length cause signal reflections for a wave travelling along the cable. These impedance variations can arise from variations in conductor thickness, spacing between the shielding, or both.
In a PCB, there are a number of design problems that can produce the same effect. Variations in trace geometry due to length tuning methods, vias along interconnects, and inconsistent return path/trace spacing due to warpage create impedance discontinuities along the length of a transmission line. If an interconnect is not designed correctly, incident signals reflect back to the source from these impedance discontinuities.
Design to Limit Structural Return Loss
In PCB manufacturing, the etching process is precisely controlled and monitored, which generally limits structural return loss due to manufacturing imperfections. Over-etching on the outside of a trace can vary the trace thickness and surface roughness, although return loss in this situation is generally not noticeable at lower frequencies.
Roughness in conductive and resistive elements is one suspected cause of passive intermodulation products, which are generated from frequency modulated signals or from multiple coherent analog waves in a single channel. At high frequencies, the current is confined closer to the rough surface, and nonlinear effects in the trace will cause harmonic generation and intermodulation products.
Vias are one common impedance discontinuity in high speed/high frequency boards
In general, impedance discontinuities and structural return loss will arise if signal paths are not properly designed. Impedance discontinuities may be capacitive or inductive, depending on the geometric variations along a conductor, producing larger structural return loss in specific frequency ranges.
Vias are a common impedance discontinuity and should generally be minimally used in high speed/high frequency interconnects. In addition to length tuning schemes and vias, a transition between different transmission line geometries with different impedance values will produce structural return loss. Compensating for these effects requires simulating and measuring the impedance spectrum of any potential discontinuity to ensure there is no impedance mismatch on an interconnect.
Simulation, Test, and Measurement
As structural return loss is effectively a geometric effect, particularly in PCBs, it cannot be accounted for in schematic simulations unless you precisely model parasitics and other impedance discontinuities and include these in your schematic directly. There are a number of commercial simulation tools available that can be used to extract parasitics from your PCB layout, but including every parasitic in a schematic quickly becomes an intractable task, especially in complex boards. Pre-layout simulations provide huge value in circuit design, but the inability to model the real geometry of a circuit in an IC or PCB layout is the primary reason pre-layout simulations fail to capture most signal integrity problems.
This is where post-layout simulation tools become invaluable. Signal integrity problems like crosstalk and structural return loss can only be accurately modeled with a post-layout simulation tool. These tools take your device layout and use a 2D or 3D field solver to examine signal behavior, typically in the time domain. If you need to move into the frequency domain, you can convert your time domain data into a Fourier spectrum and construct return loss and insertion loss profiles for various portions of your layout. This provides a useful reference once you begin measuring a device under test.
In order to measure structural return loss directly, you’ll need to use a vector network analyzer and perform time domain reflectometry (TDR) measurements. This provides a direct measurement of signal reflection along the length of an interconnect. Getting accurate measurements for the interconnect itself requires de-embedding the S-parameters for any cables and connectors used in the measurement setup. Note that this is the same procedure used for locating fractures or degraded components in installed fiber optic cable links.
Getting a structural return loss spectrum for an electrical interconnect requires sweeping through a range of frequencies in TDR tests and converting the data to return loss measurements using the incident and reflected intensity. Note that you may need to artificially compensate for attenuation if you are examining a lossy interconnect in order to get accurate structural return loss measurements, otherwise your analysis will overestimate return loss at each impedance discontinuity.
Time domain reflectometry measurement with a fiber optic cable
Whether you want to examine structural return loss in a PCB layout or in any other electronic system, you need to use the right PCB design and analysis software. Allegro PCB Designer from Cadence is ideal for creating layouts for your system, and Cadence’s full suite of analysis tools can help you simulate signal behavior in a finished PCB layout.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.