Understanding RF Reflection

Key Takeaways

• Find out what RF reflection is.

• Learn how RF reflection affects signal integrity.

• Explore ways to suppress RF reflection.

RF reflection occurs when there’s a discontinuity in characteristic impedance, functioning similarly to how a soundwave is reflected

Children are a curious lot. My young son is so fascinated by his voice echoing in a long empty corridor. It’s hard to explain to him how soundwaves bounce back at the end of the corridor and result in an echo. I tried to demonstrate this phenomenon to him by manually generating waves in a pail of water and showing him that they were eventually reflected.

During this visual demonstration, I was reminded that reflection isn’t confined to sound and water. While reflections are fascinating to my son, they can be a source of frustration to engineers who have to deal with RF reflection in electronics. In this article, we will discuss what RF reflection is, how it impacts signal integrity, and ways to suppress it.

What is RF Reflection?

When designing low-speed signals, engineers are mostly concerned with how signals are moving from the source to the destination. These concerns change when working with signals that are in the regions of hundreds of MHz or GHz. At such high frequencies, engineers need to start worrying about signals reflecting back onto the trace. 

RF reflection functions similarly to how sound is bounced back and creates an echo. It’s also analogous to waves of water crashing and bouncing back. The reflection occurs because the wave encounters a discontinuity of the medium that it’s traveling, and that’s the same for reflected RF waves.

RF reflection must be considered when discussing transmission lines. One of the basic principles of transmission lines is to ensure that the characteristic impedance of the driver, traces, and load are matched. Characteristic impedance is not defined by the resistance or the trace length but rather determined by the dielectric, trace width, and the separation between the trace and the plane. 

In order for a driver to fully transfer the RF signal to the load, the characteristic impedance must be equal and matched along the transmission line. Any discontinuity in the impedance will result in the RF wave bouncing back to the driver. This is the phenomenon known as RF reflection.

How Does RF Reflection Affect Signal Integrity?

RF reflection results in attenuation and interference

Signals can be reflected whenever there is a mismatch in characteristic impedance. However, it rarely causes any problem at low speeds. RF reflection becomes a concern when the trace or conductor’s length is equal to or larger than 1/4 of the signal’s wavelength. 

In other words, RF reflection is a concern at Gigahertz frequencies. When the signal is reflected due to impedance discontinuity, two things happen:

1. The signal is attenuated, as not all of the energy is transferred to the load. This affects transmission quality, and the receiver may need additional amplification to pick up the signal. 

2. The reflection causes the signal to bounce back and interfere with incident waves from the source. The clash causes interference and results in the waveform peaking at certain points, in what’s being termed as “incident waves”. 

There could be peaks where the amplitude goes beyond the tolerable limits of certain components, resulting in their damage. In less serious cases, the signal integrity is compromised when the reflected waves interfere with the on-going transmission.

Preventing RF Reflection by Impedance Matching

RF Reflection can be avoided with matched characteristic impedance

Preventing RF reflection isn’t an easy task. There are many factors that determine the continuity of characteristic impedance along a transmission line. The dielectric constant of the substrate, trace width, and thickness can affect the characteristic impedance of a transmission line. The trick is getting the ideal values for the parameters. 

There are also other impedance matching techniques that you can apply to a design. The series termination matching, where a terminating resistor is placed between the source and transmission line, is helpful for a source with a lower output impedance. Meanwhile, parallel terminal matching is used to match a source that has a very low impedance to a load with a higher impedance. 

You’ll definitely benefit from using PCB design software that guides you along with impedance matching. Allegro’s PCB Editor allows you to set impedance constraints and run simulations for RF reflection. You can also use InspectAR to accurately assess and improve PCBs using augmented reality and intuitive interaction. Inspecting, debugging, reworking, and assembling PCBs has never been faster or easier.

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