Termination Methods in High Speed and High Frequency PCBs
To this day I still have a tall stack of printed articles I refer to when working on a new research or design project. Even though I know I can find all of this information online, I still refer to my trusty stack of articles regularly for design information. Keeping all the relevant system design rules and research results in one place saves me a huge amount of time.
Whether you’re working with PCBs or any other electronic system, keeping track of every design aspect is a tall order. Any PCB familiar with high speed design rules is familiar with impedance matching. There are a number of termination methods available, depending on the solder and trace arrangement, component footprint, and frequency band of interest.
Looking at the impedance of traces, including microstrips and striplines, the impedance of a trace tends to saturate towards a constant value at very high frequencies (e.g., at RF levels). Simple termination can be performed using either a series or shunt resistor connected between the end of the trace and the component load. The compensation this provides will be a constant value that is independent of frequency.
Using a series resistor is rather simple as the resistor can be placed in-line with the trace. An interconnect on the surface layer will not require any vias, and the termination will be quite accurate. The series resistor should match the impedance of the signal source rather than the load. Series termination is also advantageous as it consumes a small amount of power.
Any residual reflection at the end of the link could be suppressed as the series resistor effectively increases the damping constant for reflected signals, which changes the RLC resonance condition in the link. The series resistor also effectively increases the RC time constant, which increases rise times in digital signals and adds skew in a digital circuit.
This is helpful for suppressing bit error rates, but this also increases the impedance saturation frequency. In most cases this will not be a problem; the signal frequency already tends to be high enough that the impedance of the link has already saturated at a constant value.
You’ll want to make sure that you know your board front-to-back to catch error rates in time.
Parallel Termination Schemes
A better option is termination with a shunt (i.e., parallel) resistor. This involves connecting a parallel resistor between the load and ground that is equal to the line impedance. This is a better option in components without programmable output or input impedance.
An alternative parallel termination method is the Thevenin termination method. This method connects a resistor between the component power source and the termination point, as well as a second resistor between the termination point and ground. This method is rather simple as the equivalent parallel resistance between these two resistors just needs to be set equal to the trace impedance.
Fly-by parallel termination methods typically require the use of a via to route connections between the input pin on the load and the power and ground connections. Therefore, the via stub on this connection should be made as short as possible. This sets the stub resonance frequency to a very high value and also minimizes reflection from the stub itself, which suppresses EMI. If possible, the stub should be eliminated entirely by backdrilling.
Other Termination Methods
One simple method for matching impedance between a source, trace, and load involves the use of a transformer. The turns ratio required to compensate an impedance mismatch is related to the ratio of the new impedance value.
The turns ratio in the transformer is equal to the square root of the ratio of the impedance on the secondary side to the impedance on the primary side. This allows you to easily calculate the turns ratio required to generate a specific impedance on the secondary side of the transformer.
Using a transformer to match impedance may not be practical in all PCBs due to the footprint of these components. Transformers with ferrite cores are only useful up to hundreds of MHz, and transformers will be less effective for impedance matching in higher frequency devices.
Antenna modules in a PCB will inevitably require use of an amplifier, regardless of whether an antenna is used for transmitting or receiving signals. The amplifier is usually built into the receiver or transmitter. The impedance of the antenna must equal the impedance of the transmitter/receiver in order to pass the maximum amount of power between these components.
This is typically done by adjusting the impedance of the transmitter/receiver using one of the methods above, or by using an inductor/capacitor pair. One of these components must be placed as a shunt element, and the other is placed in series. A Smith chart is the best way to visualize the level of mismatch in an antenna circuit and provides a useful guide when matching impedances.
SIM card holder and antenna on a green PCB
Working with a dual band antenna is a different beast as it requires impedance matching in both bands simultaneously. Termination in both bands requires two capacitors and two inductors. A capacitor and inductor are used as a pair to match a single band. One pair must be placed in series with the antenna/load, and the other pair will be placed as shunt elements.
Terminating one band changes the termination conditions in the other band, so each band cannot be impedance matched sequentially. Proper termination and determining the exact placement of each capacitor and inductor requires some trial and error and design experience. Simulation programs can also be helpful. Note that these steps for terminating a dual band antenna apply to any component that requires multi-band termination.
With so many termination options available in a PCB, you’ll need CAD and analysis software that allows you to precisely determine the level of impedance matching required for your traces. The built-in CAD, simulation, and component management tools in Altium Designer® can help you avoid signal integrity problems caused by improper termination.