Build Your RF Circuit Design Away From a Catastrophic Crescendo

March 20, 2018 Altium Designer

String instrument composed of musical notes floating into the air

 

 

The art of music composition combines different elements such ranging from chord progression to the melody. A composer usually creates from experience, may learn from other musicians and builds from an understanding of music theory. Successful music composition relies on tried and true methods, formulas, schemes, and models.

 

Music composition also depends on the respect for fundamentals like reading, writing, and thinking music notation while knowing and loving the basics of music theory. Great musicians persuade people through their music.

 

The art of RF circuit design demands the same attention to the basics and minute details seen with music composition. Good design begins with the expert experience of the designer and builds around fundamental theories and common elements. This basic approach recognizes that an RF PCB layout covers a specific set of frequencies and that analog signals can exist between the Min and Max levels of voltage and current.

Elements Sometimes Have a Subtle Impact in RF Circuit Designs

Take a moment and listen to Samuel Barber’s, “Adagio for Strings.” Adagios do not move at a fast pace but—using Barber’s work as an example—often have an incredibly subtle impact on human emotions.

 

A similar subtle—albeit unemotional—impact occurs because of factors such as resistance, temperature coefficient, impedance, and inductance on RF design. Resistance has a subtle but potentially powerful impact on RF circuit performance. All traces, wires, and interconnections offer resistance and have a temperature coefficient. Although both factors seem minimal, the impact on circuit performance can become measurable and can cause errors.

 

With high precision systems or with circuits that have large signal currents, trace resistance can become a factor. For example, the trace resistance can become a friendly partner to a load and form a divider. The resistive voltage drop then leads to a gain error. If the circuit operates at high frequencies, inductance adds to the error and continues to degrade overall performance.

 

At room temperatures, the temperature coefficient of PCB traces doesn’t seem worth a second look. As conditions change, though, the temperature coefficient also changes. This corresponding change in temperature coefficient leads to a change in net impedance.

 

Interactive routing in Altium Designer

You’ll need to make your traces work in unison toward the overall goal of your design.

 

In terms of PCB design, the solutions may seem obvious but introduce other complexities. Wider traces minimize resistance but consume badly-needed space. Thicker copper for the traces also cuts resistance but adds to the production cost.

 

On this last point, circuits that do not provide a low impedance return path allow signal return currents to create a path. Controlling impedance becomes a matter of paying attention to trace length and trace design. Wider traces cause a decrease in trace impedance. Increasing the spacing between a trace and ground increases impedance.

 

Another solution involves keeping the load impedance constant and using voltage sensing feedback to adjust system gain. However, this basic approach may encounter problems if the circuit needs to drive two or more loads with equal precision. We can derive feedback only from one point.

Dissonance Sometimes Works—But Doesn’t

If you have listened to Radiohead’s “How to Disappear Completely,” you’ll know how dissonance catches your attention. The unexpected clashing of tones irritates our senses to the point that we want to listen. As happens so often in RF design, ground path voltages can create the type of dissonance and detracts from good circuit performance.

 

We can hope that ground path voltages remain small and that the voltage sensing feedback will work. In practice, not all ground points have the same potential. Additional external currents can generate noise voltages between the ground points. A zero-ground voltage difference between the source ground and load ground interferes with a transmitted signal in high precision circuits when the undesired voltages appear at the load.

 

Signals return currents follow the path of least impedance or, in high frequency circuits, the path of least inductance. Think—for a moment—about a high current circuit sharing an unlooped ground return with the signal source. When the large changing current flows in the common ground return, the combination of current and resistance generates an error voltage.

 

Along with the error voltage, other bad things happen. The current-related signals also become noise that harms the performance of sensitive circuits. Any loops in the ground network introduce vulnerability to external EMF.

Pins and resistors on a white background

Ensure that your design maintains proper signal integrity.

 

Sometimes Inspiration Just Happens

The line between an instant classic and a one-hit wonder is razor thin. Music sometimes endures because a composer decided that a different approach would capture a special mood and speak to an audience.

 

Achieving low impedance and decreasing ground noise with ground planes is a recurring theme in PCB design. In high frequencies, however, harmful noise disrupts circuit operation and a designer needs to take a different approach. The next move towards eliminating noise can occur through a ground isolation and the decrease of ground errors.

 

Your design can accomplish this by employing a differential input, ground isolation amplifier. The ground isolation amplifier sets between stages, processes the signal in differential mode, and rejects common mode signals. Using a differential amplifier that uses high impedance, differential signaling techniques also decreases voltage drops.

 

Analog-to-Digital conversion can accomplish the same process with an ADC converter that operates in differential mode. Using this technique eliminates the need for an additional differential ground isolation amplifier stage. Again, high input impedance for the ADC converter is the key. High input impedance lessens the sensitivity to trace and wire resistance. Common mode rejection at the ADC eliminates the possibility of noise interference occurring between the ADC and source ground.

 

You may try to use a conductor’s baton to solve all of your PCB design problems, but when that doesn’t work look into stronger PCB design software. With features such as smart design rule checking, auto-interactive routing, and a power distribution analyzer all within the same software tool, Altium Designer has what you need to make an RF circuit design work.

If you have more questions about signal integrity and circuit strength, contact an expert at Altium today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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