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Tips for RF PCB Design

Tips for RF PCB Design

With so many things to consider when designing an RF PCB, it can be hard to know where to start. How do you choose the right layout? What tools should you use to get the job done? RF PCB design has its own set of challenges that need to be addressed to get the most out of your circuit. There are some best practices you can use to ensure that your design will function optimally and run smoothly.
The best PCB design solution can be the difference between success and failure for your product, so you want to make sure that you are getting it right the first time around. Our guide will walk you through all of the steps necessary to create an RF design that works and that makes your product even better than you envisioned.

Tips for RF PCB Design
Tips for RF PCB Design

Surface Mount Capacitors

Use surface mount capacitors when space is limited. You can incorporate them into your RF Printed Circuit Board design with no impact on performance. Try to keep track of how much space they’ll take and make sure there’s still enough room for other components on the board, like resistors and coils. The surface mount parts are more fragile than through-hole components and require an extra level of care when handling.
Consider using larger surface mount parts wherever possible, or incorporating test points into any sensitive areas if smaller parts are required. Remember that small surface-mount parts have very thin leads that could break off easily. Make sure you know what size of solder tip will be needed before moving forward with your design. If it’s too small, it could damage the delicate leads on these devices. It may also be difficult to attach them to boards after soldering as their leads are so small that they tend to slide around during assembly. When in doubt, use a slightly larger part.

Differential Pair Transformer Coupling

Choosing proper circuit components is an important part of designing and building circuits. When creating transmitter/receiver pairs, several factors influence how well each device will perform. The goal of any transmitter or receiver circuit is to accurately convert an input voltage into an output voltage with minimal noise and distortion.
These parameters are known as gain, linearity, bandwidth, noise figure, NF, return loss, RL, and intercept point, IP. A low NF results in more power being transferred from input to output. The IP value represents how much power can be handled by the front end of a given device before distorting or saturating it, all while maintaining its linearity characteristics.
In other words, if a device has high gain but poor linearity, then it may still have acceptable levels of IP. But if a device has high gain and poor linearity at lower power levels, then it won’t have good IP numbers. In general, devices with higher gains have lower bandwidths, however, there are exceptions to every rule. For example, some amplifiers have very high gains but also operate over wide frequency ranges.

Microstrip Transmission Lines

These are transmission lines in which all of their circuit elements, including those forming half-space planes, and terminations, such as capacitors and transformers, are fabricated on metal strip circuits. The strip circuit is usually etched onto an epoxy substrate using photo etching or electroplating techniques. The micro-strip designs have become very popular for many PC BOARD applications because they can be packaged in small cases with relative ease due to their thin profile. They also have good impedance matching properties over a wide frequency range.

Reference Planes, Power Planes & Ground Planes

Radio-frequency circuit boards or RF designs are often more sensitive to ground loops and signal integrity issues than regular designs because RF circuits and components are particularly susceptible to noise. One way that experienced designers combat these types of issues is by strategically adding power planes, reference planes, and ground planes to their board layouts.
Reference planes: The reference or signal planes can allow designers to focus on specific sections of a circuit without having to worry about interfering signals from other portions.
They also provide a convenient place for designers to add vias between layers of copper, which helps improve both signal quality and thermal performance. Reference planes can be especially useful when they’re directly connected to a component’s ground pin, which allows them to act as an extension of that component’s ground plane.
Power planes: they should always be connected directly to an external source of power, otherwise, they could cause voltage drops across adjacent traces and components.
Ground planes: They should always be connected directly to an external source of the earth, otherwise, they could cause the voltage rises across adjacent traces and components. It’s important to note that many high-speed applications use multiple ground planes at different potentials, so it may not be feasible to have just one global ground plane. However, it’s generally best practice to keep each section within a single board connected through at least one shared global ground plane.
By using separate grounds for different sections of a design, designers can avoid parasitic effects and increase the overall reliability by ensuring all parts of their designs have access to low impedance paths back to an external earth point.
In addition, it’s crucial to ensure that any ground or power planes are spaced far enough away from any active circuitry in order to minimize crosstalk. Generally speaking, there should be at least 1/10th of a millimeter between active circuitry and any nearby reference or power planes. The distance requirements become even more stringent with higher frequencies. When operating above 30 GHz, there should ideally be no less than 0.3 mm of separation between any ground or power plane and active circuitry.

Vias – Size, Shape & Placement

Vias are required in order for us to make electrical contact with traces on different layers. There are three types of vias, including plated through, blind, and buried. A plated through via connects one layer’s copper trace directly with another via’s copper trace or traces. A blind via creates electrical contact but no physical connection between two layers and a buried via provides both an electrical and physical connection.
Placement is generally determined by where it will be soldered or how many layers are involved. For example, if there are four or more layers involved then we recommend using plated through vias because they offer better conductivity than a blind via. If there are only two layers involved then we recommend using either type of visa, depending on its location relative to other components.
As far as shape goes, choose from square, round, or rectangular options that match your pcb fabrication layout requirements. Circular vias may also be available upon request, however, these have a higher cost associated with them due to their complexity and are not always necessary. We use wire-bondable vias wherever possible because they provide a faster assembly process. The above diagram illustrates each type of via along with its respective shape and size.

Isolating an RF trace

It is essential to isolate an RF trace from high-speed signals, including HDMI, USB differential pairs, or crystals’ clock traces. Experts do it through a method known as via stitching where vias are stitched around the RF traces to keep them away from other parts on the circuit board. But, there should be proper isolation as improper isolation can affect PCB function.

RF Circuit Board Insulation

Insulation is one of the most important factors for minimizing the signal loss in an RF transmission. If you want to ensure that your signal reaches its destination without any hiccups or interference, it’s crucial that you use good quality material for your board’s insulation, and choose one with at least 5-mil thickness.
One of our favorite options is Taconic TFEP as it offers superior heat resistance and can withstand temperatures as high as 350 degrees Celsius. It also has great mechanical properties and will last for years to come. Another great option is Rogers, which boasts similar properties but has a slightly higher temperature resistance.
You’ll also want to make sure that you’re using a good adhesive when attaching components to your board. This will ensure that everything stays intact even in tough conditions. If you want to ensure that your signal reaches its destination without any hiccups or interference, it’s crucial that you use a good, quality material for your board’s insulation.
You can create a flawless RF circuit board by considering all the above design guidelines.

Final Thoughts

RF PCB design has its own set of challenges that need to be addressed in order to get the most out of your circuit. There are some best practices you can use to ensure that your design will function optimally and run smoothly. You have to consider material, traces, surface mount capacitors, isolation, insulation, reference, ground, and power planes, vias size and shape, coupling, and micro-strip transmission line.
Would like to know more about RF design or pcb assembly services? Email us at sales@pnconline.com

Written by Sam Sangani

Sam Sangani

Sam Sangani is the President & CEO of PNC Inc., a Nutley, NJ based Printed Circuit Board manufacturer. Sam graduated from L. D. Engineering College with a BS Degree in Mechanical Engineering. He also continued his education and graduated from Steven’s Institute of Technology where he acquired a Master’s degree in Computer Science.

After completion of his BS, Sam worked as a QC Manager, for Xerox, Romania and London. He was responsible for the Quality Control of Cable and Wire Harness imports from Romania. After completing his Master’s Degree, he worked as a Senior Programmer with IBM, Tucson, Arizona. Sam was responsible for leading the Mainframe System Programming Team.

In 1997, Sam acquired PNC INC., a Nutley, NJ based PC Board fabrication Shop. From 1997-2013, Sam has made tremendous improvements and changes within PNC INC., as he added many new Products and Technologies in PNC’s portfolio. With his proven track record and leadership, PNC has never had an unprofitable year and has continued its growth yearly since 1997.

His current responsibilities are Strategic Planning, Corporate Management, New Business Ventures, Sales & Marketing, Trade Shows, Professional Services and leading productive teams to achieve peak potential. He has also utilized Lean Management techniques which have built a foundation for PNC’s high-paced growth. Sam also enjoys real-estate investing, web design & SEO, trading stocks, options, futures and Forex markets.

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