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Design and layout is an art form, and never more so than when you are designing an RF circuit. SSS Online is proud to present some tricks and hints on this important topic to help you design RF circuits that don't fall prey to the harmonics, drop out and other bugaboos that can haunt the designer.


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Layout Considerations for Texas Instruments CC1100/1101 Transceiver
by Eric Myers, Senior RF Circuit Design Engineer, Pegasus Technologies, Inc.


One of the RF chips that we have used in many of our current designs is the Texas Instruments CC1100/1101 transceiver chip. This chip is an inexpensive and very versatile sub-1GHz transceiver that is aimed at ultra-low power wireless applications in a variety of frequency bands. The versatility of this chip is enhanced by its integrated and highly configurable baseband modem, which supports a number of different modulation formats and data rates. With extensive capabilities for packet handling, data buffering, burst transmissions, clear channel assessment, and link quality indication, this chip is typically used with an on-board microcontroller.

Originally developed by ChipCon, this chip has continued as an active TI product following their acquisition of ChipCon in early 2006. The newest version of this chip, the CC1101, has been recently introduced and is the chip we are using in new designs. The pin-for-pin and code compatible CC1101 has better spurious response and improved close-in phase noise for adjacent channel performance.

We selected the CC1100/1101 for our designs due to its versatility and small form factor. We typically pair it with the MSP430 microcontroller, a decision we made before TI's acquisition of Chipcon and which works especially well now that both are manufactured by the same company. This microcontroller, an ultra-low-power 16-bit RISC mixed-signal processor, is ideal for low power and portable applications. TI has also combined the 1100 with an 8051 MCU to create a system-on-chip IC (CC1110), and is in the process of developing an IC with the 430 MCU. This provides us with the ability to offer several options to our clients while using a consistent and proven transceiver architecture.

In designing circuits using their components, Texas Instruments' website provides very helpful documentation and design examples. TI also continuously works to improve their reference designs. At the present time (August 2009), three designs have been published for the CC1100 chip, but the first two are no longer recommended. The current design has improvements to eliminate harmonic emission as well as variations based on power settings. Some of the older data sheets still show the older designs, however, so RF engineers using this chip should be sure to download the most recent version from the TI website when beginning a new design. The current version at the time this article was written is Rev D, dated May 29, 2009. The current version for the CC1101 is CC1101: Low-Power Sub-1 GHz RF Transceiver (Enhanced CC1100), of 2009.

Although TI's datasheets are invaluable in incorporating this chip into a particular design, we have still found some issues that are not addressed, particularly related to layout, and have written this article to fill in some of the gaps based on our own experience.

Filter Topology and Layout Concerns

TI shows a recommended layout for the CC1100 on page 89 of its datasheet. A copy of the picture is reproduced below:

Recommended PCB Layout for QLP 20 Package

TI notes that this diagram is an illustration only and not to scale, and that there are five 10 mil via holes distributed symmetrically in the ground pad under the package that are not shown in the diagram. What the picture does not show is how the supporting components should be placed and routed. The following sections will provide descriptions of the more important sections along with pictures showing the recommended and not recommended ways of configuring them.

Filter Topology

One of the key pieces of a design using the CC1100 is the filter balun. The design of the filter balun accomplishes three main tasks. First it converts the balanced RF signals from the CC1100 into an unbalanced signal. Second it provides a low pass filter. The third task is to match the impedance from the output of the CC1100 to the impedance of the antenna (typically 50Ω). Because of the importance and multi-tasking required by this portion of the design, there are some important considerations in layout that need to be taken into account.

The balanced portion of the filter balun is very susceptible to variations between the two sections. For this reason, it is very important to keep the component spacing and traces as equal as possible. As an example, when placing the LPF inductors that connect directly to the CC1100, it would be better to have both set slightly off from the pins. This would allow for the traces to be of equal length and shape. If one inductor is placed so that it has a very short straight trace, the other inductor will not fit in front of its corresponding pin. This will cause the trace for the second inductor to be longer with more bending. The effect of having the two sides with uneven layouts is higher harmonics (especially the 2nd and 4th) as well as reduced output power at the single-ended side of the filter balun.

Figure 1-Incorrect

Figure 2-Correct

The single-ended side of the filter balun is designed to be 50Ω impedance. The trace width for this section up to the antenna should be 50Ω. However, the trace widths for the filter balun section itself should be as wide as possible without requiring a resize at the pads of the filter balun components (in other words, you should make the traces from the CC1100 to the filter inductors the same width as the CC1100 pads). In and ideal world, the components would be right next to each other without any traces required to connect them. However since that is not possible, the next best approach is to minimize the amount of inductance within the trace by making it as wide as possible based on the size of the component pads.

Figure 3-Incorrect

Figure 4-Correct

Solid ground plane below the IC and filter balun components

Another important aspect of the filter balun layout is the ground plane. The layer directly below the components should be a solid ground. Because the CC1100 packaging has pins on all four sides, it can be difficult to connect things such as power. It is often necessary to switch between layers in order access all of the connections required. With a multi-layer board, the first inner layer can be a solid plane, which will accomplish this requirement rather easily. With a two-layer design, it becomes more complicated to achieve a solid ground. The preferred method is to route bottom side traces on the opposite side of the CC1100 from the RF outputs. If this is not possible, the trace should be routed directly below the device, but NOT under the RF pads or filter balun.

Figure 5-Incorrect

Figure 6 - Acceptable

Figure 7 - Correct

Importance of decoupling capacitors to each pin

When using the reference design provided by TI, it is important to understand that each decoupling component is included for a reason. When pins are located next to each other and have a capacitor associated with each one, these components must be connected individually. Do not simply connect the two pins together and then route a single trace to the two capacitors. By using a single trace the pins are no longer isolated from each other. The correct procedure is to route an individual trace from each pin to its associated capacitor. This will obviously make the layout a little more complicated, but extra effort will be appreciated when the circuit performs as expected without troubleshooting and modifications to the PCB.

Figure 8-Incorrect

Figure 9-Correct

Trace width variation with different PCB thicknesses

At the output of the filter balun, the RF output is now 50Ω impedance. The trace width needs to be set based on the thickness of the dielectric and maintained up to the antenna. In this case, if the width is wider than the component pads, the trace will have to be reduced just before entering the pad. As with the filter balun section, the ground plane below the trace must be solid up to the antenna. Depending on the style of antenna, the ground plane may or may not need to be removed at the antenna itself.


This article, if read in conjunction with the excellent TI application notes for this chip, will make it easier to incorporate the CC1100 into your design. While the advice presented here may seem simple, failure to apply these techniques may result in a circuit that does not perform as desired. By taking the time apply these techniques, hopefully, it will save you some layout mistakes and reworks, and thus reduce both development time and costs. You can spend all this time and money you save here on designing better firmware for the chip and its associated microcontroller - with the versatility available with the CC1100, this is where the real excitement should be!

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Layout Resources from Pegasus Technologies

TI Developer's Network. We are proud to be a member of the Texas Instruments' Low Power RF developer's network.

Design Bureau Listings. Pegasus Technologies, in conjunction with our subsidiary Design Sources, is listed as an EMA Design Automation Service Bureau using Cadence Allegro Design Software.

We are also listed as an Altium Service Bureau using the latest Altium Designer PCB design software with the Extended Feature Set. This service is also offered in conjunction with Design Sources.

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Layout Resources on the Web

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Reference Books on PCB / RF Layout

Click on a Title Below for a Direct Link to Purchase


Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers (IEEE Press Series on Electronics Technology), by Mark I. Montrose. Hardcover - 332 pages (June 2000).


PCB Design for Real-World EMI Control (The Springer International Series in Engineering and Computer Science), Bruce R. Archambeault & James Drewniak. Hardcover - 268 pages (July 2002).


Emc & the Printed Circuit Board: Design, Theory, & Layout Made Simple, by Mark I. Montrose. Hardcover - 325 pages (August 1998).


The Circuit Designer's Companion, Second Edition (EDN Series for Design Engineers) , by Tim Williams. Paperback - 368 pages (January 2005).


RF Circuit Design, Second Edition (Paperback), by Christopher Bowick, Cheryl Ajluni, John Blyler. Paperback - 256 pages, 2nd edition (November 2, 2007).


Complete PCB Design Using OrCAD Capture and PCB Editor, by Kraig Mitzner. Paperback: 488 pages, 1st Edition (June 11, 2009).

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