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Application Note AN040 Folded dipole antenna for CC2400, CC2420, CC2430, CC2431, and CC2480 By G. E. Jonsrud 1 KEYWORDS • Radiation Pattern • Line of sight range • CC2400 • CC2420 2 INTRODUCTION This application note describes the design of a folded dipole antenna for CC2400, CC2420, CC2430, CC2431, and CC2480. The CC2400 is a true single-chip, general- purpose transceiver for the 2.4 GHz SRD band for data rates up to 1 Mbps. The CC2420 is a true single-chip RF trans- ceiver designed for low power wireless networks operating in the 2.4 GHz SRD band compliant with the ZigBee™/IEEE 802.15.4 standard. CC2430 is a true SoC combining the CC2420 with a single cycle 8051 microcontroller. CC2431 is CC2430 with location engine. The CC2480 is a cost-effective, low power, Z-Accel ZigBee Processor full ZigBee functionality with a minimal development effort. The design described in this application note is based on the CC2400, but it is possible to tune the antenna impedance so it can be used with CC2420, CC2430, that provides • CC2430 • CC2431 • CC2480 • Folded dipole CC2431, and CC2480. The tuning is done by adjusting the value of the inductor placed across the RF pins. The RF front end consists of three pin connections. Two pins serve as a differential interface shared by the LNA and PA. The third pin changes voltage level in order to provide power to the PA during transmission and ground to the LNA during reception. A differential interface provides a better utilisation of the available supply voltage as well as less parasitic capacitance to ground. Design criteria for the antenna and the design process are described. Also included are test results and a comparison of the tested antenna to a balun and monopole antenna solution. Gerber files and schematics can be downloaded from www.ti.com/lpw. SWRA093D Page 1 of 26

Application Note AN040 Table of Contents KEYWORDS ................................................................................................................... 1 1 INTRODUCTION............................................................................................................. 1 2 ABBREVIATIONS........................................................................................................... 3 3 DESIGN CRITERIA......................................................................................................... 4 4 DESIGN DESCRIPTION................................................................................................. 4 5 SCHEMATICS AND LAYOUT........................................................................................ 5 6 7 TUNING........................................................................................................................... 8 TEST RESULTS ........................................................................................................... 10 8 8.1 SUMMARY OF RESULTS............................................................................................ 13 9 CONCLUSION .............................................................................................................. 13 10 APPENDIX A - RADIATION DIAGRAMS .................................................................... 14 11 DOCUMENT HISTORY ................................................................................................ 26 SWRA093D Page 2 of 26

Application Note AN040 3 ABBREVIATIONS CC243x CC2480 DC EB EIRP EM CC2400EM FCC FR4 FSK ISM LNA PA PCB RBW RF RFC SMA SRD VBW CC2430 and CC2431 Z-Accel ZigBee Processor Direct Current Evaluation Board Effective Isotropic Radiated Power Electromagnetic CC2400 Evaluation Module Federal Communications Commission Common PCB material Frequency Shift Keying Industrial Scientific and Medical Low Noise Amplifier Power Amplifier Printed Circuit Board Resolution Bandwidth Radio Frequency Radio Frequency Choke Common RF connector Short Range Device Video Bandwidth SWRA093D Page 3 of 26

Application Note AN040 4 DESIGN CRITERIA The following design criteria were important for the antenna design: • Optimum load impedance for CC2400, 110 + j130 Ohm, differential • DC-connection between RF pins and TXRX_switch pin • TXRX_switch pin isolated from RF • Few components • Manufacturability • Low spurious emission • Low losses • Omnidirectionality The optimum termination impedance is a trade-off between optimum source impedance for the internal LNA and optimum load for the internal PA. The TXRX_switch pin level is 0 V in receive mode to provide ground for the LNA and 1.8 V in transmit mode to provide the required supply voltage to the PA. This pin should be isolated from the RF signals by using a shunt capacitor and/or a series inductor (RFC). Antennas that are electrically short compared to the wavelength tend to be sensitive to component variations in the tuning network. Electrically small antennas may cause yield problems or require individual tuning. Pay special attention to the harmonic levels for operation in the 2.4 GHz SRD band. Both the second and third harmonic will fall within restricted bands as defined by FCC part 15. In typical SRD applications, it is desired that the antenna radiates equally in all directions, i.e. that the antenna is omni directional. A folded dipole is attractive because of its high impedance that makes it easier to match to the optimum impedance for the CC2400. The theoretical impedance is 292 Ohm for a half wavelength folded dipole. A shunt inductor should provide the inductive part of the optimum load impedance while reducing the real part. The folded dipole is a metal loop that will provide DC contact between the RF pins. In addition the mid point of the antenna is virtual ground, meaning that a connection can be made to the TXRX_switch pin without distorting antenna performance. The folded dipole is a resonant structure that should be less sensitive to component variations and provide low losses. The radiation pattern of a folded dipole is omni-directional in the plane normal to the antenna. 5 DESIGN DESCRIPTION An initial investigation to check the feasibility of the design was performed using the Smith chart. Plotting the 292 Ohm in the Smith chart and adding a 15 nH shunt inductor resulted in 115 + j141 Ohm. The CC2400EM reference design was selected as the base for the design. The CC2400EM is a radio module with balun and an SMA connector. The balun with the SMA connector is designed to work with 50 Ohm unbalanced devices such as a ¼ wave antenna and most RF instruments The antenna was implemented on the PCB as part of the layout. The antenna was placed relatively close to the CC2400 to keep the design compact. SWRA093D Page 4 of 26

Application Note AN040 The antenna design was simulated before the layout was made. The antenna was designed using an EM simulator and the matching circuit was simulated using a linear simulator and S- parameters from the EM simulation. The first step in the simulation was to design a folded dipole on a FR4 PCB in front of a ground plane of the same size as the CC2400EM. The length of the antenna was adjusted until the impedance was 290 Ohm. The next step was to add feed lines with pads for a shunt inductor and a transmission line to the virtual ground point of the antenna for DC connection to the TXRX switch pin. The transmission line to the TXRX switch pin was connected to ground during the simulations and was fitted with pads for a series inductor. The inductor pads were defined as ports to make it easy to simulate with various inductors in the following S-parameter simulations. Due to the PCB material and the ground plane, the antenna became shorter than the theoretical half wavelength. Finally, the inductor values were determined using a linear simulator, S-parameters from the antenna simulation and S- parameters for the inductors. 6 SCHEMATICS AND LAYOUT Figure 1 shows the schematic of the CC2400EM with the folded dipole antenna. Figure 2 shows the board layout. The distance to the antenna and extension of the ground plane behind the antenna are critical parameters. If the PCB is wider than the CC2400EM board, the ground plane, components and tracks should be pulled away from the end points of the antenna. SWRA093D Page 5 of 26

Application Note AN040 Figure 1: Schematics for CC2400EM with Folded Dipole Antenna SWRA093D Page 6 of 26

Application Note AN040 Figure 2: Layout of CC2400EM with Folded Dipole SWRA093D Page 7 of 26

Application Note AN040 7 TUNING The schematic in Figure 1 shows the recommended component values when matching the antenna to CC2400. Since the optimum impedance of CC2420, CC243x, and CC2480 is different from CC2400 is it required to tune the value of the matching inductor (L62) to obtain optimum performance when implementing the antenna with these parts. The optimum load impedance for CC2420, CC243x, and CC2480 is given in the data sheets. The size of the ground plane, encapsulation of the board and objects in the vicinity of the antenna will also affect the performance. Thus, it is important to have the antenna implemented in the environment it is going to be used when performing the tuning. It might also be needed to tune the length of the antenna to obtain optimum performance. The length of the antenna will affect the resonance frequency. Implementations with ground plane size different from the reference design and with encapsulation will most likely require slightly different antenna length to ensure optimum performance in the middle of the 2.4 GHz ISM band. To find the optimum length, test software that steps a carrier across the frequency band can be used. Measuring the radiated power by using max hold on a spectrum analyzer will identify the optimum frequency. If the frequency with highest radiation is too low, the antenna could be made slightly shorter and if the maximum radiation is at a too high frequency, the antenna should be made longer. Tuning of the antenna length could be done using a sharp knife and soldering on copper tape or a small wire. The purpose of tuning the value of L62 is to maximise output power while maintaining good spectrum properties. Figure 3 shows the spectrum when CC2400 is configured to transmit random data continuously at 1 Mbps. It is measured with a cable between the spectrum analyser and the CC2400EM. The cable and the instrument is 50 Ohm and a good impedance match for the CC2400EM. Figure 3 also illustrates how to judge a good spectrum. The marker measures the difference between the peak power level and the first null. It should be at least 25 dB, typically 28 dB, for no degradation in transmission. The difference in frequency is 760 kHz. It is important to measure with 100 kHz RBW and a 100 kHz VBW. It is also an advantage to apply averaging for the measurements over the air. (Note: The plot uses different settings on RBW and VBW) Figure 3: Reference Spectrum for CC2400 at 1 Mbps SWRA093D Page 8 of 26

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