0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 1 of 17 Anaren 0404 (BD2425N50200A00) balun optimized for Texas Instruments CC2420 Transceiver Nithya R Subramanian – RF Development Engineer Niels Kirkeby– R&D Manager August 31st, 2007 Introduction Over the last few years, the drive for miniaturization and integration has intensified the challenges concerning the trade off between repeatability, cost and time to market. The design must be robust enough to get good yields, but also have the lowest possible bill of material cost. The total cost not only depends on the number and the types of parts and their associated cost, but also on the size of the PCB and enclosure. At Anaren the focus is on developing product that addresses this trade off. Integrating 100% RF tested components increase yield and decreases size and time to market. The following application note demonstrates these objectives clearly as we present a small and simple balun solution optimized for use with the CC2420 from Texas Instruments. The CC2420 is a true single-chip 2.4 GHz ISM and IEEE 802.15.4 (ZigBee) compliant RF transceiver, designed for low-power wireless applications. The reference design presented in this application note uses only three components for the impedance matching: a 1mm square Anaren multilayer balun, a DC blocking capacitor and an inductor for final impedance matching. This results in, a design which takes up very little space and performs according to the numbers stated in the data sheet. The CC2420 is a low-cost, highly integrated solution for robust wireless communication in the 2.4 GHz unlicensed ISM band. CC2420 is designed to be compliant with SRD regulations covered by ETSI EN 300 328 and EN 300 440 class 2 (Europe), FCC CFR47 Part 15 (US) and ARIB STD-T66 (Japan). The CC2420 provides extensive hardware support for packet handling, data buffering, burst transmissions, data encryption, data authentication, clear channel assessment, link quality indication and packet timing information. For more information about this or any other products currently available in the Anaren product portfolio, please visit our website at www.anaren.com for datasheets, S parameters and general corporate information. Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 2 of 17 For more information on Low Power Wireless products from Texas Instruments please visit www.ti.com\lpw for product information. Comparisons of Different Balun Solutions The RF front end for the CC2420 is architecturally simple in that both receiver LNA and transmitter PA are attached to the same set of balanced pins. The transmitter is linear enough to not require any significant filtering. Hence an impedance matched, balanced to single ended transformation is all that is needed. The only complication being that the PA bias power, pin 7 on the CC2420, needs to be supplied into the balanced pin set. A multitude of possible balun implementations exist and Texas Instruments provides two other reference designs. One uses a 180° transmission line and 4 discrete components. The other solution is a discrete lattice balun implementation that uses 7 components. The solution from Anaren, described in this document, uses a discrete balun, one capacitor and one inductor. Below we will step through each implementation to detail the differences and benefits that each offer. Figure 2 Layout of transmission line implementation Figure 1 Schematic of transmission line implementation The transmission line implementation shown in Figure 1 and Figure 2 is straight forward and employs only four discrete components. However the PCB real estate taken up is significant and the performance is sensitive to changes of line width of the 180˚ transmission line, PCB thickness and variation in the PCB material. Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 3 of 17 Figure 4 Layout of the lumped element implementation Figure 3 Schematic of the lumped element implementation The lumped element implementation shown in Figure 3 and Figure 4 is smaller in size than the transmission line version, but uses a total of 7 discrete components. This implementation is less susceptible to line width and PCB variations, but it is sensitive to discrete component tolerances. Figure 5 Schematic of the Anaren balun implementation Figure 6 Layout of the Anaren balun implementation The Anaren balun implementation shown in Figure 5 and Figure 6 has fewer components and uses less real estate than any of the other solutions The Anaren balun implementation shown in Figure 5 and Figure 6 takes up even less PCB area and has reduced sensitivity to discrete component tolerance/variation. The recommended components are a parallel inductor of 6.8nH (Johanson L-07C6N8J) and a 12pF DC-blocking capacitor (Johanson 250R07C120JV4). Care should be taken using alternate vendors especially on the inductor as they do not have the same performance. Each vendor of inductors and capacitors has their own way of realizing the inductor/capacitor with associated differences in parasitic values, even within a single vendor, different component series are made differently with significantly different parasitic’s. Even from one value to the next in the same series there can be parasitic differences, if for instance a spiral inductor requires another turn to fit in the same footprint from one value to the next then either the trace width drops, another trace layer is used or maybe a different material set – these, from a Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 4 of 17 value point of view, subtle changes causes significant parasitic change. If the change in impedance cause by parasitic changes fall within the circle outlined in Figure 9. the performance will be acceptable. One way to evaluate alternate vendors is to compare sparameters of the components – however the s-parameters must be measured the same way from both vendors to be able to compare and this information is not often available. Another way is to use vendor or third party models but it is still important to know what each model represents; does it include the PCB pads, is the model valid for the PCB used etc. The Anaren recommended layout (can be supplied as Gerber files) is fabricated on a 39mil thick FR4 board. If a multilayer board is used it is recommended that internal power/GND planes be opened such that the effective height to GND is roughly 40mil, as illustrated in Figure 7 below,. If it is not possible in the application to open up internal GND planes then follow Table 1 for changes to the differential connecting lines, identified in Figure 8, with red arrows and the single ended connection identified in Figure 8, with a blue arrow. Figure 7 Opening in power/GND planes below RF circuitry Figure 8 Differential (red) connecting lines and single ended (blue) connecting line Distance to ground & material [mil] Differential line width Single ended line width Width/Length [mil/mil] Width/Length [mil/mil] 5, PI 8/60 8/221 8, Ro4350 8/60 10/221 10, FR4 10/60 15/221 20, FR4 10/60 30/221 30, FR4 10/60 50/221 39, FR4 10/60 80/221 60, FR4 10/75 80/221 Table 1 : Differential line and single ended line width/length for various substrate heights If a SMA connector is used and GND plane spacing other than 39 mils is used then the launch area must be opened or otherwise compensated to provide proper match. If the location of the components is changed (not recommended) then it is very important to keep the DC blocking capacitor very close to the balun (pin 1). Also the trace lines between Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 5 of 17 the CC2420 chip, the parallel inductor and balun are an integral part of the matching and should not be changed in length. Table 2 demonstrates the significant reduction in size; component count and board layout achieved with the Anaren 0404 balun solution. Table 2 Comparison of the three different balun implementations Solution Transmission line design Lumped element design Anaren/TI 0404 internal bias solution Component Count and type 4 Total 7 Total 3 Total 1 Capacitor 4 Capacitors 1 Balun 3 Inductors 3 Inductors 1 Inductor 1 Capacitor PCB Area 0.1448 sq. inch 0.0423 sq. inch 0.0174 sq inch Space savings based on lumped element design -242% 0% 59% The Anaren balun performance is consistent and tolerant to PCB manufacturing tolerances. Production average and worst case RF performance data for the BD2425N50200A00 is illustrated in Table 3. Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 6 of 17 Table 3 Balun Performance Max 20 15 1 10 0.5 5 Frequency [MHz] Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 3200 3100 3000 2900 2800 2700 2600 Frequency [MHz] 3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2100 2000 1900 3200 3100 3000 2900 2800 2700 2600 2500 2400 -20 2300 -15 -2 2200 -10 2100 -1 2000 2500 0 -1.5 1900 Max -5 1800 -0.5 1700 [Deg] 2 0 Min Ave Phase Balance Ave 2200 Min 1.5 1800 2400 2200 2100 2000 1900 1800 1700 3200 3100 3000 2900 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 2800 Frequency [MHz] Am plitude Balance 1700 Max Ave 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 -36 Frequency [MHz] [dB] Min Return Loss [dB] 0 -0.3 -0.6 -0.9 -1.2 -1.5 -1.8 -2.1 -2.4 -2.7 -3 -3.3 -3.6 1700 [dB] Min Ave 2300 Max Insertion Loss 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 7 of 17 Figure 8 Implementation of CC2420 using Anaren balun. Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 8 of 17 Application Verification Measurements verify that the reference designs presented in this application note has the same performance as given in the data sheet. These measurements include; • Transmit Power • Receive Sensitivity • Harmonics These measurements were performed independently by both Anaren and TI. In addition TI also performed a Vector Error measurement. Through further testing at Anaren the optimum impedance for the CC2420 Chip is found to be inside the range (ellipse) shown in Smith chart below. Anaren balun design TRL design Lumped element design Figure 9 Optimum Impedance Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 9 of 17 The Impedance Matching The following describes the matching steps. Optimum differential impedance as seen from the RF ports towards antenna is stated as 95+j187 Ω in the data sheet for the CC2420. The BD2425N50200A00 balun has 50 Ω single ended port impedance and 200 Ω balanced port impedance. A 6.8nH inductor is connected across the balanced ports for matching purposes and a 12pF capacitor is used at pin 1 as a DC block (RF GND) to allow biasing through this pin to the differential ports, pin 3 and pin 4 of the balun. This is illustrated in Figure 10. Figure 10 Anaren balun schematic The impedance matching steps, with a SMA connector, illustrated in the Smith chart, Figure 11 performs the match as follows. • The connector and the connecting line at the input transform 50 Ω to 45-j2 Ω (Note 1). • The balun transforms 45-j2 Ω into 239+j29 Ω • The inductor at the differential arm of the balun brings the impedance to 53+j98 Ω • The transmission line to interface the chip to the balun brings the impedance to 62+j135 Ω. • The DC-blocking capacitor and transmission line from the Tx/Rx switch to the bias point of the balun (pin 1) transforms the impedance to 84+j172 Ω Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 10 of 17 Figure 11 Smith chart showing the impedance matching steps The value of the inductor and the capacitor also depends on the length of the transmission line used between the CC2420 chip and the balun. If the length of the transmission line between chip and the balun is increased then the inductor and the capacitor values should be decreased. If the length of the line is decreased then the inductor and capacitor should be increased. It is strongly recommended to use the same line length, width, inductor and capacitor values as shown in the 0404 internal bias design schematic. Any change in inductor value, DCblocking capacitor value or layout will give only similar but not exactly the same performance. Note 1: If a SMA connector is not used then a 50 Ω transmission line should be used to connect to the balun, this change will cause a negligible shift in performance. References 1. http://focus.ti.com/docs/prod/folders/print/cc2420.html - CC2420EM Reference Design 2. http://focus.ti.com/docs/prod/folders/print/cc2420.html - CC2420 Development kit user manual Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 11 of 17 Ultra Low Profile 0404 Balun 50Ω to 200Ω Balanced Description The BD2425N50200A00 is a low cost, low profile sub-miniature unbalanced to balanced transformer designed for differential inputs and output locations on modern chipsets in an easy to use surface mount package. The BD2425N50200A00 is ideal for high volume manufacturing and delivers higher performance than traditional ceramic baluns. The BD2425N50200A00 has an unbalanced port impedance of 50Ω and a 200Ω balanced port impedance. This transformation enables single ended signals to be applied to differential ports on modern integrated chipsets. The output ports have equal amplitude (-3dB) with 180 degree phase differential. The BD2425N50200A00 is available on tape and reel for pick and place high volume manufacturing. Detailed Electrical Specifications: Specifications subject to change without notice. ROOM (25°C) Features: • • • • • • • • • • • • 2400 – 2500 MHz 0.65mm Height Profile 50 Ohm to 2 x 100 Ohm Low Insertion Loss 802.11 b+g MIMO b+g Bluetooth Zigbee Surface Mountable Tape & Reel Non-conductive RoHS Compliant Parameter Min. Frequency 2400 Typ. Max 2500 Unit MHz Unbalanced Port Impedance 50 Ω Balanced Port Impedance 200 Ω 27 dB Return Loss 21 Insertion Loss* 0.6 0.7 dB Amplitude Balance 0.5 1.0 dB Phase Balance 2 6 Degree s CMRR 29 dB Power Handling Operating Temperature -55 1 Watts +85 ºC * Insertion Loss stated at room temperature (Insertion Loss is approximately 0.1 dB higher at +85 ºC) Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 12 of 17 Outline Drawing Top View (Near-side) .041+.003 -.002 1.05+0.08 -0.06 [ ] Side View .041+.003 -.002 1.05+0.08 -0.06 [ Bottom View (Far-side) .012 [0.30] .024±.0025 [0.62±0.064] 0.12 [0.30] 1 ] 2 3x.0 [0.2 4 4X .020 [0.50] 3 3x.010 [0.25] Pin Designation GND / DC Feed 1 + RF GND Dimensions are in Inches [Millimeters] Mechanical Outline 2 Unbalanced Port 3 Balanced Port 4 Balanced Port Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 Tolerances are Non-Cumula 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 13 of 17 Typical Performance:2400 MHz. to 2500 MHz. Insertion Loss -3 -0.3 -6 -0.6 -9 -0.9 -12 -1.2 -15 -1.5 1.5 15 1 10 0.5 5 2600 2550 2500 -20 2450 -15 -2 2400 -10 2350 -1 -1.5 Frequency [MHz] Frequency [MHz] CMRR 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 2600 2550 2500 2450 -36 2400 2450 -5 2400 0 -0.5 2350 0 2300 deg 20 2350 2600 Phase Balance 2 2300 [dB] Amplitude Balance [dB] 2600 Frequency [MHz] Frequency [MHz] 2300 2550 2300 2600 2550 2500 -3.6 2450 -36 2400 -3.3 2350 -3 -33 2550 -2.7 -30 2500 -2.4 -27 2500 -2.1 -24 2450 -1.8 -21 2400 -18 2350 [dB] 0 2300 [dB] Return Loss - Input 0 Frequency [MHz] Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun Application note (ANN-2001) Page 14 of 17 Wide Band Performance: 500 MHz. to 8500 MHz. Return Loss - Input 0 -0.3 -3 -0.6 -6 -9 -1.2 -12 -1.5 -15 [dB] -0.9 -1.8 -18 Frequency [MHz] CMRR -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 -36 3000 6500 7000 7500 8000 8500 7500 8000 8500 5000 4500 4000 3500 3000 2500 2000 Frequency [MHz] 0 2500 1500 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 -20 3000 -15 -2 2500 -10 2000 -1 -1.5 500 -5 1000 0 -0.5 1500 7000 5 0 2000 6000 10 1500 6500 1 0.5 500 5500 15 500 6000 1.5 deg 20 1000 5000 Phase Balance 2 1000 [dB] 4500 Frequency [MHz] Amplitude Balance [dB] 5500 Frequency [MHz] 4000 3500 3000 2500 2000 500 8500 8000 7500 7000 6500 6000 5500 5000 4000 3500 3000 -36 2500 -33 -3.6 2000 -30 -3.3 1500 -27 -3 500 -24 -2.7 1000 -2.4 1500 -21 ` 1000 -2.1 4500 [dB] Insertion Loss 0 Frequency [MHz] Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 15 of 17 Mounting Configuration: In order for Xinger surface mount components to work optimally, the proper impedance transmission lines must be used to connect to the RF ports. If this condition is not satisfied, insertion loss, Isolation and VSWR may not meet published specifications. All of the Xinger components are constructed from ceramic filled PTFE composites which possess excellent electrical and mechanical stability having X and Y thermal coefficient of expansion (CTE) of 17 ppm/oC. An example of the PCB footprint used in the testing of these parts is shown below. An example of a DC-biased footprint is also shown below. In specific designs, the transmission line widths need to be adjusted to the unique dielectric coefficients and thicknesses as well as varying pick and place equipment tolerances With No DC Bias 3X .011 [0.29] .008 [0.21] 3X .011 [0.29] With DC Bias Plated thru hole to ground .014 [0.36] .008 [0.21] .020 [0.50] 3X Transmission Line .020 [0.50] Circuit Pattern .008 [0.21] 3X .011 [0.29] .014 [0.36] .008 [0.21] .020 [0.50] 3X Transmission Line .020 [0.50] Circuit Pattern Footprint Pad (s) Solder Resist Plated thru hole to ground 3X .011 [0.29] Footprint Pad (s) Dimensions are in Inches [Millimeters] Mounting Footprint Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 Solder Resist Dimensions are in Inches [Millimeters] Mounting Footprint 0404 Balun Application note (ANN-2001) Page 16 of 17 Packaging and Ordering Information Parts are available in reel and are packaged per EIA 481-2. Parts are oriented in tape and reel as shown below. Minimum order quantities are 4000 per reel. See Model Numbers below for further ordering information. ØA ØC ØD QUANTITY/REEL 4000 TABLE 1 REEL DIMENSIONS (inches [mm]) ØA 7.00 [177.8] B 0.32 [8.0] 2.0 [50.8] ØC ØD 0.512 [13.0] B Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369 0404 Balun App. Note 0404 Balun Application note (ANN-2001) Page 17 of 17 Sales Desk USA: Voice: (800) 544-2414 Fax: (315) 432-9121 Sales Desk Europe: Voice: (+44) 2392 232392 Fax: (+44) 2392 251369