Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 nAN24-17 Application Note v1.0 All rights reserved. Reproduction in whole or in part is prohibited without the prior written permission of the copyright holder. 2011-05-18 nAN24-17 Contents 1 2 3 3.1 3.2 3.3 4 Introduction ................................................................................................. Antenna matching network overview........................................................ Implementation............................................................................................ Schematic ............................................................................................. Layout ................................................................................................... Measurement results on the 2450BM14A0002 balun........................... Conclusion................................................................................................... Revision 1.0 Page 2 of 11 3 4 5 5 5 6 10 Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 1 Introduction This application note describes how to use Johanson Technology’s 2450BM14A0002 balun (an SMD sixpin device with a small footprint of 1.6x0.8 mm (EIA 0603)) with the nRF24LE1 QFN32. The 2450BM14A0002 balun greatly simplifies the RF front-end by considerably reducing component count, system variability, implementation size area, and PCB sensitivity. Johanson Technology’s 2450BM14A0002 balun was originally designed for the nRF24L01(+) transceivers, and a technical note on how to use it with nRF24L01(+) can be downloaded from Johanson Technology’s website (http:// www.johansontechnology.com/en/technical-notes.html) Since nRF24LE1 uses nRF24L01+ as its radio core, it means the 2450BM14A0002 balun can also be used on some of the nRF24LE1 variants. nRF24LE1 has a different packet size to the nRF24L01(+) standalone chips, so the length of the bonding wires is different. This means the impedance load on the antenna pins is different for nRF24LE1 when compared to nRF24L01(+). This application note describes how to compensate for this difference. Note: This application note does not apply to nRF24LE1 QFN24 or nRF24LE1 QFN48. The 2450BM14A0002 balun is not suitable for these devices because of the different packet sizes. Revision 1.0 Page 3 of 11 nAN24-17 2 Antenna matching network overview The antenna matching network has four main tasks: • • • • Balanced to unbalanced tranformation (balun) Impedance transformation from a high impendance PA (Power Amplifier) load to 50 Ω Low pass filtering to suppress harmonics Power supply feed to the PA All these tasks are handled by the antenna matching network’s three inductors and three capacitors, see Figure 1.. The layout is critical since parasitic capacitance and inductance can be high. L3 ANT2 ANT1 VDD_PA J1 C5 4.7n L1 6.8n 1.5p C6 L2 1.0p GND 6.8n C3 C4 2.2nF NA GND GND GND Figure 1. Standard antenna matching network If space is limited, you can replace the antenna matching network with a single component; an integrated balun. This performs the same tasks as the discrete balun, but with a single component only. Revision 1.0 Page 4 of 11 Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 3 Implementation The balun replaces all RF components in the antenna matching network and performs the same tasks as the antenna matching network (see Figure 1. on page 4) but with reduced space requirements. 3.1 Schematic The schematic of the nRF24LE1 QFN32 and the 2450BM14A0002 balun are shown in Figure 2.. XC1 XC2 VCC_nRF P1.5 P1.6 P0.0 C11 100nF GND 22k 1% U1 GND GND VCC_nRF C10 33nF 1 C3 C4 2.2nF N.A GND B1 2450BM14A0002 GND 6 DC GND NC Unbal 2 3 Bal2 Bal1 =Value 5 nRF24LE1 24 23 22 21 20 19 18 17 J1 GND P1.3 P1.4 RESET GND C8 33nF VDD VSS ANT2 ANT1 VDD_PA RESET P1.4 P1.3 VDD P0.4 P0.5 P0.6 P0.7 P1.0 P1.1 P1.2 GND C7 100nF P0.1 VDD DEC1 DEC2 P0.2 PROG P0.3 VSS 9 10 11 12 13 14 15 16 C9 100nF P0.2 PROG P0.3 1 2 3 4 5 6 7 8 4 P0.1 P0.4 P0.5 P0.6 P0.7 P1.0 P1.1 P1.2 VCC_nRF Ant RF I/O GND P0.0 XC1 XC2 P1.6 P1.5 VDD VSS IREF 32 31 30 29 28 27 26 25 R1 GND Figure 2. nRF24LE1 QFN32 schematic and the balun The capacitor C3 (2.2nF) is needed to filter and decouple the power supply to the PA. All other matching network components are replaced by the balun B1. You can connect the 50 Ω unbalanced output directly to a 50 Ω antenna, or to the antenna match. 3.2 Layout The nRF24LE1 QFN32 has shorter internal bonding wires than nRF24L01(+), so to compensate for this difference, the balun is placed 2.34 mm away from nRF24LE1. This is a longer distance than used for the nRF24L01(+) standalone chipsets and this extra length increases the inductance of the tracks from the ANT1 and ANT2 pins to the balun, creating a more optimal load for nRF24LE1. See Figure 3. on page 6. Revision 1.0 Page 5 of 11 nAN24-17 Figure 3. Balun layout Note: The distance is measured from the edge of the nRF24LE1 pad to the edge of the balun pad. 3.3 Measurement results on the 2450BM14A0002 balun Figure 4. on page 7 shows the output power, measured on the 50 Ω unbalanced port on the balun. The carrier is swept across the 2.4 GHz band (markers are placed at 2.40 GHz, 2.44 GHz and 2.48 GHz). Figure 5. on page 7 to Figure 7. on page 8 shows the harmonic components (markers are placed at maximum power). Figure 8. on page 9 shows the LO leakage in RX mode (a marker is placed at the maximum power). Revision 1.0 Page 6 of 11 Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 Figure 4. Output power, carrier sweep Figure 5. 2nd harmonic, carrier sweep Revision 1.0 Page 7 of 11 nAN24-17 Figure 6. 3rd harmonic, carrier sweep Figure 7. 4th harmonic, carrier sweep Revision 1.0 Page 8 of 11 Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 Figure 8. RX LO, channel sweep Frequency (GHz) 2.40 2.44 2.48 Measurement level (dBm) 0.351 -0.552 -1.022 Pass ETSI/FCC? Yes Yes Yes Table 1.Output power levels on the 2450BM14A0002 balun Name 2nd harmonic 3rd harmonic 4th harmonic RX LO Frequency (GHz) 4.802 7.299 9.876 2.761 Measurement level (dBm) -38.63 -54.95 -59.22 -55.24 Pass ETSI/FCC? Yes Yes Yes Yes Table 2.Harmonics and RX LO levels on the 2450BM14A0002 balun Revision 1.0 Page 9 of 11 nAN24-17 4 Conclusion As the measurement results show (see Table 2. on page 9), the output power is close to 0 dBm across the band. The harmonics are well below the regulation limits for the ETSI and FCC regulatory standards and so is the spurious in RX mode. The discrete antenna matching network has similar measurement results: Frequency (GHz) 2.400 2.480 Measurement level (dBm) 0.3 -0.6 Pass ETSI/FCC? Yes Yes Table 3.Output power levels on the discrete antenna matching network Name 2nd harmonic 3rd harmonic 4th harmonic RX LO Frequency (GHz) 4.802 7.299 9.876 2.761 Measurement level (dBm) -44.8 -54.9 -51 -57.4 Pass ETSI/FCC? Yes Yes Yes Yes Table 4.Harmonics and RX LO levels on the discrete antenna matching network These measurement results prove that the discrete balun can be replaced with an integrated balun without any performance loss. Revision 1.0 Page 10 of 11 Using a 2450BM14A0002 Balun with nRF24LE1 QFN32 Liability disclaimer Nordic Semiconductor ASA reserves the right to make changes without further notice to the product to improve reliability, function or design. Nordic Semiconductor ASA does not assume any liability arising out of the application or use of any product or circuits described herein. Life support applications Nordic Semiconductor’s products are not designed for use in life support appliances, devices, or systems where malfunction of these products can reasonably be expected to result in personal injury. Nordic Semiconductor ASA customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify Nordic Semiconductor ASA for any damages resulting from such improper use or sale. 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