APPLICATION NOTE Voltage Detector Filter Optimization for the SKY65137 WLAN Power Amplifier Introduction Notch Filter The Skyworks SKY65137 Power Amplifier (PA) provides ideal features for Wireless Local Area Network (WLAN) applications. The device provides an output voltage signal, V_DET (pin 14), the voltage detector output filter. This signal can be used to control the input level of a signal source that is driving the RF_IN port (pin 18). In most applications, this is achieved by a baseband or controller circuit. Circuit Description A low frequency notch filter is recommended to remove the coupled signal from V_DET. Depending on the baseband or controller requirements, the coupled signal can be completely filtered, but with a trade-off in rise time of the V_DET signal. A schematic of the SKY65137 Evaluation Board is shown in Figure 2. The V_DET signal has two external components for the detector output filter, both shown with nominal values. However, a recommended notch filter would have three external components to completely filter out the coupled modulated signal. These three components are illustrated in Figure 3. When a modulated signal is used as an input to the SKY65137, the modulated signal is coupled on the V_DET signal, itself. Depending on the baseband or controller used in the application, this signal may have to be filtered. This is illustrated in Figure 1. This Application Note describes how to optimize the V_DET output filter for the SKY65137. Refer to the device Data Sheet (document number 200851) for additional electrical and mechanical details. Signal Source RF Input SKY65137 RF Output V_DET Signal Filter Oscilloscope S1562 Figure 1. SKY65137/Voltage Detector Block Diagram PA_EN VC_BIAS N/C N/c 1 GND 2 GND DC Supply2 19 18 17 GND 20 RF_IN N/C GND 1 2 3 4 5 6 GND J3 C5 8.2 pF VCC1 R7 0Ω N/C 3 4 Bias Voltage 15 V_DET VCC1 N/C L2 10 nH 14 Detector Output VC_BIAS VCC2 13 C10 8.2 pF N/C 7 × 8 9 C11 3300 pF 6 5 4 3 2 1 J4 C12 10 μF Collector Voltage (Stage 2) VCC3 DC Supply 1 12 R4 51 Ω GND 6 VDET_CNTL RF_OUT × N/C GND 5 V_DET VCC2 C9 8.2 pF C1 3300 pF × Collector Voltage (Stage 1) × SKY65137 VREF3 C17 10 μF PA_EN C7 10 μF 16 PA Enable C18 8.2 pF C6 3300 pF VCC3 11 C13 DNI 10 C14 8.2 pF C15 3300 pF Note: Some component labels may be different than the corresponding component symbol shown here. Component values, however, are accurate as of the date of this Data Sheet. C16 10 μF Collector Voltage (Stage 3) S1243 Figure 2. SKY65137 Evaluation Board Schematic Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201105A • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • July 15, 2009 1 APPLICATION NOTE • OPTIMIZING THE SKY65137 VOLTAGE DETECTOR FILTER 100 kΩ 220 nH V_DET 47 pF S1563 Figure 3. Recommended Notch Filter Topology Figure 4. V_DET Signal With No Filter Figure 5. Filtered V_DET Signal The IEEE 802.11a standard requires the V_DET signal to switch on and off. This switching has a rise and fall time, for which the baseband may impose timing requirements. Figure 4 plots the result of V_DET voltage measurements (in mV) over time (in μs) without filters. When this signal is filtered with the recommended notch filter, the resulting V_DET voltage measurements over time is shown in Figure 5. The filtered signal, however, has a rise time of 18 μs, which may not be acceptable to the baseband. Table 1 provides a list of resistor values against the rise time trade-off and voltage ripple. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 2 July 15, 2009 • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • 201105A APPLICATION NOTE • OPTIMIZING THE SKY65137 VOLTAGE DETECTOR FILTER Conclusion The nominal filter implemented on the reference design is appropriate for most applications. However if an application has more restrictions on the noise present on the voltage detector signal, a notch filter scheme can be implemented with much flexibility. Table 1. Notch Filter Series Resistor Values, and Corresponding Rise Time and Voltage Ripple V_DET Series Resistor Value (kΩ) Rise Time (μs) Voltage Ripple (mV) No Match <0.140 >200 1 0.146 100 10 1.45 50 20 3.00 20 50 9.30 15 100 18.0 10 Copyright © 2009 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. 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Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201105A • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • July 15, 2009 3