Crystal Filter Yields Pure Sine Waves Application Note June 1998 Introduction 2 V 2 T 3.3 R ≥ ---------------- = ----------- K ≈ 2.2K P MAX 5 Electronic applications such as distortion and communications measurements require pure (distortionless) sine waves as input test signals. Distortion contained in test signals causes two problems. First, the test signal distortion content must be calibrated so it can be subtracted out of the measurement. Second, processing a distorted test signal usually creates unique harmonics which cause false readings because they can’t be calibrated out. R and C1 form a low pass filter which kills the high frequency response and filters out noise. The -3dB point for R, C1 should be set at one tenth the crystal frequency or 5MHz/10 = 500KHz. C1 is calculated from Equation 2. 1 1 C 1 = -------------- = --------------------------------------------------- = 144pF ≈ 150pF 2πfR 2π ( 5EE5 )(2.2EE3) TEST SIGNAL R 2.2k C1 150pF (EQ. 2) Load capacitance changes tend to pull the crystal, and C3 should be large so changes are a small percentage of C3 . C3 is usually selected as approximately 3 times C1 . Let C3 = 510pF. The series combination of C1 , C2 , and C3 should equal CL = 32pF specified by the crystal vendor. (EQ. 3) 1 1 1 1 ------- = ------- - ------- - ------C2 CL C1 C3 The crystal is surrounded by a π network which preserves the crystals Q, and makes it less susceptible to loading. The crystal must be parallel resonant to function with the π circuit. The manufacturer specifies the crystal loading capacitance, CL, and the maximum input power, PMAX . The series resistor, R, limits the crystal power, and although it is an optimistic approximation, Equation 1 can be used to select R. 5MHz CRYSTAL (EQ. 1) For V T = 3.3V and P MAX = 5mW Signal sources which approach distortionless operation are available, but they are expensive, bulky, hard to use, and overkill for simple applications. The crystal filter described here utilizes any inexpensive sine wave generator to supply the test signal, and it filters the distortion out of the test signals prior to the measurement. This crystal filter is inexpensive, simple to design, and it reduces 2nd harmonic distortion by 70dB. Configured as shown in the schematic with the HFA1112 buffer, it can drive back terminated 50Ω loads while reducing distortion 63dB. Furthermore, driving the crystal filter with a square wave test signal only increases the 2nd harmonic content by a fraction of a dB. C2 43pF AN9815 C2 calculates 44.2pF, so it was selected as 43pF. With the component values shown in the schematic, the -6dB bandwidth is 144Hz, and this equates to a crystal Q exceeding 45,000. The crystal filter must be constructed using a ground plane and other similar high frequency techniques. Reducing the value of C2 by 10pF, and adding a 20pF variable capacitor in parallel with C2 yields a 0.1% adjustment of the filter’s center frequency. This adjustment range can compensate for normal manufacturing tolerances. +5V HFA1112 7 3 + 6 C3 510pF 2 4 RT FILTERED TEST SIGNAL -5V FIGURE 1. CRYSTAL FILTER 1 1-888-INTERSIL or 321-724-7143 | Copyright © Intersil Corporation 1999 Application Note 9815 All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification. Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see web site www.intersil.com Sales Office Headquarters NORTH AMERICA Intersil Corporation P. O. Box 883, Mail Stop 53-204 Melbourne, FL 32902 TEL: (321) 724-7000 FAX: (321) 724-7240 EUROPE Intersil SA Mercure Center 100, Rue de la Fusee 1130 Brussels, Belgium TEL: (32) 2.724.2111 FAX: (32) 2.724.22.05 2 ASIA Intersil (Taiwan) Ltd. 7F-6, No. 101 Fu Hsing North Road Taipei, Taiwan Republic of China TEL: (886) 2 2716 9310 FAX: (886) 2 2715 3029