Application Bulletin Comparing Vectron’s Crystal-Based 125MHz Oscillator with a SAW-based solution Jitter is a difficult parameter to clearly understand and compare by reviewing manufactures data sheets. Although largely unintentional, variations in jitter specifications include the method of test and equipment used as well as the number of samples acquired. In order to alleviate this, a side by side test was performed using the same test equipment, methods, fixturing and number of samples of Vectron’s crystal-based 125MHz CMOS oscillator, VCC1-B3A-125M000, versus a equivalent SAW-based 125.000MHz CMOS oscillator. The following jitter results were measured using a Tektronix TDS7254D and differential probe, acquiring 25,000 samples. Period jitter compares the length of each cycle to the average period of an ideal clock using the long term averaged frequency. SAW-based Vectron Period Jitter rms, ps 1.103 0.962 Period Jitter p/p, ps 8.716 8.160 Cycle to cycle jitter compares the difference in the cycle length of adjacent cycles. SAW-based Vectron Cycle to Cycle Jitter rms, ps 1.744 1.665 Cycle to Cycle Jitter p/p, ps 15.795 14.842 TIE or accumulated jitter is the variation in a clock’s transition from its ideal position over many cycles. SAW-based Vectron Accumulated Jitter rms, ps 2.800 2.377 Accumulated Jitter p/p, ps 18.766 15.219 While data shows excellent results for both devices, the crystal based VCC1 series has an advantage in all three measurements; period, cycle to cycle and TIE jitter. Another key parameter for reference clocks is temperature stability. Unlike VCXO’s, which are used in a self-correcting PLL’s rendering temperature drift negliable, temperature stability Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1• http://www.vectron.com becomes more critical in fixed frequency clocks. The graph below shows relatively good stability in the -20 to 70˚C range, and while the SAW based device meets rated specification, performance quickly degrades below -25˚C. Vectron versus Epson 125.000 MHz Frequency, PPM 40 20 -60 -40 0 -20 0 -20 20 40 60 80 100 -40 -60 -80 -100 Temperature, C Compare this to the crystal-based VCC1 performance curve, which is the data in blue, and advantages over extended temperature become apparent. In fact even +/-20ppm – including aging – can be provided which is Vectron’s VCC1-103 and VCC1-107 series. The third critical characteristic which effects stability and jitter performance is power supply sensitivity. Measurements were made under DC conditions and the power supply was varied by +/-10% and then compared to measurements made at 3.3volts. The results are graphed below and the advantage of the crystal based VCC1 solution is almost 5 times better. For tight stability/low cost/low jitter applications, a crystal based solution should be the preferred choice. Change in Output Frequency versus 10% Change in Supply Voltage Vectron VCC1 1 SAW based 0 2 4 6 8 10 change in ppm For Additional Information Please Contact: USA: Tel: 1-88-VECTRON-1 • Fax: 1-888-FAX-VECTRON EUROPE: Tel: 49 (0) 3328 4784 17 · Fax: 49 (0) 3328 4784 30 ASIA: Tel: +86 21 28909740 / 41 / 42 · Fax: +86 21 28909240 / 2890999 Vectron International reserves the right to make changes to the product(s) and/or information contained herein without notice. No liability is assumed as a result of their use or application. No rights under any patent accompany the sale of any such product(s) or information. June 15, 2004