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FS6127 Datasheet(PDF) 2 Page - List of Unclassifed Manufacturers |
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FS6127 Datasheet(HTML) 2 Page - List of Unclassifed Manufacturers |
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2 / 6 page  XT February 1999 2 2.4.99 )6 9&;2 &ORFN*HQHUDWRU,& $GYDQFH ,QIRUPDWLRQ ,62 Table 2: Pin Descriptions Key: AI = Analog Input; AO = Analog Output; DI = Digital Input; DI U = Input with Internal Pull-Up; DI D = Input with Internal Pull-Down; DIO = Digital Input/Output; DI-3 = Three-Level Digital Input, DO = Digital Output; P = Power/Ground; # = Active Low pin PIN TYPE NAME DESCRIPTION 1 AI XIN VCXO Crystal Feedback 2 P VDD Power Supply (+5V) 3 AI XTUNE VCXO Tune 4 P VSS Ground 5 DO CLK Clock Output 6 P VDD Power Supply (+5V) 7 P VSS Ground 8 AO XOUT VCXO Crystal Drive 3.0 Functional Block Description 3.1 Phase-Locked Loop (PLL) The on-chip PLL is a standard frequency- and phase- locked loop architecture. The PLL multiplies the reference oscillator to the desired frequency by a ratio of integers. The frequency multiplication is exact with a zero synthe- sis error. 3.2 Voltage-Controlled Crystal Oscillator (VCXO) The VCXO provides a tunable, low-jitter frequency refer- ence for the rest of the FS6127 system components. Loading capacitance for the crystal is internal to the FS6127. No external components (other than the crystal resonator itself) are required for operation of the VCXO. Continuous fine-tuning of the VCXO frequency is accom- plished by varying the voltage on the XTUNE pin. The total change (from one extreme to the other) in effective loading capacitance is t.b.d. nominal. The oscillator operates the crystal resonator in the paral- lel-resonant mode. Crystal warping, or the “pulling” of the crystal oscillation frequency, is accomplished by altering the effective load capacitance presented to the crystal by the oscillator circuit. The actual amount that changing the load capacitance alters the oscillator frequency will be dependent on the characteristics of the crystal as well as the oscillator circuit itself. Specifically, the motional capacitance of the crystal (usu- ally referred to by crystal manufacturers as C1), the static capacitance of the crystal (C0), and the load capacitance (CL) of the oscillator determine the “warping” or “pulling” capability of the crystal in the oscillator circuit. A simple formula to obtain the warping capability of a crystal oscillator is: () () ( ) C C C C C C C ppm f L L L L 1 0 2 0 6 1 2 1 2 10 ) ( + × + × × − × = ∆ where CL1 and CL2 are the two extremes of the applied load capacitance. EXAMPLE: A crystal with the following parameters is used. With C1 = 0.02pF, C0 = 5pF, CL1 = 10pF, and CL2 = 22.66pF, the tuning range is () () ( ) ppm . . . f 305 10 5 66 22 5 2 106 10 66 22 02 0 = + × + × × − × = ∆ . |
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