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SP8855DIGHCAR Datasheet(PDF) 3 Page - Zarlink Semiconductor Inc |
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SP8855DIGHCAR Datasheet(HTML) 3 Page - Zarlink Semiconductor Inc |
3 / 14 page SP8855D 11 FROM CHARGE PUMP REFERENCE C1 C2 R2 TO VCO FROM CHARGE PUMP OUTPUT Fig. 8 Third order loop filter circuit diagram – + Loop Filter Design Generally the third order filter configuration shown in Fig.7 gives better results than the more commonly used second order because the reference sidebands are reduced. Three equations are required to determine values for the three constants where; t1 = C1 t2 = R2 (C1 + C2) t3 = C2 R2 The equations are; 1 t 2 + 1 w n 2 t 3 2 t 3 + * tan F 0 ) 1 cos F 0 wn 3 t 1 + Kf K0 N wn2 1 ) wn 2 t 2 2 1 ) wn 2 t 3 2 ½ Where; Kf is the phase detector gain factor in mA/radian K0 is the VCO gain factor in radian/second/Volt N is the total division ratio from VCO to reference frequency wn is the natural loop bandwidth F0 is the phase margin normally set to 45° Since the phase detector is linear over a range of 2 p radian, Kf can be calculated from Kf = Phase comparator current setting/2p mA/radian These values can now be substituted in equation 1 to obtain a value for C1 and equation 2 and 3 used to determine values for C2 and R2 EXAMPLE Calculate values for a loop with the following parameters Frequency to be synthesised: 1000MHz Reference frequency 10MHz Division ratio 1000MHz/10MHz = 100 wn natural loop frequency 100kHz K0 VCO gain factor 2 p x 10MHz/Volt F0 phase margin 45 ° Phase comparator current 6.3mA The phase detector gain factor Kf = 6.3mA /2 p = 1mA/radian From equation 3: t 3 + * tan 45° ) 1 cos 45 ° 100kHz 2 p + 0 . 4142 628319 From equation 2: t 2 + 1 (100kHz 2 p)2 659 10*9 t 2 + 3 . 844 10*6 Using these values in equation 1: t 1+ 1x10 *3 2 p 10MHz V 100 (2 p 100kHz)2 [A] ½ t3 + 659 10*9 Where A is : t 1 + 1. 59 10*9 x 2 . 415 t 1 + 3. 84 10*9 Now t 1 + C1 N C1 + 3 . 84nF t 2 + R2 (C1 ) C2) t 3 + C2 R2 t 1 + 62832 39 . 48 1012 6. 833 1 . 1714 ½ 1 ) wn2 t22 1 ) wn2 t32 + 1 ) (2 p 100k Hz) 2 (3 . 844 x 10*6) 2 1 ) (2p 100k Hz) 2 (659 x 10 *9) 2 Substituting for C2 t 2 + R 2 C1 ) t3 R2 N t 2 + R2 C1 ) t3 NR2 + t2 * t3 C1 + 3.844 10*6 * 659 10*9 9.61 10*9 R2 + 829 .4W t3 + C2 R2 N C2 + t3 R2 + 659 10*9 829 . 4 C2 + 0.794nF |
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