Electronic Components Datasheet Search |
|
SP8855 Datasheet(PDF) 11 Page - Mitel Networks Corporation |
|
SP8855 Datasheet(HTML) 11 Page - Mitel Networks Corporation |
11 / 14 page 11 SP8855E A minimum value for the pull down resistor is 330 Ohms. When the F pd and F ref outputs are disabled the output level will be at the logic low level of about 3.5V so that the additional supply current due to the load resistors will be present even when the outputs are disabled. Reference input The reference input circuit functions as an input amplifier or crystal oscillator. When an external reference signal is used this is simply AC coupled to pin 28, the base of the input emitter follower. When a low phase noise synthesiser is required the reference signal is critical since any noise present here will be multiplied by the loop. To obtain the lowest possible phase noise from the SP8855E it is best to use the highest possible reference input frequency and to divide this down internally to obtain the required frequency at the phase detector. The amplitude of the reference input is also important, and a level close to the maximum will give the lowest noise. When the use of a low reference input frequency say 4-10MHz is essential some advantage may be gained by using a limiting amplifier such as a CMOS gate to square up the reference input. In cases where a suitable reference signal is not available, it may be more convenient to use the input buffer as a crystal oscillator in this case the emitter follower input transistor is connected as a Colpitts oscillator with the crystal connected from the base to ground and with the feedback necessary for oscillation provided by a capacitor tap at the emitter. The arrangement is shown inset in Fig. 5. FROM CHARGE PUMP OUTPUT C 1 C 2 R 2 TO VCO - + FROM CHARGE PUMP REFERENCE Fig. 8 third order loop filter circuit diagram Loop Filter Design Generally the third order filter configuration shown in Fig.8 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; τ 1 = C1 τ 2 = R 2 (C1 + C2) τ 3 = C 2 R2 The equations are 2 τ 2 = 1 ω n 2 τ 3 3 τ 3 = - tan φ ο + 1 cos ω n Where; K φ is the phase detector gain factor in mA/radian K 0 is theVCO gain factor in radian/second/Volt N is the total division ratio from VCO to reference frequency ω n is the natural loop bandwidth φ ο is the phase margin normally set to 45 ° Since the phase detector is linear over a range of 2 π radian, K φ can be calculated from K φ = Phase comparator current setting/2π mA/radian These values can now be substituted in equation 1 to obtain a value for C 1 and equation 2 and 3 used to determine values for C 2 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 ω n natural loop frequency 100KHz K 0 VCO gain factor 2 π x 10MHz/Volt φ 0 phase margin 45 ° Phase comparator current 6.3mA The phase detector gain factor K φ = 6.3mA /2 π = 1mA/radian 1 τ 1 = K φ K0 N ω n 2 1 + ω n 2 τ 2 2 1 + ω n 2 τ 3 2 1/2 φ ο |
Similar Part No. - SP8855 |
|
Similar Description - SP8855 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |