Si5345-44-42-D-RM
12
Rev. 1.0
3.1. Dividers
There are five divider classes within the Si5345/4/2. See Figure 1 for a block diagram that shows all of these
dividers.
Wide range input dividers P3, P2, P1, P0
MultiSynth divider
48 bit numerator, 32 bit denominator
Min value is 1
Practical range limited by phase detector and VCO range
Each divider has an update bit that must be written to cause a newly written divider value to take effect.
Narrow range input divider Pxaxb
Only divides by 1, 2, 4, 8
Feedback M divider
MultiSynth divider
Integer or fractional divide values
56 bit numerator, 32-bit denominator
Practical range limited by phase detector and VCO range
Each divider has an update bit that must be written to cause a newly written divider value to take effect.
Output N divider
MultiSynth divider
Integer or fractional divide values
44 bit numerator, 32 bit denominator
Each divider has an update bit that must be written to cause a newly written divider value to take effect.
Output R divider
Only even integer divide values
Min value is 2
Maximum value is 2
25
– 2
3.2. DSPLL Loop Bandwidth
The DSPLL loop bandwidth determines the amount of input clock jitter attenuation and wander filtering. Register
configurable DSPLL loop bandwidth settings in the range of 0.1 Hz to 4 kHz are available for selection. Since the
loop bandwidth is controlled digitally, the DSPLL will always remain stable with less than 0.1 dB of peaking
regardless of the loop bandwidth selection. The DSPLL loop bandwidth is set in registers 0x0508-0x050D and are
determined using ClockBuilder Pro.
The higher the PLL bandwidth is set relative to the phase detector frequency (F
pfd
), the more chance that F
pfd
will
cause a spur in the Phase Noise plot of the output clock and increase the output jitter. To guarantee the best phase
noise/jitter it is recommended that the normal PLL bandwidth be kept less than F
pfd
/160 although ratios of F
pfd
/100
will typically work fine.
Table 2. PLL_BW Registers
Register Name
Hex Address [Bit
Field]
Function
BW_PLL
0x0508[7:0]-
0x050D[7:0]
Determines the loop BW for the DSPLL.