JAJSFA5E September 2015 – April 2018 LMK03318
PRODUCTION DATA.
These spurs appear at a fraction of fPD/DEN and depend on modulator order. With the first order modulator, there are no sub-fractional spurs. The second order modulator can produce 1/2 sub-fractional spurs if the denominator is even. A third order modulator can produce sub-fractional spurs at 1/2, 1/3, or 1/6 of the offset, depending if it is divisible by 2 or 3. For instance, if the phase detector frequency is 100 MHz and the fraction is 3/100, no sub-fractional spurs for a first order modulator or sub-fractional spurs at multiples of 1.5 MHz for a second or third order modulator would be expected. Aside from strategically choosing the fractional denominator and using a lower order modulator, another tactic to eliminate these spurs is to use dithering and express the fraction in larger equivalent terms. Since dithering also adds phase noise, its level must be managed to achieve acceptable phase noise and spurious performance.
Table 19 gives a summary of the spurs discussed so far and techniques to mitigate them.
SPUR TYPE | OFFSET | WAYS TO REDUCE | TRADE-OFFS |
---|---|---|---|
Phase Detector | fPD | Reduce Phase Detector Frequency. | Although reducing the phase detector frequency does improve this spur, it also degrades phase noise. |
Integer Boundary | fVCO mod fPD | Methods for PLL Dominated Spurs
-Avoid the worst case VCO frequencies if possible. -Ensure good slew rate and signal integrity at reference input. -Reduce loop bandwidth or add more filter poles to suppress out of band spurs. |
Reducing the loop bandwidth may degrade the total integrated noise if the bandwidth is too narrow. |
Methods for VCO Dominated Spurs
-Avoid the worst case VCO frequencies if possible. -Reduce Phase Detector Frequency. -Ensure good slew rate and signal integrity at reference input. |
Reducing the phase detector may degrade the phase noise. | ||
Primary Fractional | fPD/DEN | -Decrease Loop Bandwidth.
-Change Modulator Order. use Larger Unequivalent Fractions. |
Decreasing the loop bandwidth may degrade in-band phase noise. Also, larger unequivalent fractions don’t always reduce spurs. |
Sub-Fractional | fPD/DEN/k k=2,3, or 6 | use Dithering.
use Larger Equivalent Fractions. use Larger Unequivalent Fractions. -Reduce Modulator Order. -Eliminate factors of 2 or 3 in denominator. |
Dithering and larger fractions may increase phase noise. |