SNAS805 June 2020 LMK61E08
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. Because dithering also adds phase noise, its level needs to be managed to achieve acceptable phase noise and spurious performance.
Table 4 summarizes spur and mitigation techniques.
| 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 | Reducing the loop bandwidth may degrade the total integrated noise if the bandwidth is too narrow. |
| - 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. | |||
| Methods for VCO Dominated Spurs | Reducing the phase detector may degrade the phase noise. | ||
| - Avoid the worst case VCO frequencies if possible. | |||
| - Reduce Phase Detector Frequency. | |||
| - Ensure good slew rate and signal integrity at reference input. | |||
| Primary Fractional | fPD/DEN | - Decrease Loop Bandwidth. | Decreasing the loop bandwidth may degrade in-band phase noise. Also, larger unequivalent fractions don’t always reduce spurs. |
| - Change Modulator Order. | |||
| - Use Larger Unequivalent Fractions. | |||
| Sub-Fractional | fPD/DEN/k k=2,3, or 6 | - Use Dithering. | Dithering and larger fractions may increase phase noise. |
| - Use Larger Equivalent Fractions. | |||
| - Use Larger Unequivalent Fractions. | |||
| - Reduce Modulator Order. | |||
| - Eliminate factors of 2 or 3 in denominator. |