SNAS668E August 2015 – September 2024 LMK03328
PRODUCTION DATA
The programmable fractional modulator order gives the opportunity to better optimize phase noise and spurs. Theoretically, higher order modulators push out phase noise to farther offsets, as described in Table 8-5.
ORDER | APPLICATIONS |
---|---|
Integer Mode (Order = 0) | If the fractional numerator is zero, running the PLL in integer mode is best to minimize phase noise and spurs. |
First Order Modulator | When the equivalent fractional denominator is 6 or less, the first order modulator theoretically has lower phase noise and spurs, and is the preferred choice. When the fractional denominator is between 6 and about 20, consider using the first order modulator because the spurs can be far enough outside the loop bandwidth that the spurs are filtered. The first order modulator also does not create any sub-fractional spurs or phase noise. |
Second and Third Order Modulator | The choice between 2nd and 3rd order modulator tends to be a little more application specific. If the fractional denominator is not divisible by 3, then the second and third order modulators has spurs in the same offsets, so the third order modulator is typically better for spurs. However, if stronger levels of dithering is used, the third order modulator creates more close-in phase noise than the second order modulator. |
Figure 8-19 and Figure 8-20 provide an idea of the theoretical impact of the delta sigma modulator order on the shaping of the phase noise and spurs. In terms of phase noise, this is theoretically expected if strong dithering is used for a well-randomized fraction. Dithering can be set to different levels or even disabled and the noise can be eliminated. The spurs can change based on fraction, but the spurs can theoretically pushed out to higher phase detector frequencies. The following graphs are just theoretical and for offsets that are less than 5% of the phase detector frequency, other factors can impact the noise and spurs. In Figure 8-19, the curves all cross at 1/6th of the phase detector frequency and that this transfer function peaks at half of the phase detector frequency, which is assumed to be well outside the loop bandwidth. Figure 8-20 shows the impact of the phase detector frequency on the modulator noise.