JAJSFA5E September 2015 – April 2018 LMK03318
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 6.
ORDER | APPLICATIONS |
---|---|
Integer Mode (Order = 0) | If the fractional numerator is zero, it is best to run the PLL in integer mode 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, so it always makes sense in these situations. When the fractional denoninator is between 6 and about 20, consider using the first order modulator because the spurs might be far enough outside the loop bandwidth that they will be 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 will have spurs in the same offsets, so the third is generally better for spurs. However, if stronger levels of dithering is used, the third order modulator will create more close-in phase noise than the second order modulator. |
Figure 59 and Figure 60 give 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 what one would theoretically expect if strong dithering was used for a well-randomized fraction. Dithering can be set to different levels or even disabled and the noise can be eliminated. In terms of spurs, they can change based on fraction, but they will theoretically pushed out to higher phase detector frequencies. However, one must be aware that these are just THEORETICAL graphs and for offsets that are less than 5% of the phase detector frequency, other factors can impact the noise and spurs. In Figure 59, 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 60 shows the impact of the phase detector frequency on the modulator noise.