8.3.11 Phase Adjust
The MASH_SEED word can use the sigma-delta modulator to shift output signal phase with respect to the input reference. If a SYNC pulse is sent (software or pin) or the MASH is reset with MASH_RST_N, then this phase shift is from the initial phase of zero. If the MASH_SEED word is written to, then this phase is added. Use Equation 5 to calculate the phase shift.
Equation 5. Phase shift in degrees = 360 × ( MASH_SEED / PLL_DEN) × ( IncludedDivide / CHDIV )
Mash seed = 1
Denominator = 12
Channel divider = 16
Phase shift (VCO_PHASE_SYNC = 0) = 360 × (1/12) × (1/16) = 1.875 degrees
Phase Shift (VCO_PHASE_SYNC = 1) = 360 × (1/12) × (4/16) = 7.5 degrees
There are several considerations with phase shift with MASH_SEED:
- Phase shift can be done with a FRAC_NUM = 0, but MASH_ORDER must be greater than zero. For MASH_ORDER = 1, the phase shifting only occurs when MASH_SEED is a multiple of PLL_DEN.
- For the phase adjust, the condition PLL_DEN > PLL_NUM + MASH_SEED must be satisfied.
- When MASH_SEED and Phase SYNC are used together with IncludedDivide > 1, additional constraints may be necessary to produce a monotonic relationship between MASH_SEED and the phase shift, especially when the VCO frequency is below 10 GHz. These constraints are application specific, but some general guidelines are to reduce modulator order and increase the N divider. One possible guideline is for PLL_N ≥ 45 (2nd order modulator), PLL_N ≥ 49 (3rd Order modulator), PLL_N ≥ 54 (4th Order Modulator).