SNAS764 May 2018 LMX2572LP
PRODUCTION DATA.
Manual ramping is enabled when the user sets RAMP_EN = 1 and RAMP_MANUAL = 1. In this mode, the ramp is clocked by the rising edges applied to the RampClk pin. The size of the frequency change is defined by RAMP0_INC and RAMP1_INC. If a LOW is seen at the RampDir pin on the rising edge of RampClk, the output frequency will be incremented by RAMP0_INC. On the contrary, the output frequency will be incremented by RAMP1_INC if a HIGH is captured. If a rising edge is seen on the RampClk pin while the VCO is calibrating, then this rising edge is ignored. The frequency for the RampClk must be limited to a frequency of 250 kHz or less, and the rising edge of the RampDir signal must be targeted to the falling edge of the RampClk pin. The necessary register fields for use in manual ramping mode are shown in Table 137.
REGISTER FIELD | VALUE | DESCRIPTION |
---|---|---|
RAMP_EN | 1 = Enable ramp | Set this bit to 1 to enable frequency ramping. |
RAMP_MANUAL | 1 = Manual ramping mode | To select manual ramping mode, set this bit to 1. |
RAMP_LIMIT_HIGH | Greater than the highest VCO ramp frequency | This sets the upper ramp limit that the ramp cannot go above. Suppose fHigh is this frequency and fStart is the starting VCO ramp frequency, then, for fHigh> fStart,
RAMP_LIMIT_HIGH = 224 × (fHigh – fStart) / fPD |
RAMP_LIMIT_LOW | Smaller than the lowest VCO ramp frequency | This sets the lower ramp limit that the ramp cannot go below. Suppose fLow is this frequency and fStart is the starting VCO ramp frequency, then, for fStart> fLow,
RAMP_LIMIT_LOW = 233 – 224 × (fStart – fLow) / fPD |
RAMP0_INC
RAMP1_INC |
Equal to the ramp size | Suppose the ramp size is Δf, then
RAMPx_INC = (Δf / fPD) × 224 or = 230 – (Δf / fPD) × 224 if Δf is a negative number. |
RAMP_THRESH | Suggest less than 50 MHz | If the amount of frequency ramp exceeds this threshold, a VCO calibration will be initiated. For example, if the ramp size is 50 MHz while this threshold is 30 MHz, then VCO calibration will be executed every time it ramps. Suppose the threshold frequency is fTH, then
RAMP_THRESH = (fTH / fPD) × 224 |
RAMP_TRIGA
RAMP_TRIGB |
1 = RampClk rising edge trigger | In manual ramping mode, the ramp is triggered by the rising edges applied to the RampClk pin.
Either RAMP_TRIGA or RAMP_TRIGB can be selected as the trigger source for the next ramp. |
RAMP0_NEXT_TRIG
RAMP1_NEXT_TRIG |
Equal to the selected RAMP_TRIGx | These fields define what triggers the next ramp. They must be set to the same trigger source selected above. |