LMX2572LP datasheet(64/82 Pages) TI1

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RAMPx_INC

LMX2572LP

SNAS764 – MAY 2018

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Table 137. Manual Ramping Mode Programming (continued)

VALUE

DESCRIPTION

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.

Figure 161. Manual Ramp Waveform

8.1.7.2 Automatic Ramping Mode

Automatic ramping mode is enabled when RAMP_EN = 1 with RAMP_MANUAL = 0. In this mode, there are two

ramps profiles that one can use to set the length and frequency change. In addition to this, there are ramp limits

that can be used to create more complicated waveforms. The output frequency will ramp once on each phase

detector cycle.

Automatic ramping can really be divided into two classes depending on whether the VCO must calibrate in the

middle of the ramping or not. If the VCO can go the entire range without calibrating, this is calibration-free

ramping. Note that this range is less at hot temperatures and less for lower frequency VCOs. This range is not

ensured, so margin must be built into the design.

For ramping that are not calibration free, the ramp waveform is more like a staircase ramp.

Table 138. Automatic Ramping Mode Programming

VALUE

DESCRIPTION

RAMP_EN

1 = Enable ramp

Set this bit to 1 to enable frequency ramping.

RAMP_MANUAL

0 = Automatic

ramping mode

To select automatic ramping mode, set this bit to 0.

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 lowest 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) × 2

24 or = 230 – (Δf / f

PD) x 2

24 if Δf is a negative number.

RAMP_THRESH

Suggest less than 50

MHz

If the amount of frequency ramp exceed this threshold, a VCO calibration will be initiated.

For example, if the ramp size is 15 MHz while this threshold is 20 MHz, then VCO

calibration will be executed every two ramps. Suppose the threshold frequency is fTH, then

RAMP_THRESH = (fTH / fPD) × 2

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