JAJSFI5 May 2018 LMX2572LP
PRODUCTION DATA.
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.
REGISTER FIELD | 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) × 224 or = 230 – (Δf / fPD) x 224 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) × 224 |
RAMP0_LEN
RAMP1_LEN |
0 to 216 | Set the number of ramp required in each ramp profile. Maximum value is 216. If this number is exceeded, enable the RAMPx_DLY bit or reduce the phase detector frequency.
RAMPx_LEN = Ramp duration of a ramp profile × fPD |
RAMP0_DLY
RAMP1_DLY |
0 or 1 | If this bit is set to 1, the output frequency will ramp every two fPD cycles. |
RAMP0_NEXT
RAMP1_NEXT |
Equal to the next ramp | Set the next ramping profile when the present profile is finished. |
RAMP0_NEXT_TRIG
RAMP1_NEXT_TRIG |
0 = RAMP_LENx time out counter | Set these bits to 0 in order to start the next ramp immediately after the previous ramp. |
RAMP0_RST
RAMP1_RST |
0 or 1 | If the stop frequency of the present ramp profile is different from the start frequency of the next ramp profile, set this bit to 1. |
RAMP_SCALE_COUNT
RAMP_DLY_CNT |
Suggest a minimum pause time of 50 µs | These two register fields set the minimum pause time when RAMP_THRESH is hit. This pause time must be sufficient to allow the VCO to complete a calibration, otherwise it will be overwritten by the actual VCO calibration time.
Minimum pause time = RAMP_DLY_CNT × 2RAMP_SCALE_COUNT × 2CAL_CLK_DIV / fOSCin |