LMX2572LP datasheet(65/83 Pages) TI2

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1 / fPD

Configurable

pause time

Accumulated

ramp frequency.

RAMP_THRESH

Total ramp time = Desired ramp duration + sum of all pause time.

RAMP_THRESH

RAMPx_INC

LMX2572LP

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Table 138. Automatic Ramping Mode Programming (continued)

VALUE

DESCRIPTION

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_COU

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

Figure 162. Auto Ramp Waveform

8.1.8 Application for FSK

In fractional mode, the finest delta frequency difference between two programmable output frequencies is equal

to Equation 7:

f1 – f2 = Δfmin = fPD × {[(PLL_N + 1) / PLL_DEN] – (PLL_N / PLL_DEN)} = fPD / PLL_DEN

(7)

In other words, when the fractional numerator is incremented by 1 (one step), the output frequency will change

by Δfmin. A two steps increment will therefore change the frequency by 2 × Δfmin.

In FSK operation, the instantaneous carrier frequency is kept changing among some pre-defined frequencies. In

general, the instantaneous carrier frequency is defined as a certain frequency deviation from the nominal carrier

frequency. The frequency deviation could be positive and negative.

LMX2572LP Datasheet(PDF) 65 Page - Texas Instruments

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