Typical Measurement
8
SNAU235 – May 2018
Copyright © 2018, Texas Instruments Incorporated
LMX2572LPEVM Evaluation Instructions
Figure 8. Phase Adjustment
3.2.2
Calibration-Free Automatic Ramping
The LMX2572LP supports linear frequency ramp without the need of VCO calibration in the middle of the
ramp. The output waveform is a continuous frequency sweep between the start and the end frequencies.
However, the frequency ramp range is limited. When using ramp, these parameters must be set
accordingly:
•
OUT_FORCE = 1
•
LD_DLY = 0
•
PLL_DEN = 2
32
– 1
Table 3. Calibration-free Automatic Ramp Example
PARAMETER
EXAMPLE VALUE
Ramping start frequency
4795 MHz
Ramping stop frequency
4805 MHz
Phase detector frequency
50 MHz
Ramp up / down time
200 µs
RAMP_LIMIT_HIGH
4995 MHz
RAMP_LIMIT_LOW
4595 MHz
This is a triangular ramp example. Ramp up is defined by RAMP0 while ramp down is defined by RAMP1.
RAMP_THRESH, RAMP_DLY_CNT, and RAMP_SCALE_COUNT are set to "don't care" because there is
no plan to trigger VCO calibration. RAMP_MANUAL = 0 means Automatic Ramping mode.
Set RAMP_EN = 1 to start ramping. Set RAMP_EN = 0 to turn off ramping.
