Analog Devices ADRV9001 User Manual page 81

Preliminary Technical Data
EXT LO IN±
RF PLL Loop Filter Recommendations
For optimal phase noise and EVM performance, a lookup table of RF PLL loop filter bandwidth settings is implemented in ADRV9001
firmware. ADRV9001 automatically selects best RF PLL loop filter configuration based on LO frequency. Alternatively, user can program
its own RF PLL loop filter bandwidth following instruction outlined in Loop Filter Configuration paragraph.
PLL Phase Noise
The Figure 78 shows the typical PLL phase noise contributors. For low offset frequencies reference clock dominates the phase noise, and
for high offset frequencies, VCO noise dominates the phase noise. User can optimize the phase noise by:
Provide better reference clock source
•
Provide higher reference clock frequency (PFD)
•
Adjust loop filter bandwidth to trade-off between close-in band and far-out band noise
•
When changing the loop filter bandwidth, typically consideration is the wider the bandwidth, the better close-in band noise, but the
worse the far-out band noise. User should trade-off between the two to find the optimal setting for the specific application.
Following is an example that PLL phase noise is highly dependent on the PFD frequency (REF_CLK). With a higher PFD frequency, a
better phase noise can be achieved.
RF
VCO n = 1,2,3,...255
SINGLE-ENDED
EXT LO
n = 1,2,3,...255
DIFFERENTIAL
EXT LO
Figure 77. RF LO Generation Diagram
INPUT
REFERENCE
PHASE DETECTOR
AND CHARGE PUMP
VCO
–20dB/dec
F
LOOP
FREQUENCY OFFSET
Figure 78. PLL Phase Noise Contributors
Rev. PrA | Page 81 of 253
/1, /2n,
/2
/1, /2n,
INPUT REFERENCE NOISE
PHASE DETECTOR AND
CHARGE PUMP NOISE
VCO NOISE
AMPLITUDE NOISE
OVERALL PLL NOISE
SPUR
AMPLITUDE
NOISE
LOG F
OFFSET
UG-1828
RF LO