L M X 2 5 3 1 L Q 1 2 2 6 E E V A L U A T I O N B O A R D O P E R A T I N G I N S T R U C T I O N S
Free-Running VCO Phase Noise (Internal Divide by 2 Enabled)
Fout = 592
MHz (1184 MHz/2
)
Fout = 613
MHz (1226 MHz/2
)
Fout = 634
MHz (1268 MHz/2
)
The plots to the left show the true phase noise capability of the
VCO. In order to take these plots, the E5052 phase nose
analyzer was used. The method was to lock the PLL to the
proper frequency, then disable the EN_PLL, EN_PLLLDO1, and
EN_PLLLDO2 bits. The equipment needs to be able to track
the VCO phase noise to measure in this way, and one can not
let the VCO drift too far off in frequency. If this kind of equipment
is not available, the VCO phase noise can also be measured by
making a very narrow loop bandwidth filter.
When divide by 2 is enabled, the phase noise at lower offsets is
about 6 dB better; but at high offsets, the phase noise
improvement may be less because the divider is noise floor is
adding to the phase noise.
7
