L M X 2 5 3 1 L Q 1 3 1 2 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 Noise (Internal Divide by 2 Enabled)
Fout = 634 MHz (1268 MHz/2)
Fout = 657 MHz (1314 MHz/2)
Fout = 680 MHz (1360 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
noise 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. At
high offsets, the overall phase
noise improvement may be
lower because the divider is
noise floor is adding to the
phase noise.
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