Q1. When the output divider by 2 is off, the second harmonic amplitude is
around 15dBc (the ADF4360-6 show the typical value is 20dBc). But when they
turn on the internal output divider on the ADF4360-6, the 3rd harmonic is
amplitude is very close to the main frequency (~-5.6dBc).Does this is the
internal VCO and output divider limitation or not. Because we try to use the
ADF4360-6 as the mixer for GSM 900MHz/1800MHz repeater. The result can not meet
2. The Spectrum of the spurs shown the spurs performance is not good enough
(58dBc only, the specification need at least 65dBc). This is relative to the
output filter design. Any suggestions to improve it?
A1.This will happen, as the ADF4360-x parts will output a square wave when the
divider is enabled, leading to high harmonic levels. A solution is to filter
the output with an LC filter (described on datasheet) or use a bandpass filter,
this will at least attenuate the unwanted 3rd harmonic. The ADF4360-7 at the
same frequencies performs a little better (See graphs, with divide by two both
enabled and disabled).(see file SCREN555 and SCREN556).2.Those levels have been
seen on the silicon, although they seem a little high. If possible I recommend
using the ADF4360-7, which will have considerably lower spur levels (See
graph). The external inductor value needs to be approximately 1.6 nH - 2 nH.
See attached plot from ADF4360-7. It performs better and takes lower
current(see file SCREN554) .