SP5669 Datasheet, PDF (11/15 Pages) Mitel Networks Corporation – 2.7GHz I2C Bus Controlled Synthesiser

Loop Bandwidth
The majority of applications for which the SP5669 is
intended require a loop filter bandwidth of between
2kHz and10kHz.
Typically the VCO phase noise will be specified at both
1kHz and10kHz offset. It is common practice to arrange
the loop filter bandwidth such that the 1kHz figure lies
within the loop bandwidth. Thus the phase noise de-
pends on the synthesiser comparator noise floor, rather
than the VCO.
The 10kHz offset figure should depend on the VCO
providing the loop is designed correctly, and is not
underdamped.
Reference Source
The SP5669 offers optimal LO phase noise perform-
ance when operated with a large step size. This is due
to the fact that the LO phase noise within the loop
bandwidth is:
( ) ^{phase comparator}
LO frequency
noise floor + 20 log ₁₀ phase comparator frequency
Assuming the phase comparator noise floor is flat
irrespective of sampling frequency, this means that the
best performance will be achieved when the overall LO
to phase comparator division ratio is a minimum.
SP5669
There are two ways of achieving a higher phase compa-
rator sampling frequency:–
A) Reduce the division ratio between the reference
source and the phase comparator
B) use a higher reference source frequency.
Approach B) may be preferred for best performance
since it is possible that the noise floor of the reference
oscillator may degrade the phase comparator perform-
ance if the reference division ratio is very small.
Driving Two Devicesfrom A Common
Reference
As mentioned earlier in the Datasheet, the SP5669 has
a REF/COMP output which allows two synthesisers to
be driven from a common reference. To do this, the
‘‘Master” should be programmed by setting RE = 1 and
RTS = 0. The driven device should be programmed for
normal operation i.e. RE = 0, and RTS = 0. The two
devices should be connected as shown below.
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