SP5658
LOOP BANDWIDTH
The majority of applications for which the SP5658 is
intended require a loop filter bandwidth of between 2kHz and
10kHz.
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 depends 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 SP5658 offers optimal LO phase noise performance
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
noise floor
+ 20 log
10
(
LO frequency
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.
There are two ways of achieving a higher phase
comparator 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 performance if the reference
division ratio is very small.
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MITEL [ MITEL NETWORKS CORPORATION ]
