10

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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