SP5748 datasheet(9/15 Pages) MITEL

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SP5748 Advance Information

LOOP BANDWIDTH

The majority of applications for which the SP5748 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

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 SP5748 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

LO frequency

noise floor

+ 20 log

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.

APPLICATION NOTES

A generic set of application notes AN168 for designing

withsynthesisers such as the SP5748 has been written.

This covers aspects such as loop filter design and

decoupling. Thisapplication note is also featured in the

Media Data Book, or refer to the Mitel Semiconductor

Internet Site http://www.Mitelsemi.com. A generic test/

demo board has been produced which can be used for

the SP5748. A circuit diagram is shown in Figure 12.

The board can be used for the following purposes:

(A) Measuring RF sensitivity performance.

(B) Indicating port function.

(D) Testing of external reference.

(E) Measurement of phase noise performance.

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