10

SP5669

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 depends on the

synthesiser comparator noise floor, rather than the VCO.

The 10kHz offset figure should depend on the VCO provid-

ing the loop is designed correctly, and is not underdamped.

REFERENCE SOURCE

The SP5669 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

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.

There are two ways of achieving a higher phase compara-

tor 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.

DRIVING TWO DEVICES FROM 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.

4MHz 18pF

1nF

2

3

4

5

6

7

13

12

11

10

89

14

15

116

116

2

3

4

5

6

7

13

12

11

10

89

14

15

116

116

0.5

0.2

1

0

+j0.2

+j0.5

+j1

+j2

+j5

2

5

–j5

–j2

–j1

–j0.5

–j0.2

FREQUENCY MARKERS AT 100MHz,

X

X

X

X

NORMALISED TO 50

500MHz, 1GHz AND 2.7GHz

Fig. 12 Driving two devices from a common reference

Fig. 13 typical RF input impedance