9

SP5659

LOOP BANDWIDTH

Most applications for which the SP5659 is intended require

a loop filter bandwidth of between 2kHz and 10kHz.

Typically, the VCO phase noise will be specified at both

1kHz and 10kHz offset. It is common practice to arrange the

loop filter bandwidth such that the 1kHz figure lies within the

loop bandwidth. The phase noise therefore depends on the

synthesiser comparator noise floor rather than the VCO

The 10kHz offset figure should depend on the VCO provided

that the loop has been designed correctly and is not

underdamped.

REFERENCE SOURCE

The SP5659 offers optimal local oscillator phase noise

performance when operated with a large step size. This is

because the local oscillator phase noise within the loop

bandwidth is:

phase comparator frequency.





Assuming the phase comparator noise floor is flat regardless

of sampling frequency, this means that the best performance

will be achieved when the overall local oscillator to phase

comparator division ratio is a minimum.

The are two ways of achieving a higher phase comparator

sampling frequency:

1. Reduce the division ratio between the reference source

and the phase comparator

2. Use a higher reference source frequency

The second approach 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 SP5659s FROM A COMMON

REFERENCE

The REF/COMP output on pin 3 allows two synthesisers to be

driven from a common reference. To do this, the first device

should be programmed by setting RE = 1 and RTS = 0. The

driven device should be programmed for normal operation with

RE = 0 and RTS = 0. The two devices should be connected as

shown in Fig. 9.

SP5659

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

4MHz 18p

SP5659

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

1n

Fig. 9 Two SP5659 devices using a common reference

Fig. 10 Typical RF input impedance

Ω

NORMALISED TO 50

Ω

FREQUENCY MARKERS AT 100MHz,

500MHz, 1GHz AND 2·7GHz

j 2

j 1

j 0.5

j 0.2

0

2

j 0.2

2

j 0.5

2

j 1

2

j 2

1

0.5

0.2

j 5

2

j 5

2

5