SP5659 Datasheet, PDF (9/12 Pages) Mitel Networks Corporation – 2·7GHz I2C Bus Low Phase Noise Synthesiser

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 noise floor 120log10}
F_LO
F_COMP
where F_LO is the local oscillator frequency and F_COMP is the
phase comparator frequency.
SP5659
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.
4MHz 18p
1
16
2
15
3
14
4
13
SP5659
5
12
6
11
7
10
8
9
1n
1
16
2
15
3
14
4
13
SP5659
5
12
6
11
7
10
8
9
Fig. 9 Two SP5659 devices using a common reference
j1
j 0.5
j2
j 0.2
j5
0
0.2
0.5
1
2
5
2j 5
2j 0.2
S_11:Z_O = 50Ω
2j 0.5
NORMALISED TO 50Ω
2j 1
2j 2
FREQUENCY MARKERS AT 100MHz,
500MHz, 1GHz AND 2·7GHz
Fig. 10 Typical RF input impedance
9