SP5669_97 Datasheet, PDF (10/12 Pages) Mitel Networks Corporation – 2.7GHz I2C Bus Controlled Synthesiser

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
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
1nF
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
Fig. 12 Driving two devices from a common reference
+j1
+j0.5
+j2
+j0.2
+j5
0
0.2
0.5
1
2
5
X
S₁₁:Z₀ = 50
NORMALISED TO 50
–j0.2
X
–j5
X
–j0.5
X
–j2
–j1
Fig. 13 typical RF input impedance
FREQUENCY MARKERS AT 100MHz,
500MHz, 1GHz AND 2.7GHz
10