Datasheet

Data Sheet ADF4106
Rev. E | Page 19 of 24
APPLICATIONS
LOCAL OSCILLATOR FOR LMDS BASE STATION
TRANSMITTER
Figure 22 shows the ADF4106 being used with a VCO to
produce the LO for an LMDS base station.
The reference input signal is applied to the circuit at FREF
IN
and, in this case, is terminated in 50 Ω. A typical base station
system would have either a TCXO or an OCXO driving the
reference input without any 50 Ω termination.
To achieve a channel spacing of 1 MHz at the output, the
10 MHz reference input must be divided by 10, using the
on-chip reference divider of the ADF4106.
The charge pump output of the ADF4106 (Pin 2) drives the
loop filter. In calculating the loop filter component values, a
number of items need to be considered. In this example, the
loop filter was designed so that the overall phase margin for
the system would be 45°.
Other PLL system specifications include:
K
D
= 2.5 mA
K
V
= 80 MHz/V
Loop Bandwidth = 50 kHz
F
PFD
= 1 MHz
N = 5800
Extra Reference Spur Attenuation = 10 dB
These specifications are needed and used to derive the loop
filter component values shown in Figure 22.
The circuit in Figure 22 shows a typical phase noise
performance of −83.5 dBc/Hz at 1 kHz offset from the carrier.
Spurs are better than −62 dBc.
The loop filter output drives the VCO, which in turn is fed
back to the RF input of the PLL synthesizer and also drives the
RF output terminal. A T-circuit configuration provides 50 Ω
matching between the VCO output, the RF output, and the RF
IN
terminal of the synthesizer.
In a PLL system, it is important to know when the system
is in lock. In Figure 22, this is accomplished by using the
MUXOUT signal from the synthesizer. The MUXOUT pin
can be programmed to monitor various internal signals in the
synthesizer. One of these is the LD or lock-detect signal.
ADF4106
CE
CLK
DATA
LE
1000pF
1000pF
REF
IN
100pF
CP
MUXOUT
CPGND AGND DGND
100pF
1.5nF
20pF
100pF
51Ω
6.2kΩ
4.3kΩ
100pF
18Ω
NOTE
DECOUPLING CAPACITORS (0.1
µ F/10pF) ON AV
DD
, DV
DD
, AND
V
P
OF THE ADF4106 AND ON V
CC
OF THE V956ME03 HAVE
BEEN OMITTED FROM THE DIAGRAM TO AID CLARITY.
SPI
®
-COMPATIBLE SERIAL BUS
R
SET
RF
IN
A
RF
IN
B
AV
DD
DV
DD
V
P
FREF
IN
V
DD
V
P
LOCK
DETECT
V
CC
V956ME03
1, 3, 4, 5, 7, 8,
9, 11, 12, 13
18Ω
18Ω
100pF
RF
OUT
5.1kΩ
7
15
16
8
2
14
6
5
1
9
4
3
14
2
10
51Ω
02720-027
Figure 22. Local Oscillator for LMDS Base Station