Datasheet

ADF4360-8
Rev. A | Page 21 of 24
APPLICATIONS
CHOOSING THE CORRECT INDUCTANCE VALUE
The ADF4360-8 can be used at many different frequencies
simply by choosing the external inductors to give the correct
output frequency. Figure 22 shows a graph of both minimum
and maximum frequency vs. the external inductor value. The
correct inductor should cover the maximum and minimum
frequencies desired. The inductors used are 0603 CS or 0805 CS
type from Coilcraft. To reduce mutual coupling, the inductors
should be placed at right angles to one another.
The lowest center frequency of oscillation possible is approxi-
mately 65 MHz, which is achieved using 560 nH inductors. This
relationship can be expressed by
()
EXT
O
L
F
+
=
nH0.9pF9.32π
1
where F
O
is the center frequency and L
EXT
is the external induc-
tance.
0
150
50
100
350
250
300
200
450
400
0 100 200 300 400 600500
INDUCTANCE (nH)
FREQUENCY (MHz)
04763-025
Figure 22. Output Center Frequency vs. External Inductor Value
The approximate value of capacitance at the midpoint of the
center band of the VCO is 9.3 pF, and the approximate value of
internal inductance due to the bond wires is 0.9 nH. The VCO
sensitivity is a measure of the frequency change vs. the tuning
voltage. It is a very important parameter for the low-pass filter.
Figure 23 shows a graph of the tuning sensitivity (in MHz/V) vs.
the inductance (nH). It can be seen that as the inductance in-
creases, the sensitivity decreases. This relationship can be de-
rived from the equation above; that is, since the inductance has
increased, the change in capacitance from the varactor has less
of an effect on the frequency.
0
4
2
10
8
6
12
0 100 200 300 400 600500
INDUCTANCE (nH)
SENSITIVITY (MHz/V)
04763-026
Figure 23. Tuning Sensitivity (in MHz/V) vs. Inductance (nH)
FIXED FREQUENCY LO
Figure 24 shows the ADF4360-8 used as a fixed frequency LO at
200 MHz. The low-pass filter was designed using ADIsimPLL
for a channel spacing of 2 MHz and an open-loop bandwidth of
100 kHz. The maximum PFD frequency of the ADF4360-8 is
8 MHz. Since using a larger PFD frequency allows the use of a
smaller N, the in-band phase noise is reduced to as low as pos-
sible, −109 dBc/Hz. The typical rms phase noise (100 Hz to
100 kHz) of the LO in this configuration is 0.09°. The reference
frequency is from a 16MHz TCXO from Fox; thus, an R value of
2 is programmed. Taking into account the high PFD frequency
and its effect on the band select logic, the band select clock
divider is enabled. In this case, a value of 8 is chosen. A very
simple shunt inductor and dc-blocking capacitor complete the RF
output stage.
SPI-COMPATIBLE SERIAL BUS
ADF4360-8
V
VCO
V
VCO
FOX
801BE-160
16MHz
V
VCO
CPGND AGND DGND L1 L2
RF
OUT
B
RF
OUT
A
CP
1nF
47pF
68nH470Ω
68nH
470Ω
22nF
56nH 56nH
680pF
51Ω
100pF
100pF
1nF1nF
10µF
4.7kΩ
6.8kΩ
15kΩ
R
SET
C
C
LE
DATA
CLK
REF
IN
C
N
V
TUNE
DV
DD
AV
DD
CE MUXOUT
5
4
24
7
2023221
6
14
16
17
18
19
13
1 3 8
9
1011 22 15
12
V
VDD
LOCK
DETECT
04763-027
Figure 24. Fixed Frequency LO