Datasheet
LTC6946
12
6946fa
BLOCK DIAGRAM
RF
–
28
2
3
11
GND
10
MUTE
9
RF
+
12
V
RF
+
13
27
REF
–
REFO
≤250MHz
≤100MHz
÷1 TO 1023
÷1 TO 6, 50%
÷32 TO 65535
0.373GHz TO
5.79GHz
MUTE
1
V
REFO
+
REF
+
26
V
REF
+
R_DIV
LOCK
PFD
O_DIV
N_DIV
B_DIV
CAL, ALC
CONTROL
2.24GHz TO 3.74GHz (LTC6946-1) OR
3.08GHz TO 4.91GHz (LTC6946-2) OR
3.84GHz TO 5.79GHz (LTC6946-3)
TUNE
15
CP
V
VCO
+
GND
CM
A
CM
B
CM
C
GND
250μA TO
11.2mA
25
22
21
20
19
18
17
24
V
CP
+
23
GND
16
TB
6946 BD
14
BB
SERIAL
PORT
STAT
CS
7
SDO
SDI
SCLK
8
V
D
+
6
5
4
OPERATION
The LTC6946 is a high performance PLL complete with
a low noise VCO available in three different frequency
range options. The output frequency range may be further
extended by utilizing the output divider (see Available Op-
tions table, for more details). The device is able to achieve
superior integrated phase noise by the combination of
its extremely low in-band phase noise performance and
excellent VCO noise characteristics.
REFERENCE INPUT BUFFER
The PLL’s reference frequency is applied differentially on
pins REF
+
and REF
–
. These high impedance inputs are
self-biased and must be AC-coupled with 470pF capacitors
(see Figure 1 for a simplified schematic). Alternatively, the
inputs may be used single ended by applying the refer-
ence frequency at REF
+
and bypassing REF
–
to GND with
a 470pF capacitor.
A high quality signal must be applied to the REF
±
inputs as
they provide the frequency reference to the entire PLL. To
achieve the part’s in-band phase noise performance, apply
27
28
4.2k
REF
+
REF
–
4.2k
6946 F01
1.9V
BST
BIAS
V
REF
+
V
REF
+
LOWPASS
FILT[1:0]
Figure 1. Simplified REF Interface Schematic
a CW signal of at least 6dBm into 50Ω, or a square wave
of at least 0.5V
P-P
with slew rate of at least 40V/μs.
Additional options are available through serial port register
h08 to further refine the application. Bits FILT[1:0] control
the reference input buffer’s lowpass filter, and should be
set based upon f
REF
to limit the reference’s wideband
noise. The FILT[1:0] bits must be set correctly to reach
the L
M(NORM)
normalized in-band phase noise floor. See
Table 1 for recommended settings.