Datasheet

LTC6946
21
6946fa
APPLICATIONS INFORMATION
INTRODUCTION
A PLL is a complex feedback system that may conceptually
be considered a frequency multiplier. The system multiplies
the frequency input at REF
±
and outputs a higher frequency
at RF
±
. The PFD, charge pump, N divider, and external VCO
and loop filter form a feedback loop to accurately control
the output frequency (see Figure 14). The R and O divider
are used to set the output frequency resolution.
Using the above equations, the output frequency resolu-
tion f
STEP
produced by a unit change in N is given by
Equation 6:
f
STEP
=
f
REF
R•O
(6)
LOOP FILTER DESIGN
A stable PLL system requires care in selecting the external
loop filter values. The Linear Technology PLLWizard ap-
plication, available from www.linear.com, aids in design
and simulation of the complete system.
The loop design should use the following algorithm:
1. Determine the output frequency, f
RF
, and frequency step
size, f
STEP
, based on application requirements. Using
Equations 3, 4, 5 and 6, change f
REF
, N, R, and O until
the application frequency constraints are met. Use the
minimum R value that still satisfies the constraints.
Then calculate B using Equation 1 and Table 7.
2. Select the open-loop bandwidth, BW, constrained by
f
PFD
. A stable loop requires that BW is less than f
PFD
by at least a factor of 10.
3. Select loop filter component R
Z
and charge pump cur-
rent I
CP
based on BW and the VCO gain factor, K
VCO
.
BW (in Hz) is approximated by the following equation:
BW
I
CP
•R
Z
•K
VCO
2•π •N
(7)
or
R
Z
=
2•π •BW•N
I
CP
•K
VCO
where K
VCO
is in Hz/V, I
CP
is in Amps, and R
Z
is in Ohms.
KVCO is obtained from the VCO tuning sensitivity in the
Electrical Characteristics. Use I
CP
= 11.2mA to lower in-
band noise unless component values force a lower setting.
R_DIV
N_DIV
÷R
÷N
O_DIV
÷O
f
PFD
LTC6946
REF
±
(f
REF
)
f
VCO
K
PFD
K
VCO
25
RF
±
(f
RF
)
15
CP
R
Z
C
I
C
P
LOOP FILTER
LF(s)
6946 F14
TUNE
I
CP
Figure 14. PLL Loop Diagram
OUTPUT FREQUENCY
When the loop is locked, the frequency f
VCO
(in Hz)
produced at the output of the VCO is determined by the
reference frequency, f
REF
, and the R and N divider values,
given by Equation 3:
f
VCO
=
f
REF
•N
R
(3)
Here, the PFD frequency f
PFD
produced is given by the
following equation:
f
PFD
=
f
REF
R
(4)
and f
VCO
may be alternatively expressed as:
f
VCO
= f
PFD
• N
The output frequency f
RF
produced at the output of the O
divider is given by Equation 5:
f
RF
=
f
VCO
O
(5)