
AD9910
Rev. 0 | Page 26 of 60
PFD
CP
PLL_LOOP_FILTER
VCO
÷N
PLL OUT
PLL IN
AVDD
REFCLK PLL
2
R1
C1
C2
0
Figure 35. REFCLK PLL External Loop Filter
In the prevailing literature, this configuration yields a third-
order, Type II PLL. To calculate the loop filter component
values, begin with the feedback divider value (N), the gain of
the phase detector (K
D
), and the gain of the VCO (K
V
) based on
the programmed VCO SEL bit settings (see Table 1 for K
V
). The
loop filter component values depend on the desired open-loop
bandwidth (f
OL
) and phase margin (
φ
), as follows:
( )
+
=
K
K
Nf
π
R1
V
D
OL
sin
1
1
(4)
( )
)
2
(
f
2
tan
OL
V
D
N
K
K
C1
=
(5)
(
)
( )
( )
1
=
f
2
N
K
K
C2
OL
V
D
cos
sin
2
(6)
where:
K
D
is equal to the programmed value of I
CP
.
K
V
is taken from Table 1.
Ensure that proper units are used for the variables in Equation 4
through Equation 6. I
CP
must be in amps, not μA as appears in
Table 9; K
V
must be in Hz/V, not MHz/V as listed in Table 1; the
loop bandwidth (f
OL
) must be in Hz; the phase margin (φ) must
be in radians.
For example, suppose the PLL is programmed such that
I
CP
= 287 μA, K
V
= 625 MHz/V, and N = 25. If the desired loop
bandwidth and phase margin are 50 kHz and 45°, respectively,
then the loop filter component values are R1 = 52.85 Ω, C1 =
145.4 nF, and C2 = 30.11 nF.
PLL LOCK INDICATION
When the PLL is in use, the PLL_LOCK pin provides an active
high indication that the PLL has locked to the REFCLK input
signal. When the PLL is bypassed the PLL_LOCK pin defaults
to Logic 0.
OUTPUT SHIFT KEYING (OSK)
The OSK function (Figure 36) allows the user to control the
output signal amplitude of the DDS. Both a manual and an
automatic mode are available under program control. The
amplitude data generated by the OSK block has priority over
any other functional block that is programmed to deliver
amplitude data to the DDS. Hence, the OSK data source, when
enabled, overrides all other amplitude data sources.
0
OSK ENABLE
AMPLITUDE SCALE FACTOR
(ASF<15:2>)
AMPLITUDE RAMP RATE
(ASF<31:16>)
AMPLITUDE STEP SIZE
(ASF<1:0>)
MANUAL OSK EXTERNAL
AUTO OSK ENABLE
OSK
60
DDS CLOCK
TO DDS
AMPLITUDE
CONTROL
PARAMETER
LOAD ARR AT I/O_UPDATE
OSK
CONTROLLER
14
16
14
2
Figure 36. OSK Block Diagram
The operation of the OSK function is governed by four control
register bits, the external OSK pin, and the entire 32 bits of the
ASF register. The primary control for the OSK block is the OSK
enable bit. When the OSK function is disabled, the OSK input
controls are ignored and the internal clocks shut down.
When the OSK function is enabled, automatic and manual
operation is selected using the select auto OSK bit.
Manual OSK
In manual mode, output amplitude is varied by successive write
operations to the amplitude scale factor portion of the ASF
register. The rate at which amplitude changes can be applied to
the output signal is limited by the speed of the serial I/O port.
In manual mode, the OSK pin functionality depends on the
state of the manual OSK external bit. When the OSK pin is
Logic 0, the output amplitude is forced to zero; otherwise, the
output amplitude is set by the amplitude scale factor value.
Automatic OSK
In automatic mode, the OSK function automatically generates a
linear amplitude vs. time profile (or amplitude ramp). The
amplitude ramp is controlled via three parameters: the maximum
amplitude scale factor, the amplitude step size, and the time interval
between steps. The amplitude ramp parameters reside in the 32-bit
ASF register and are programmed via the serial I/O port. The time
interval between amplitude steps is set via the 16-bit amplitude
ramp rate portion of the ASF register (Bits<31:16>). The maximum
amplitude scale factor is set via the 14-bit amplitude scale factor in
the ASF register (Bits<15:2>). The amplitude step size is set via the
2-bit amplitude step size portion of the ASF register (Bits<1:0>).
Additionally, the direction of the ramp (positive or negative slope)
is controlled by the external OSK pin.