AD9952YSVZ Datasheet, PDF (19/28 Pages) Analog Devices – 400 MSPS 14-Bit, 1.8 V CMOS Direct Digital Synthesizer

AD9952
The shaped on-off keying function can be bypassed (disabled)
by clearing the OSK enable bit (CFR1 [25] = 0).
The modes are controlled by two bits located in the most
significant byte of the control function register (CFR). CFR1
[25] is the shaped on-off keying enable bit. When CFR1 [25] is
set, the output scaling function is enabled and CFR1 [25]
bypasses the function. CFR1 [24] is the internal shaped on-off
keying active bit. When CFR1 [24] is set, internal shaped on-off
keying mode is active; when CFR1 [24] is cleared, external
shaped on-off keying mode is active. CFR1 [24] is a don’t care if
the shaped on-off keying enable bit (CFR1 [25]) is cleared. The
power-up condition of the shaped on-off keying is disabled
(CFR1 [25] = 0). Figure 23 shows the block diagram of the OSK
circuitry.
Auto Shaped On-Off Keying Mode Operation
The auto shaped on-off keying mode is active when CFR1 [25]
and CFR1 [24] are set. When auto shaped on-off keying mode is
enabled, a single-scale factor is internally generated and applied
to the multiplier input for scaling the output of the DDS core
block (see Figure 23). The scale factor is the output of a 14-bit
counter that increments/decrements at a rate determined by the
contents of the 8-bit output ramp rate register. The scale factor
increases if the OSK pin is high and decreases if the OSK pin is
low. The scale factor is an unsigned value such that all 0s
multiply the DDS core output by 0 (decimal) and 0x3FFF
multiplies the DDS core output by 16,383 (decimal).
To use the full amplitude (14-bits) with fast ramp rates, the
internally generated scale factor step size is controlled via the
ASF [15:14]. Table 6 describes the increment/decrement step
size of the internally generated scale factor per the ASF [15:14].
A special feature of this mode is that the maximum output
amplitude allowed is limited by the contents of the amplitude
scale factor register. This allows the user to ramp to a value less
than full scale.
Table 6. Auto Scale Factor Internal Step Size
ASF [15:14] (Binary)
Increment/Decrement Size
00
1
01
2
10
4
11
8
OSK Ramp Rate Timer
The OSK ramp rate timer is a loadable down counter that
generates the clock signal to the 14-bit counter, which, in turn,
generates the internal scale factor. The ramp rate timer is loaded
with the value of the ASFR every time the counter reaches 1
(decimal). This load and countdown operation continues for as
long as the timer is enabled, unless the timer is forced to load
before reaching a count of 1.
If the load OSK timer bit (CFR1 [26]) is set, the ramp rate timer
is loaded upon an I/O UPDATE or upon reaching a value of 1.
The ramp timer can be loaded before reaching a count of 1 by
three methods.
The first method of loading is by changing the OSK input pin.
When the OSK input pin changes state, the ASFR value is
loaded into the ramp rate timer, which then proceeds to count
down as normal.
The second method in which the sweep ramp rate timer can be
loaded before reaching a count of 1 is if the load OSK timer bit
(CFR1 [26]) is set and an I/O UPDATE is issued.
The last method in which the sweep ramp rate timer can be
loaded before reaching a count of 1 is when going from the
inactive auto shaped on-off keying mode to the active auto
shaped on-off keying mode; that is, when the sweep enable bit is
being set.
DDS CORE
COS(X)
TO DAC
AUTO DESK
ENABLE
CFR1[24]
01
AMPLITUDE SCALE
FACTOR REGISTER
(ASF)
0
OSK ENABLE
CFR[25]
OSK PIN
SYNC_CLK
LOAD OSK TIMER
CFR1[26]
AMPLITUDE RAMP
RATE REGISTER
(ASF)
OUT
HOLD
UP/DN
INC/DEC ENABLE
LOAD DATA
EN
AUTO SCALE
FACTOR GENERATOR
RAMP RATE TIMER
Figure 23. On-Off Shaped Keying, Block Diagram
CLOCK
Rev. B | Page 19 of 28