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AD9951

Rev. A | Page 18 of 28

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 18). 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 16383 (decimal).

For those users who use the full amplitude (14-bits) but need

fast ramp rates, the internally generated scale factor step size

is controlled via the ASF<15:14> bits. Table 6 describes the

increment/decrement step size of the internally generated scale

factor per the ASF<15:14> bits.

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

OSK Ramp Rate Timer

The OSK ramp rate timer is a loadable down counter, which

generates the clock signal to the 14-bit counter that 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.

Method one 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.

OSK PIN

LOAD OSK TIMER

CFR1<26>

SYNC_CLK

AUTO DESK

ENABLE

CFR1<24>

TO DAC

AUTO SCALE

FACTOR GENERATOR

RAMP RATE TIMER

CLOCK

DDS CORE

OSK ENABLE

CFR<25>

AMPLITUDE SCALE

FACTOR REGISTER

(ASF)

HOLD

INC/DEC ENABLE

OUT

COS(X)

AMPLITUDE RAMP

RATE REGISTER

(ASF)

UP/DN

DATA

LOAD

Figure 18. On-Off Shaped Keying, Block Diagram

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