PRELIMINARY TECHNICAL DATA

AD9953

Phase dithering is independently controlled on the four least significant bits of the phase word

routed to the angle rotation function. That is, any or all of the phase word four least significant bits

may be dithered or not dithered, controlled by the user via the serial port. Specifically, the

CFR1<19> bit controls the phase dithering enable function of the phase word <16> bit. The

CFR1<18> bit controls the phase dithering enable function of the phase word <15> bit. The

CFR1<17> bit controls the phase dithering enable function of the phase word <14> bit. The

CFR1<16> bit controls the phase dithering enable function of the phase word <13> bit. This

enable function is such that if the bit is high, dithering is enabled. If the bit is low, dithering is not

enabled.

Amplitude dithering uses one control bit to enable or disable dithering. If the amplitude dither

enable bit (CFR1<20>) is logic 0, no amplitude dithering is enabled and the data from the DDS

core is passed unchanged. When high, amplitude dithering is enabled.

Shaped On-Off Keying

General Description: The Shaped On-Off keying function of the AD9953 allows the user to

control the ramp-up and ramp-down time of an “on-off” emission from the DAC. This function is

used in “burst transmissions” of digital data to reduce the adverse spectral impact of short, abrupt

bursts of data.

AUTO and MANUAL Shaped On-Off Keying modes are supported. The AUTO mode generates a

linear scale factor at a rate determined by the Amplitude Ramp Rate (ARR) Register controlled by

an external pin (OSK). MANUAL mode allows the user to directly control the output amplitude by

writing the scale factor value into the Amplitude Scale Factor (ASF) Register (ASF).

The Shaped On-Off keying function may 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; 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; CFR1<24> cleared is external Shaped On-Off Keying mode active. CFR1<24> is a “Don’t

care” if the Shaped On-Off Keying enable bit (CFR1<25>) is cleared. The power up condition is

Shaped On-Off Keying disabled (CFR1<25> = 0). Figure C below shows the block diagram of the

OSK circuitry.

REV. PrB 1/30/03

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Analog Devices, Inc.