AD9854SLASHPCB datasheet(18/44 Pages) AD

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AD9854

–18–

REV. 0

MODE

TW1

TW2

FSK DATA (PIN 29)

001 (FSK NO RAMP)

000 (DEFAULT)

Figure 36. Traditional FSK Mode

MODE

TW1

TW2

FSK DATA (PIN 29)

010 (RAMPED FSK)

000 (DEFAULT)

Figure 37. Ramped FSK Mode

Where N is the phase accumulator resolution (48 bits in this

instance), frequency is expressed in Hertz, and the FTW, Fre-

quency Tuning Word, is a decimal number. Once a decimal

number has been calculated, it must be rounded to an integer

and then converted to binary format—a series of 48 binary-

weighted 1s or 0s. The fundamental sine wave DAC output

frequency range is from dc to 1/2 SYSCLK.

Changes in frequency are phase continuous—that is, the new

frequency uses the last phase of the old frequency as the reference

point to compute the ﬁrst new frequency phase.

The I and Q DACs of the AD9854 are always 90 degrees out-

of-phase. The 14-bit phase registers (discussed elsewhere in this

data sheet) do not independently adjust the phase of each DAC

output. Instead, both DAC’s are affected equally by a change in

phase offset.

The single-tone mode allows the user to control the following

signal qualities:

• Output Frequency to 48-Bit Accuracy

• Output Amplitude to 12-Bit Accuracy

– Fixed, User-Deﬁned, Amplitude Control

– Variable, Programmable Amplitude Control

– Automatic, Programmable, Single-Pin-Controlled, “Shaped

On/Off Keying”

• Output Phase to 14-Bit Accuracy

Furthermore, all of these qualities can be changed or modulated

via the 8-bit parallel programming port at a 100 MHz parallel-byte

rate, or at a 10 MHz serial rate. Incorporating this attribute will

permit FM, AM, PM, FSK, PSK, ASK operation in the single-

tone mode.

Unramped FSK (Mode 001)

When selected, the output frequency of the DDS is a function

of the values loaded into Frequency Tuning Word registers 1

and 2 and the logic level of Pin 29 (FSK/BPSK/HOLD). A logic

low on Pin 29 chooses F1 (frequency tuning word 1, parallel

address 4–9 hex) and a logic high chooses F2 (frequency tuning

word 2, parallel register address A–F hex). Changes in frequency

are phase-continuous and practically instantaneous. (Please

refer to pipeline delays in speciﬁcation table.) Other than F2 and

Pin 29 becoming active, this mode is identical to single-tone.

The unramped FSK mode, Figure 36, is representative of

traditional FSK, RTTY (Radio Teletype) or TTY (Teletype)

transmission of digital data. Frequency transitions occur nearly

instantaneously from F1 to F2. This simple method works

extremely well and is the most reliable form of digital communica-

tion, but it is also wasteful of RF spectrum.

See the following Ramped FSK section for an alternative FSK

method that conserves bandwidth.

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