AD9854 Analog Devices, AD9854 Datasheet - Page 18

• Manufacturer Part Number: AD9854
• Description: CMOS 300 MHz Quadrature Complete-DDS
• Manufacturer: Analog Devices

AD9854
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
• Output Phase to 14-Bit Accuracy
– Fixed, User-Defined, Amplitude Control
– Variable, Programmable Amplitude Control
– Automatic, Programmable, Single-Pin-Controlled, “Shaped
On/Off Keying”
FSK DATA (PIN 29)
FSK DATA (PIN 29)
MODE
MODE
TW1
TW2
TW1
TW2
000 (DEFAULT)
000 (DEFAULT)
F2
F1
F2
F1
0
0
0
0
0
0
Figure 36. Traditional FSK Mode
Figure 37. Ramped FSK Mode
–18–
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.
010 (RAMPED FSK)
001 (FSK NO RAMP)
F2
F1
F1
F2
REV. 0