Datasheet

AD9954
Rev. B | Page 19 of 40
03374-003
FTW0
SINGLE-TONE
MODE
LINEAR SWEEP MODE
AT POINT A: LOAD RISING RAMP RATE REGISTER, APPLY RISING DFTW.
AT POINT B: LOAD FALLING RAMP RATE REGISTER, APPLY FALLING DFTW.
PS<0> = 1PS<0>=0 PS<0>=0
TIME
FTW1
A
B
f
OUT
Figure 21. Linear Sweep Frequency Plan
03374-004
FTW0
SINGLE-TONE
MODE
LINEAR SWEEP MODE ENABLE–NO DWELL BIT SET
FTW1
AA A
BBB
f
OUT
TIME
PS<0> = 1
PS<0> = 0PS<0> = 0 PS<0> = 1 PS<0> = 1PS<0> = 0
Figure 22. Linear Sweep Using No-Dwell Frequency Plan
Linear Sweep No-Dwell Feature
See CFR1<2> in the register maps (see Table 12 and Table 13)
for general details of the no-dwell mode. Figure 22 depicts the
linear sweep mode operation when the linear sweep no-dwell
bit is set. The Label A points indicate where a rising edge on
PS0 is detected; the Label B points indicate where the AD9954
has determined f
OUT
has reached the terminal frequency and
automatically returns to the starting frequency. Note that in this
mode, only sweeps from FTW0 to FTW1 using the positive
linear sweep control word are supported. Toggling PS0 from 1
to 0 neither initiates a falling sweep when the no-dwell bit is set,
nor interrupts a positive sweep already underway.
Resetting the Ramp Rate Timer
The ramp timer can be reset before reaching a count of 1 by
three methods.
Method one is by changing the PS0 input pin. When the PS0
input pin toggles from 0 to 1, the RSRRW value is loaded into
the ramp rate timer, which then proceeds to countdown as
normal. When the PS0 input pin toggles from 0 to 1, the falling
sweep ramp rate word (FSRRW) value is loaded into the ramp
rate timer, which then proceeds to countdown as normal.
The second method uses the LOAD SRR @ I/O UD bit
(CFR1<15>), see Table 12 for details.