Datasheet

Data Sheet ADF4159
Rev. D | Page 27 of 36
WAVEFORM DEVIATIONS AND TIMING
Figure 38 shows a version of a ramp.
TIMER
f
DEV
FREQUENCY
TIME
10849-033
Figure 38. Waveform Timing
The key parameters that define a ramp are
• Frequency deviation
• Timeout interval
• Number of steps
Frequency Deviation
The frequency deviation for each frequency hop is set by
f
DEV
= (f
PFD
/2
25
) × (DEV × 2
DEV_OFFSET
) (7)
where:
f
PFD
is the PFD frequency.
DEV is a 16-bit word (Bits DB[18:3] in Register R5).
DEV_OFFSET is a 4-bit word (Bits DB[22:19] in Register R5).
Timeout Interval
The time between each frequency hop is set by
Timer = CLK
1
× CLK
2
× (1/f
PFD
) (8)
where:
CLK
1
and CLK
2
are the 12-bit clock values (12-bit CLK
1
divider in
Register R2 and 12-bit CLK
2
divider in Register R4). Bits DB[20:19]
in Register R4 must be set to 11 for ramp divider.
f
PFD
is the PFD frequency.
Number of Steps
A 20-bit step value (Bits DB[22:3] in Register R6) defines the
number of frequency hops that take place. The INT value cannot
be incremented by more than 2
8
= 256 from its starting value.
SINGLE RAMP BURST
The most basic waveform is the single ramp burst. All other
waveforms are variations of this waveform. In the single ramp
burst, the ADF4159 is locked to the frequency defined in the
FRAC/INT register (R0). When the ramp mode is enabled, the
ADF4159 increments the N divider value by DEV × 2
DEV_OFFSET
,
causing a frequency shift, f
DEV
, on each timer interval. This shift
is repeated until the set number of steps has taken place. The
ADF4159 then retains the final N divider value.
SINGLE TRIANGULAR BURST
The single triangular burst is similar to the single ramp burst.
However, when the steps are completed, the ADF4159 begins
to decrement the N divider value by DEV × 2
DEV_OFFSET
on each
timeout interval.
SINGLE SAWTOOTH BURST
In the single sawtooth burst, the N divider value is reset to its
initial value on the next timeout interval after the number of
steps has taken place. The ADF4159 retains this N divider value.
SAWTOOTH RAMP
The sawtooth ramp is a repeated version of the single sawtooth
burst. The waveform is repeated until the ramp is disabled.
TRIANGULAR RAMP
The triangular ramp is a repeated version of the single triangu-
lar burst. However, when the steps are completed, the ADF4159
begins to decrement the N divider value by DEV × 2
DEV_OFFSET
on
each timeout interval. When the number of steps has again been
completed, the part reverts to incrementing the N divider value.
Repeating this pattern creates a triangular waveform. The wave-
form is repeated until the ramp is disabled.
FMCW RADAR RAMP SETTINGS WORKED EXAMPLE
This example describes a frequency modulated continuous wave
(FMCW) radar system that requires the RF LO to use a sawtooth
ramp over a 50 MHz range every 2 ms. The PFD frequency is
25 MHz, and the RF output range is 5800 MHz to 5850 MHz.
The frequency deviation for each hop in the ramp is set to
~250 kHz.
The frequency resolution of the ADF4159 is calculated
as follows:
f
RES
= f
PFD
/2
25
(9)
Using Equation 9, f
RES
is calculated as follows:
f
RES
= 25 MHz/2
25
= 0.745 Hz
DEV_OFFSET is calculated after rearranging Equation 7.
DEV_OFFSET = log
2
(f
DEV
/(f
RES
× DEV
MAX
)) (10)
Expressed in log
10
(x), Equation 10 can be rearranged into
the following equation:
DEV_OFFSET = log
10
(f
DEV
/(f
RES
× DEV
MAX
))/log
10
(2) (11)
where:
f
DEV
is the frequency deviation.
DEV
MAX
= 2
15
(maximum value of the deviation word).
DEV_OFFSET is a 4-bit word.
Using Equation 11, DEV_OFFSET is calculated as follows:
DEV_OFFSET = log
10
(250 kHz/(0.745 Hz × 2
15
))/log
10
(2) = 3.356
After rounding, DEV_OFFSET = 4.
From DEV_OFFSET, the resolution of the frequency deviation
can be calculated as follows:
f
DEV_RES
= f
RES
× 2
DEV_OFFSET
(12)
f
DEV_RES
= 0.745 Hz × 2
4
= 11.92 Hz