Datasheet

Data Sheet ADF4158
Rev. E | Page 27 of 36
Waveform Deviations and Timing
Figure 35 shows a version of a burst or ramp. The key parameters
that define a burst or ramp are

Frequency deviation

Timeout interval

Number of steps
TIMER
f
DEV
FREQUENCY
TIME
08728-023
Figure 35. Waveform Timing
Frequency Deviation
The frequency deviation for each frequency hop is set by
f
DEV
= (f
PFD
/2
25
) × (DEV × 2
DEV_OFFSET
) (9)
where:
DEV is a 16-bit word.
DEV_OFFSET is a 4-bit word.
Timeout Interval
The time between each frequency hop is set by
Timer = CLK
1
× CLK
2
× (1/f
PFD
) (10)
where:
CLK
1
and CLK
2
are 12-bit clock values (12-bit CLK
1
divider in
R2, 12-bit clock divider in R4—CLK DIV set as RAMP DIV).
f
PFD
is the PFD frequency.
Number of Steps
A 20-bit step value 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 slight variations on this.
In the single ramp burst, the ADF4158 is locked to the fre-
quency defined in the FRAC/INT register. When the ramp
mode is enabled, the ADF4158 increments the N-divide value
by DEV × 2
DEV_OFSET
, causing a frequency shift, f
DEV
, on each
timer interval. This happens until the set number of steps has
taken place. The ADF4158 then retains the final N-divide value.
Single Sawtooth Burst
In the single sawtooth burst, the N-divide value is reset to its
initial value on the next timeout interval after the number of
steps has taken place. The ADF4158 retains this N-divide value.
Sawtooth Ramp
The sawtooth ramp is a repeated version of the single sawtooth
burst. The waveform repeats until the ramp is disabled.
Triangular Ramp
The triangular ramp is similar to the single ramp burst. However,
when the steps have been completed, the ADF4158 begins to
decrement the N-divide value by DEV × 2
DEV_OFFSET
on each
timeout interval. When the number of steps has again been
completed, it reverts to incrementing the N-divide value.
Repeating this creates a triangular waveform. The waveform
repeats until the ramp is disabled.
FMCW Radar Ramp Settings Worked Example
Take as an example, an FMCW radar system requiring the RF
LO to 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 ADF4158 is calculated as follows:
f
RES
= f
PFD
/2
25
(11)
Numerically:
f
RES
= 25 MHz/2
25
= 0.745 Hz
The DEV_OFFSET is calculated after rearranging Equation 9:
DEV_OFFSET = log
2
(f
DEV
/(f
RES
× DEV
MAX
)) (12)
Expressed in log
10
(x), Equation 10 can be transformed into the
following equation:
DEV_OFFSET = log
10
(f
DEV
/(f
RES
× DEV
MAX
))/log
10
(2) (13)
where:
DEV
MAX
(Maximum of the Deviation Word) = 2
15
f
DEV
= frequency deviation.
DEV_OFFSET = a 4-bit word.
Using Equation 13, 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 frequency deviation can
be calculated as follows
f
DEV_RES
= f
RES
× 2
DEV_OFFSET
(14)
f
DEV_RES
= 0.745 Hz × 2
4
= 11.92 Hz