Datasheet

ADF4159 Data Sheet
Rev. D | Page 28 of 36
To calculate the DEV word, use Equation 13.
DEV = f
DEV
/(f
RES
× 2
DEV_OFFSET
) (13)
52.971,20
2
2
MHz25
zkH250
4
25
=
×
=DEV
Rounding this value to 20,972 and recalculating using Equation 7
to obtain the actual deviation frequency, f
DEV
, thus produces the
following:
f
DEV
= (25 MHz/2
25
) × (20,972 × 2
4
) = 250.006 kHz
The number of f
DEV
steps required to cover the 50 MHz range
is 50 MHz/250.006 kHz = 200. To cover the 50 MHz range in
2 ms, the ADF4159 must hop every 2 ms/200 = 10 µs.
Rearrange Equation 8 to set the timer value (and set CLK
2
to 1):
CLK
1
= Timer × f
PFD
/CLK
2
= 10 µs × 25 MHz/1 = 250
To summarize the settings,
• DEV = 20,972
• Number of steps = 200
• CLK
1
= 250
• CLK
2
= 1 (Bits DB[20:19] = 11, ramp divider, in Register R4)
Using these settings, program the ADF4159 to a center frequency
of 5800 MHz and enable the sawtooth ramp to produce the
required waveform. If a triangular ramp is used with the same
settings, the ADF4159 sweeps from 5800 MHz to 5850 MHz and
back down again, taking 4 ms for the entire sweep.
ACTIVATING THE RAMP
After setting all required parameters, the ramp must be activated by
choosing the desired type of ramp (Bits DB[11:10] in Register R3)
and starting the ramp (Bit DB31 = 1 in Register R0).
Ramp Programming Sequence
The setting of parameters described in the FMCW Radar Ramp
Settings Worked Example section and the activation of the ramp
described in the Activating the Ramp section must be completed
in the following register write order:
1. Delay register (R7)
2. Step register (R6)
3. Deviation register (R5)
4. Clock register (R4)
5. Function register (R3)
6. R divider register (R2)
7. LSB FRAC register (R1)
8. FRAC/INT register (R0)
OTHER WAVEFORMS
Dual Ramps with Different Ramp Rates
The ADF4159 can be configured for two ramps with different
step and deviation settings. It also allows the ramp rate to be
reprogrammed while another ramp is running.
Example
In this example, the PLL is locked to 5790 MHz and
f
PFD
= 25 MHz. Two ramps are configured, as follows:
• Ramp 1 jumps 100 steps; each step lasts 10 µs and has a
frequency deviation of 100 kHz.
• Ramp 2 jumps 80 steps; each step lasts 10 µs and has a
frequency deviation of 125 kHz.
To enable the two ramp rates, follow these steps:
1. Activate the dual ramp rates mode by setting Bit DB24
in Register R5 to 1.
2. Program the ramp rate for Ramp 1 by setting the following
values:
• Register R5: set Bit DB23 = 0, Bits DB[18:3] = 16,777,
and Bits DB[22:19] = 3
• Register R6: set Bit DB23 = 0 and Bits DB[22:3] = 100
3. Program the ramp rate for Ramp 2 by setting the following
values:
• Register R5: set Bit DB23 = 1, Bits DB[18:3] = 20,972,
and Bits DB[22:19] = 3
• Register R6: set Bit DB23 = 1 and Bits DB[22:3] = 80
Figure 39 shows the resulting ramp with two ramp rates. To
activate the ramp, see the Activating the Ramp section.
FREQUENCY
TIME
SWEEP RATE SET BY OTHER REGISTER
SWEEP RATE SET BY ONE REGISTER
10849-134
Figure 39. Dual Ramp with Two Sweep Rates