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Data Sheet

ADF4159

Rev. B | Page 27 of 36

To calculate the DEV word, use Equation 13.

DEV = fDEV/(fRES × 2DEV_OFFSET)

(13)

971

MHz

250

DEV

Rounding this value to 20,972 and recalculating using Equation 7

to obtain the actual deviation frequency, fDEV, thus produces the

following:

fDEV = (25 MHz/225) × (20,972 × 24) = 250.006 kHz

The number of fDEV 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 CLK2 to 1):

CLK1 = Timer × fPFD/CLK2 = 10 µs × 25 MHz/1 = 250

To summarize the settings,

•

DEV = 20,972

•

Number of steps = 200

•

CLK1 = 250

•

CLK2 = 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

fPFD = 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:

•

and Bits DB[22:19] = 3

•

3. Program the ramp rate for Ramp 2 by setting the following

values:

•

and Bits DB[22:19] = 3

•

Figure 39 shows the resulting ramp with two ramp rates. To

activate the ramp, see the Activating the Ramp section.

TIME

SWEEP RATE SET BY OTHER REGISTER

SWEEP RATE SET BY ONE REGISTER

Figure 39. Dual Ramp with Two Sweep Rates

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