ADF4158 datasheet(27/36 Pages) AD

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ADF4158

Rev. 0 | Page 27 of 36

To calculate the DEV word, use Equation 12.

DEV = fDEV/(fRES × 2DEV_OFFSET)

(15)

971

MHz

250

DEV

Rounding this to 20,972 and recalculating using Equation 9

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

following:

fDEV = (25 MHz/225) × (20,972 × 24) = 249.986 kHz

The number of fDEV steps required to cover the 50 MHz range

is 50 MHz/249.986 kHz = 200. To cover the 50 MHz range in

2 ms, the ADF4158 must hop every 2 ms/200 = 10 μs.

Rearrange Equation 10 to set the timer value (and fix 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. Using these settings, program the

ADF4158 to a center frequency of 5800 MHz, and enable the

sawtooth ramp to produce the required waveform. If a triangu-

lar ramp was used with the same settings, the ADF4158 would

sweep from 5800 MHz to 5850 MHz and back down again. The

entire sweep would take 4 ms.

Activating the Ramp

After setting all of the previous parameters, the ramp must be

activated. It is achieved by choosing the desired type of ramp

(DB[11:10] in Register R3) and starting the ramp (DB31 = 1

in Register R0).

ADDITIONAL FEATURES

Two Ramp Rates

This feature allows for two ramps with different step and devia-

tion settings. It also allows the ramp rate to be reprogrammed

while another ramp is running.

Example

For example, if

•

PLL is locked to 5790 MHz.

•

Ramp 1 jumps 100 steps, each of which lasts 10 μs and has

a frequency deviation of 100 kHz.

•

Ramp 2 jumps 80 steps, each of which lasts 10 μs and has a

frequency deviation of 125 kHz.

Then,

DB24 in Register R5 should be set to 1, which activates

Ramp 2 rates mode.

Program Ramp 1 and Ramp 2 as follows to get two ramp

rates:

Ramp 1: Register R5 DB[18:3] = 16,777, DB[22:19] = 3

with DB23 = 0; Register R6 DB[22:3] = 100, DB23 = 0.

Ramp 2: Register R5 DB[18:3] = 20,972, DB[22:19] = 3

with DB23 = 1; Register R6 DB[22:3] = 80, DB23 = 1.

The resulting ramp with two various rates is shown in

Figure 35. Eventually, the ramp must be activated as

described in Activating the Ramp section.

TIME

SWEEP RATE SET BY OTHER REGISTER

SWEEP RATE SET BY ONE REGISTER

Figure 35. Dual Sweep Rate

Ramp Mode with FSK Signal on Ramp

In traditional approaches a FMCW radars used either linear

frequency modulation (LFM) or FSK modulation. These

modulations used separately introduce ambiguity between

measured distance and velocity, especially in multitarget

situations. To overcome this issue and enable unambiguous

(range − velocity) multitarget detection, use a ramp with

FSK on it.

Example

For example, if

•

PLL is locked to 5790 MHz.

•

There are 100 steps each of which lasts 10 μs and has a

deviation of 100 kHz.

•

The FSK signal is 25 kHz.

Then,

Program the ramp as described in the FMCW Radar Ramp

Settings Worked Example section. While doing that DB23

in Register R5 and DB23 in Register R6 should be set to 0.

Set the bits in Register R5 as follows to program FSK on

ramp to 25 kHz:

DB[18:3] = 4194 (deviation word), DB[22:19] = 3

(deviation offset), DB23 = 1 (deviation select for FSK on

ramp), and DB25 = 1 (ramp with FSK enabled).

An example of ramp with FSK on the top of it is shown in

Figure 36. Eventually, the ramp must be activated as described

in Activating the Ramp section.

TIME

RAMP END

FSK

SHIFT

LFM

STEP = FREQUENCY

SWEEP/(N – 1)

Figure 36. Combined FSK and LFM Waveform (N Corresponds to the Number

of LFM Steps)

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