ADF4158 datasheet(28/35 Pages) AD

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ADF4158

Data Sheet

Rev. I | Page 28 of 35

To calculate the DEV word, use Equation 15.

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) = 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 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 (12-bit CLK1 divider in R2), CLK2 = 1 (R4—CLK DIV

set as RAMP DIV). 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 triangular 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

Ramp Programming Sequence

Set parameters as described in the FMCW Radar Ramp Settings

Worked Example section and activate the ramp as described in

the Activating the Ramp section in the following register write

order.

1. Delay register (R7)

2. Step register (R6)

3. Deviation register (R5)

4. Test register (R4)

5. Function register (R3)

6. R-divider register (R2)

7. LSB FRAC register (R1)

8. FRAC/INT register (R0)

OTHER WAVEFORMS

Two Ramp Rates

This feature allows for two ramps with different step and deviation

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

another ramp is running.

Example

If, for example

•

PLL is locked to 5790 MHz and fPFD = 25MHz.

•

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,

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

Ramp 2 rates mode.

2. 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 36.

Eventually, the ramp must be activated as described in the

Activating the Ramp section.

TIME

SWEEP RATE SET BY OTHER REGISTER

SWEEP RATE SET BY ONE REGISTER

Figure 36. 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

If, for example

•

PLL is locked to 5790 MHz. fPFD = 25MHz

•

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

deviation of 100 kHz.

•

The FSK signal is 25 kHz.

Then,

1. 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.

2. 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).

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