ADF4159 datasheet(26/36 Pages) AD

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ADF4159

Data Sheet

Rev. B | Page 26 of 36

WAVEFORM DEVIATIONS AND TIMING

Figure 38 shows a version of a ramp.

TIMER

fDEV

TIME

Figure 38. Waveform Timing

The key parameters that define a ramp are



Frequency deviation



Timeout interval



Number of steps

Frequency Deviation

The frequency deviation for each frequency hop is set by

fDEV = (fPFD/225) × (DEV × 2DEV_OFFSET)

(7)

where:

fPFD is the PFD frequency.

DEV is a 16-bit word (Bits DB[18:3] in Register R5).

DEV_OFFSET is a 4-bit word (Bits DB[22:19] in Register R5).

Timeout Interval

The time between each frequency hop is set by

Timer = CLK1 × CLK2 × (1/fPFD)

(8)

where:

CLK1 and CLK2 are the 12-bit clock values (12-bit CLK1 divider in

in Register R4 must be set to 11 for ramp divider.

fPFD is the PFD frequency.

Number of Steps

A 20-bit step value (Bits DB[22:3] in Register R6) defines the

number of frequency hops that take place. The INT value cannot

be incremented by more than 28 = 256 from its starting value.

SINGLE RAMP BURST

The most basic waveform is the single ramp burst. All other

waveforms are variations of this waveform. In the single ramp

burst, the ADF4159 is locked to the frequency defined in the

FRAC/INT register (R0). When the ramp mode is enabled, the

ADF4159 increments the N divider value by DEV × 2DEV_OFFSET,

causing a frequency shift, fDEV, on each timer interval. This shift

is repeated until the set number of steps has taken place. The

ADF4159 then retains the final N divider value.

SINGLE TRIANGULAR BURST

The single triangular burst is similar to the single ramp burst.

However, when the steps are completed, the ADF4159 begins

to decrement the N divider value by DEV × 2DEV_OFFSET on each

timeout interval.

SINGLE SAWTOOTH BURST

In the single sawtooth burst, the N divider value is reset to its

initial value on the next timeout interval after the number of

steps has taken place. The ADF4159 retains this N divider value.

SAWTOOTH RAMP

The sawtooth ramp is a repeated version of the single sawtooth

burst. The waveform is repeated until the ramp is disabled.

TRIANGULAR RAMP

The triangular ramp is a repeated version of the single triangu-

lar burst. However, when the steps are completed, the ADF4159

begins to decrement the N divider value by DEV × 2DEV_OFFSET on

each timeout interval. When the number of steps has again been

completed, the part reverts to incrementing the N divider value.

Repeating this pattern creates a triangular waveform. The wave-

form is repeated until the ramp is disabled.

FMCW RADAR RAMP SETTINGS WORKED EXAMPLE

This example describes a frequency modulated continuous wave

(FMCW) radar system that requires the RF LO to use a 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 the ADF4159 is calculated

as follows:

fRES = fPFD/225

(9)

Using Equation 9, fRES is calculated as follows:

fRES = 25 MHz/225 = 0.745 Hz

DEV_OFFSET is calculated after rearranging Equation 7.

DEV_OFFSET = log2(fDEV/(fRES × DEVMAX))

(10)

Expressed in log10(x), Equation 10 can be rearranged into

the following equation:

DEV_OFFSET = log10(fDEV/(fRES × DEVMAX))/log10(2)

(11)

where:

fDEV is the frequency deviation.

DEVMAX = 215 (maximum value of the deviation word).

DEV_OFFSET is a 4-bit word.

Using Equation 11, DEV_OFFSET is calculated as follows:

DEV_OFFSET = log10(250 kHz/(0.745 Hz × 215))/log10(2) = 3.356

After rounding, DEV_OFFSET = 4.

From DEV_OFFSET, the resolution of the frequency deviation

can be calculated as follows:

fDEV_RES = fRES × 2DEV_OFFSET

(12)

fDEV_RES = 0.745 Hz × 24 = 11.92 Hz

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