ADF4158 datasheet(27/35 Pages) AD

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Data Sheet

ADF4158

Rev. I | Page 27 of 35

Waveform Deviations and Timing

Figure 35 shows a version of a burst or ramp. The key parameters

that define a burst or ramp are

•

Frequency deviation

•

Timeout interval

•

Number of steps

TIMER

fDEV

TIME

Figure 35. Waveform Timing

Frequency Deviation

The frequency deviation for each frequency hop is set by

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

(9)

where:

fPFD is the PFD frequency.

DEV is a 16-bit word.

DEV_OFFSET is a 4-bit word.

Timeout Interval

The time between each frequency hop is set by

Timer = CLK1 × CLK2 × (1/fPFD)

(10)

where:

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

R2, 12-bit clock divider in R4—CLK DIV set as RAMP DIV).

fPFD is the PFD frequency.

Number of Steps

A 20-bit step value 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 slight variations on this.

In the single ramp burst, the ADF4158 is locked to the fre-

quency defined in the FRAC/INT register. When the ramp

mode is enabled, the ADF4158 increments the N-divide value

by DEV × 2DEV_OFSET, causing a frequency shift, fDEV, on each

timer interval. This happens until the set number of steps has

taken place. The ADF4158 then retains the final N-divide value.

Single Sawtooth Burst

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

initial value on the next timeout interval after the number of

steps has taken place. The ADF4158 retains this N-divide value.

Sawtooth Ramp

The sawtooth ramp is a repeated version of the single sawtooth

burst. The waveform repeats until the ramp is disabled.

Triangular Ramp

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

when the steps have been completed, the ADF4158 begins to

decrement the N-divide value by DEV × 2DEV_OFFSET on each

timeout interval. When the number of steps has again been

completed, it reverts to incrementing the N-divide value.

Repeating this creates a triangular waveform. The waveform

repeats until the ramp is disabled.

FMCW Radar Ramp Settings Worked Example

Take as an example, an FMCW radar system requiring the RF

LO to 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 ADF4158 is calculated as follows:

fRES = fPFD/225

(11)

Numerically:

fRES = 25 MHz/225 = 0.745 Hz

The DEV_OFFSET is calculated after rearranging Equation 9:

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

(12)

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

following equation:

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

(13)

where:

DEV_OFFSET = a 4-bit word.

fDEV = frequency deviation.

DEVMAX (Maximum of the Deviation Word) = 215

Using Equation 13, 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 frequency deviation can

be calculated as follows

fDEV_RES = fRES × 2DEV_OFFSET

(14)

fDEV_RES = 0.745 Hz × 24 = 11.92 Hz

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