ADF4158 datasheet(25/36 Pages) AD

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ADF4158

Rev. 0 | Page 25 of 36

Setting Bit DB28 in the R-divider register (Register R2) to 1

enables cycle slip reduction. Note that a 45% to 55% duty cycle

is needed on the signal at the PFD in order for CSR to operate

correctly. The reference divide-by-2 flip-flop can help to

provide a 50% duty cycle at the PFD. For example, if a 100 MHz

reference frequency is available and the user wants to run the

PFD at 10 MHz, setting the R-divide factor to 10 results in a

10 MHz PFD signal that is not 50% duty cycle. By setting the

R-divide factor to 5 and enabling the reference divide-by-2 bit,

a 50% duty cycle 10 MHz signal can be achieved.

Note that the cycle slip reduction feature can only be operated

when the phase detector polarity setting is positive (DB6 in

negative.

MODULATION

The ADF4158 can operate in frequency shift keying (FSK) or

phase shift keying (PSK) mode.

Frequency Shift Keying (FSK)

FSK is implemented by setting the ADF4158 N-divider up for

the center frequency and then toggling the TXDATA pin. The

deviation from the center frequency is set by

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

(7)

where:

DEV is a 16-bit word.

DEV_OFFSET is a 4-bit word.

fPFD is the PFD frequency.

The ADF4158 implements this by incrementing or decre-

menting the set N-divide value by DEV × 2DEV_OFFSET.

Phase Shift Keying (PSK)

When the ADF4158 is set up in PSK mode, it is possible to

toggle the output phase of the ADF4158 between 0° and 180°.

The TXDATA pin controls the phase.

FSK Settings Worked Example

For example, take an FSK system operating at 5.8 GHz, with

a 25 MHz PFD, 250 kHz deviation and DEV_OFFSET = 4.

Rearrange Equation 4 as follows

OFFSET

DEV

PFD

DEV

(8)

971

MHz

kHz

250

DEV

The DEV value is rounded to 20,972. Toggling the TXDATA pin

causes the frequency to hop between ±250 kHz frequencies

from the programmed center frequency.

WAVEFORM GENERATION

The ADF4158 is capable of generating four types of waveforms

in the frequency domain: single ramp burst, single sawtooth

burst, sawtooth ramp, and triangular ramp. Figure 30 through

Figure 33 show the types of waveforms available.

TIME

Figure 30. Single Ramp Burst

TIME

Figure 31. Single Sawtooth Burst

TIME

Figure 32. Sawtooth Ramp

TIME

Figure 33. Triangular Ramp

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