Datasheet

ADF4158 Data Sheet
Rev. E | Page 26 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
Register R3). It cannot be used if the phase detector polarity is
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 TX
DATA
pin. The
deviation from the center frequency is set by
f
DEV
= (f
PFD
/2
25
) × (DEV × 2
DEV_OFFSET
) (7)
where:
DEV is a 16-bit word.
DEV_OFFSET is a 4-bit word.
f
PFD
is the PFD frequency.
The ADF4158 implements this by incrementing or decrementing
the set N-divide value by DEV × 2
DEV_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 TX
DATA
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
OFFSETDEV
PFD
DEV
f
f
DEV
_
25
2
2
×
=
(8)
52.971,20
2
2
MHz25
kHz250
4
25
=
×
=DEV
The DEV value is rounded to 20,972. Toggling the TX
DATA
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 31 through
Figure 34 show the types of waveforms available.
FREQUENCY
TIME
08728-022
Figure 31. Single Ramp Burst
FREQUENCY
TIME
08728-021
Figure 32. Single Sawtooth Burst
FREQUENCY
TIME
08728-019
Figure 33. Sawtooth Ramp
FREQUENCY
TIME
08728-020
Figure 34. Triangular Ramp