Datasheet

Data Sheet ADF4158
Rev. E | Page 29 of 36
An example of ramp with FSK on the top of it is shown in
Figure 37. Eventually, the ramp must be activated as described
in Activating the Ramp section.
08728-025
FREQUENCY
0 RAMP END
FREQUENCY SWEEP
FSK SHIFT
TIME
LFM STEP =
FREQUENCY
SWEEP/NUMBER
OF STEPS
Figure 37. Combined FSK and LFM Waveform (N Corresponds to the
Number of LFM Steps)
Delayed Start
A delayed start can be used with two different parts to control
the start time. The idea of delayed start is shown in Figure 38.
FREQUENCY
TIME
RAMP WITH
DELAYED START
RAMP WITHOUT
DELAYED START
08728-126
Figure 38. Delayed Start of Sawtooth Ramp
Example
For example, to program a delayed start with two different parts
to control the start time,
1. Set DB15 in Register R7 to 1 to enable the delayed start of
ramp option.
2. Set Bit DB16 in Register R7 to 0 and the 12-bit delay start
word (DB[14:3] in Register R7) to 125 to delay the ramp on
the first part is delayed by 5 µs, f
PFD
= 25 MHz. The delay is
calculated as follows:
Delay = t
PFD
× Delay Start Word
= 40 ns × 125 = 5 µs
3. Set Bit DB16 in Register R7 to 1 and the 12-bit delay start
word (DB[14:3] in Register R7) to 125 to delay the ramp
on the second part is delayed by 125 µs. Use the following
formula for calculating the delay:
Delay = t
PFD
× CLK
1
× Delay Start Word
= 40 ns × 25 × 125 = 125 µs
Eventually, the ramp must be activated as described in
Activating the Ramp section.
Delay Between Ramps
This feature adds a delay between bursts in ramp. Figure 39
shows a delay between ramps in sawtooth mode.
FREQUENCY
TIME
DELAY
08728-028
Figure 39. Delay Between Ramps for Sawtooth Mode
Example
For example, to add a delay between bursts in a ramp,
1. Set DB17 in Register R7 to 1 to enable delay between
ramps option.
2. Set Bit DB16 in Register R7 to 0 and the 12-bit delay start
word (DB[14:3] in Register R7) to 125 to delay the ramp
by 5 µs, f
PFD
= 25 MHz. The delay is calculated as follows:
Delay = t
PFD
× Delay Start Word
= 40 ns × 125 = 5 µs
If a longer delay is needed, for example, 125 µs, Bit DB16
in Register R7 should be set to 1 and the 12-bit delay start
word (DB[14:3] in Register R7) should be set to 125. The
delay is calculated as follows
Delay = t
PFD
× CLK
1
× Delay Start Word
= 40 ns × 25 × 125 = 125 µs
There is also a possibility to activate fast-lock operation for the
first period of delay. This is done by setting Bit DB18 in Register R7
to 1. This feature is useful for sawtooth ramps to mitigate the
frequency overshoot on the transition from one sawtooth to the
next. Eventually, the ramp must be activated as described in
Activating the Ramp section.
Nonlinear Ramp Mode
The ADF4158 is capable of generating a parabolic ramp. The
output frequency is generated according to the following
equation:
f
OUT
(n + 1) = f
OUT
(n) + n × f
DEV
(16)
where:
f
OUT
is output frequency.
f
DEV
is frequency deviation.
n is step number.