User Manual | PE11S100X Series Synthesizer
10
4.4 CW Sweeper Mode
The internal PLL features a built-in frequency sweeper function, useful for test instrumentation, FMCW
sensors, automotive radars, and other applications.
Sweeper modes include:
a. Single-Step Ramp Mode
b. 1-Way Sweep Mode
c. 2-Way Sweep Mode (alternating positive and negative frequency ramps)
The sweep generator is enabled with ramp_enable in
<1>. The sweep function cycles
through a series of discrete frequency values, which may be:
a. Single-stepped by individual triggers if ramp_singlestep (
<6>) is set, or
b. Stepped by an automatic sequencer if ramp_singlestep (
<6>) is cleared
Triggering of sweeps, or of steps in single-step mode, may be configured to operate as follows:
a. Triggered from TTL input on GPIO3 if
<5> = 1, or
b. Triggered by setting ramp_trigg (
<2>), if ramp_trigg has previously been cleared, or
c. Triggered with an automatically-generated internal trigger, if ramp_repeat_en
<3>) is set.
The sweep will begin at the current frequency value of the synthesizer, denoted as f0. The frequency step
size for the ramp is set by ramp_step (
), with
Δf
step
= ramp_step * f
REF
/ 2
24
The total number of ramp steps taken in a single sweep is given by ramp_steps_number (
), and
the initial ramp direction is set to be increasing or decreasing in frequency by clearing or setting ramp_
startdir_dn (
<4>) respectively. Setting ramp_singledir (
<7>) restricts the direction of the
sweep to the initial sweep direction only.
The final ramp frequency, ff, is given by f
f
= f
0
+ Δf
step
* ramp_steps_number for increasing frequency ramps
and f
f
= f
0
-
Δf
step
* ramp_steps_number for decreasing-frequency ramps. Unless in single-step mode, the
sweeper timebase, TREF, is the period of the divided reference f
REF
at the phase detector. So in and the
total time to ramp from f
0
to f
f
is T
ramp
= T
REF
* ramp_steps_number.
The user should be aware that the synthesized ramp is subject to normal phase-locked-loop dynamics. If
the loop bandwidth in use is much wider than the rate of the steps, then the locking will be very fast and the
ramp will have a staircase shape. As the update rate approaches the loop bandwidth, the loop will not fully
settle before a new frequency step is received. In this case, the swept output will have a small lag and will
sweep in a near-continuous fashion.
4.4.1 One-Way Sweeps
One-way sweeps are selected by enabling ramp_singledir (
<7>). At the end of the ramp time, T
ramp
,
the sweeper will dwell at the final frequency, f
f
, until a new trigger is received. The second trigger will hop
the synthesizer back to the initial frequency, f
0
. The third trigger will restart the sweep from f
0
.
