New Focus LB1005 - Typical Operation

Getting Started LB1005 Operating Manual
10 Revision 1 Bookham
waveform from a function generator sweeps the optical frequency transducer over multiple
resonances. A photodetector measures the corresponding transmission response. b) The
Sweep Span reduces the frequency sweep so that only one resonance is shown on the Error
Monitor signal. The Sweep Center tunes the frequency to locate a particular resonance, and
“center” the lock point prior to applying the feedback. The Input Offset locates the lock
point on the slope of the resonance. Note that the two lock points have opposite gain signs.
Typical Operation
The controls of the LB1005 are conveniently arranged for acquiring and
optimizing lock. In this section, five steps are suggested that will meet the needs
of most applications. Figure 5 shows the controls that are most likely to be
adjusted for each step. The solid lines indicate controls that are used most often,
while the dashed lines show controls that are less frequently adjusted.
1. Reset integrator: Before locking, reset the integrator by moving the Acquire
switch to the Lock Off position.
2. Find the locking point: Many applications will require the user to search for
the locking point, such as the side or center of an optical resonance. Turn on
sweeping by clicking the Sweep Span knob from its Off position, adjusting it
for a wide span. Use the Sweep Center knob to adjust the transducer bias so
that the lock feature is “centered” within the scan. If needed, the desired lock
point can be adjusted to zero voltage with the Input Offset knob while
monitoring the error signal. Adjust both the Sweep Center and Sweep Span
controls so that the oscilloscope scan shows the discriminator slope of the
lock feature. Figure 4 illustrates these concepts for a “side lock” to a
resonance.
3. Acquire lock: The Acquire toggle switch offers two gain settings for turning
on the feedback control. When acquiring lock, sometimes it is helpful to limit
the DC gain of the integrator. The LFGL position is an intermediate lock
mode that applies the P-I filter with a low frequency gain limit (see section
Filter Transfer Functions of Chapter 3 for details). The LF Gain Limit front
panel switch determines the setting for this gain limit, and is typically only
set once for a specific servo application.