New Focus LB1005 - Using the Integrator Hold; Applying Modulation Inputs

Detailed Operation LB1005 Operating Manual
16 Revision 1 Bookham
Using the Integrator Hold
The LB1005 has a feature that allows the user to electronically trigger the
integrator output to “hold” its current value. When a TTL digital “high” signal is
applied to the rear panel Int Hold BNC connector, the error signal input to the
filter stage integrator is disabled until the TTL signal goes to a “low” state. (The
Int Hold BNC can remain unconnected for normal operation.) This feature is
useful for dealing with exceptional perturbations to the controls system that
would force it to misbehave, such as coming permanently unlocked.
One possible application of the Int Hold function uses a digital signal that toggles
to its high state when the error signal exceeds some maximum allowable
excursion limit. For example, if certain operating regions of the error signal
result in indeterminate behavior of the controls system (such as railing the
integrator), more reliable operation can be obtained if the feedback is temporarily
switched off during the time that the error signal is in these regions. When a
disturbance causes the error to exceed a specified limit, the Int Hold is toggled
high to disable the feedback and hold the integrator output (control signal) at its
current value. The (uncontrolled) output is now prevented from following the
large error perturbation that would otherwise have driven the control system
into an unrecoverable state. Instead, the control voltage is maintained at a safe,
fixed value until the disturbance recedes. When the disturbance passes and the
error signal returns to its normal operating window, the digital signal to Int Hold
reverts to its low state and feedback control resumes.
Applying Modulation Inputs
The LB1005 has two external modulation inputs that are independently available
for such applications as adding a modulation frequency to the output for lock-in
detection or summing in a feedforward correction signal for improving the
command response of the feedback control. The signals applied to these inputs
are summed with the sweep and control signals to form the output signal. Rear
panel trimpots located near each of the modulation BNC connectors provide
adjustable attenuation of these signals. Turn fully clockwise for unity gain and
fully counter-clockwise to turn off the modulation. When a modulation input is
not in use, its attenuation trimpot should be turned fully counter-clockwise to
prevent unwanted “pick-up” from the other modulation channel.