14
Chapter 3. Feedback control loops
3.1
Input stage
The input stage of the
FSC
(figure 3.2) generates an
error signal
as
V
ERR
=
V
A
− V
B
− V
OFFSET
.
V
A
is taken from the “
A IN
”
SMA
con-
nector, and
V
B
is set using the
CHB
selector switch, which chooses
between the “
B IN
”
SMA
connector,
V
B
= 0 or
V
B
=
V
REF
as set by
the adjacent trimpot.
The controller acts to servo the error signal towards zero, which
defines the lock point. Some applications may benefit from small
adjustments to the DC level to adjust this lock point, which can be
achieved with the 10-turn knob
ERR OFFSET
for up to
±
0
.
1 V shift,
provided the
INPUT
selector is set to “offset” mode (∆). Larger offsets
can be achieved with the
REF
trimpot.
B IN
0v
+
–
CHB
+
+
–
V
REF
A IN
ERR OFFSET
FAST SIGN
SLOW SIGN
INPUT
FE
SE
SLOW ERR
Fast error
Slow error
FAST ERR
DC block
0v
0v
INPUT
B
A
Fast AC [7]
∫
AC
Δ
DC
Figure 3.2:
Schematic of the
FSC
input stage showing coupling, offset
and polarity controls. Hexagons are monitored signals available via the
front-panel monitor selector switches.
3.2
Slow servo loop
Figure 3.3 shows the slow feedback configuration of the
FSC
. A vari-
able gain stage is controlled with the front-panel
SLOW GAIN
knob.
The action of the controller is either a single- or double-integrator
