3.2 Slow servo loop
17
3.2.2 Slow output voltage swing
The output of the slow servo control loop is configured for a range of
0 to 2.5 V for compatibility with a
MOGL
abs
DLC
. The
DLC SWEEP
piezo control input has a voltage gain of 48 so that the maximum
input of 2.5 V results in 120 V on the piezo. When the slow servo
loop is engaged, the slow output will only swing by
±
25 mV relative
to its value prior to engagement. This limitation is intentional, to
avoid laser mode hops. When the slow output of the
FSC
is used with
a
MOGL
abs
DLC
, a 50 mV swing in the output of the slow channel
of the
FSC
corresponds to a 2.4 V swing in the piezo voltage which
corresponds to a change in laser frequency of around 0.5 to 1 GHz,
comparable to the free spectral range of a typical reference cavity.
For use with different laser controllers, a larger change in the locked
slow output of the FSC can be enabled via a simple resistor change.
The gain on the output of the slow feedback loop is defined by
R82/R87, the ratio of resistors R82 (500 Ω) and R87 (100 kΩ). To in-
crease the slow output, increase R82/R87, most easily accomplished
by reducing R87 by piggybacking another resistor in parallel (
SMD
package, size 0402). For example, adding a 30 kΩ resistor in parallel
with the existing 100 kΩ resistor would give an effective resistance of
23 kΩ providing an increase in the slow output swing from
±
25 mV
to
±
125 mV. Figure 3.4 shows the layout of the
FSC
PCB around
opamp U16.
R82
C
7
1
C
3
6
2
R
8
5
C
4
4
R
8
7
C
3
6
R
3
3
1
R81
C
3
5
U16
R329
Figure 3.4:
The
FSC
PCB layout around the final slow gain opamp U16,
with gain setting resistors R82 and R87 (circled); size 0402.
