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1.2 Quick Start Guide
15
1.2.4 Use with a Lock-In Amplifier
The SR542 can be used with a lock‑in as a
follower
or a
reference
. To
follow
the lock‑in, simply connect the reference output from the lock‑in to the
Ext Sync Input
of the SR542 as described above in Section 1.2.3: Lock
to External Frequency. Alternatively, the lock‑in can follow the SR542
as described here. In this example,
Outer Slots Ref Out
is used as the
reference signal input to the lock‑in amplifier.
To perform the steps in this section, in addition to the setup from the
previous sections you will need:
• a lock‑in amplifier (e.g. SR860)
• a light source (optional)
• a photodiode, phototransistor, or other light‑sensitive detector
Beginning with the setup described in Section 1.2.1:
1. Select
Internal Freq
as the
Source
by pressing
Source
→
Int
repeatedly.
2. Select
Outer
as the
Control
by pressing
Control
Shaft
repeatedly.
3. Set the internal frequency to 75 Hz by selecting
Int Freq
with
and
and then entering
7
+
5
+
.
4. Connect the
Outer Slots Ref Out
from the rear panel of the SR542 to
the reference input of the lock‑in amplifier using a standard BNC
cable (not included).
5. Configure the lock‑in reference settings to use an external refer‑
ence with TTL input and 50
Ω
termination, if available.
The
Outer Slots Ref Out
signal is derived from an opto‑interrupter
and is not available until the chopper is running. As a result, the
lock‑in amplifier will not yet be able to lock to this signal.
6. Press and hold
Run /
Stop
until the chopper starts.
Now the lock‑in amplifier should be able to lock to the
Outer Slots
Ref Out
signal.
7. Connect the photodiode to the lock‑in input and position the pho‑
todiode lens near the chopper blade.
A light source, such as a laser, can be used to illuminate the pho‑
todiode detector through the chopper blade but ambient light can
often produce a large enough signal if the detector is placed very
close to the blade.
SR542 Precision Optical Chopper