2 – 6
Introduction
2.2.1.5
Phase shift
A commandable phase shift, between 0
◦
and 360
◦
, is applied between
the reference oscillator and the phase-sensitive detector. This phase
shift determines the phase at which the lock-in will respond. If the
Ref. Out signal is directly connected to the Signal Input (A), and the
phase is set to 90
◦
, the output will be near zero; setting the phase to
180
◦
will result in a negative output signal.
2.2.2
Signal section
The SR124 Signal section provides the amplification and signal con-
ditioning of the analog signal that is applied to the phase sensitive
detector. Inputs can be either voltage or current, and single-ended
or di
ff
erential.
2.2.2.1
Voltage preamplifier
The SR124 front-end signal input stage consists of a JFET-based low-
noise di
ff
erential voltage preamplifier. Inputs can be configured as
either single-ended (A) or di
ff
erential (A–B). To preserve the low
noise performance of the input JFET’s, this front-end input stage am-
plifies the input by a gain of 50
×
(5
×
for the largest scale sensitivities).
In single-ended operation, the SR124 internally measures the signal
as a voltage on the center pin of the A input, referenced to internal
ground in the instrument. This is indicated as
A
on the front panel.
In fully di
ff
erential operation, the preamplifier measures the volt-
age di
ff
erence between the center pins of the A and B inputs. This
configuration is indicated as
A–B
on the front panel.
When using di
ff
erential input mode, it is important that both input
cables travel the same path between the experiment and the lock-in.
Specifically, there should not be a large loop area enclosed by the two
cables. Such loops are susceptible to magnetic pickup. Ideally, the
two coax cables are equal length and fastened to each other along
their length.
When used with an SRS external preamp, such as any model from
the SR55x series, the SR124 should always be configured for
A–B
input, and two equal-length BNC cables should be used to connect
the preamp output to the lock-in signal input.
2.2.2.2
Current preamplifier
The current input on the SR124 uses the A input BNC. Two internal
gain settings are available: 10
6
Volts
/
Amp and 10
8
Volts
/
Amp. The
current input is always DC coupled and includes an input (burden)
SR124
Analog Lock-In Amplifier
