2 – 4
Introduction
A / I
B
PREAMP
AC GAIN
DC GAIN
OUTPUT
REF OUT
SIGNAL
MONITOR
OFFSET
DC
BIAS
INTERNAL
TIME
CONSTANT
PSD
FLOAT
A
A–B
LOCK-IN
AC VOLT
INPUT
FILTER
PHASE
SHIFT
REF
ATTEN
+
–
+
–
×10
+
–
×1
REAR VCO
EXT. IN
PLL
VCO
90°
0°
SQUARE
600 Ω
600 Ω
50 Ω
Figure 2.1: The SR124 block diagram.
2.2.1.1
Internal mode
Operating in Internal mode, the SR124 oscillator is programmed by
an internally-generated DC voltage source. Within the oscillator
frequency range (set by the
Range knob), the oscillator frequency is
controlled with the front-panel control knob or remote command.
2.2.1.2
Rear-panel VCO mode
When configured for Rear VCO operation, the SR124 oscillator pro-
gramming voltage is directly controlled by the user through a rear-
panel BNC input. Input voltage from 0 to
+
10 V will set the oscillator
frequency between the lower and upper limits set by the range, with
an approximately linear transfer function.
2.2.1.3
External mode
The SR124 oscillator can lock to an external reference signal applied
to the Ext. In BNC connector. This input operates in two distinct
modes, sine input and TTL input (indicated by the
TTL
indicator
just above the connector).
SR124
Analog Lock-In Amplifier
