2.2
Instrument overview
2 – 3
harmonics). This process is also known as phase-sensitive detection,
and the square-wave mixer is also referred to as the phase-sensitive
detector (PSD) or the synchronous detector.
A separate
AC Volt
mode allows the square-wave mixer to be con-
trolled by the signal polarity of the input signal itself, essentially con-
verting the SR124 into an absolute-value averaging measurement.
When a phase reference signal is not available, the AC Volt mode
(together with the input filter) can be used to recover specific AC sig-
nals as well, although the lock-in mode will almost always produce
superior results when a reference is available.
The final measured value is displayed on the jeweled panel meter,
and output as a proportional DC voltage. Full-scale sensitivity is
set from the front panel in 1 – 2 – 5 steps from 100 nV to 500 mV. A
full scale input in-phase with the reference will generate
+
10 V at the
output BNC, while a full scale signal 180
◦
out of phase will generate
−
10 V. Lock-in amplifiers as a general rule display the input signal
in volts RMS, and this is the basis for the SR124 calibration as well.
For example, if the SR124 is configured for 2 mV sensitivity and a
1 mVrms sine wave is input, at the reference frequency, then the BNC
output will read
+
5 V and the panel meter will show 50% positive
deflection.
2.2
Instrument overview
An overview of the SR124 with its main sections is given below.
Further details of each block are in chapter 3. A block diagram of
the SR124 is given in Figure 2.1.
2.2.1
Reference section
Operating in Lock-In mode, the SR124 requires the reference oscil-
lator to control the frequency and phase of the square-wave mixer.
The analog reference oscillator of the SR124 generates a stable sine
wave with outputs at 0
◦
, 90
◦
, 180
◦
, and 270
◦
(all four outputs can
be monitored from rear panel BNC connectors). The oscillator is
based on an analog voltage-controlled oscillator (VCO) that can tune
across a factor of 100 in frequency; 5 overlapping frequency ranges
are available for operation from 0.2 Hz – 21 Hz, up to 2 kHz – 210 kHz.
The oscillator is controlled in one of three ways: internal, rear-panel
VCO, and external reference. When the SR124 operates in AC Volt
mode, the reference oscillator is available for excitation outputs, but
is not routed to the PSD.
SR124
Analog Lock-In Amplifier