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2.2 Instrument overview
2.2 Instrument overview
2.2.1 Reference section
SR124 Analog Lock-In Amplifier
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.
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.
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.
2 – 3
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