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Chapter 3, TECHNICAL DESCRIPTION
3.2.04 Single Harmonic / Dual Harmonic
3.2.05 Internal / External Reference Mode
3.2.06 Virtual Reference Mode
3.3 Principles of Operation
3.3.01 Block Diagram
3-2
Normally, a lock-in amplifier measures the applied signal at the reference frequency.
However, in some applications such as Auger Spectroscopy and amplifier
characterization, it is useful to be able to make measurements at some multiple n, or
harmonic, of the reference frequency, f. The model 7124 allows this multiple to be
set to any value between 2 (i.e. the second harmonic) and 127, as well as unity,
which is the normal mode. The only restriction is that the product n × f cannot
exceed 150 kHz.
Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal. As with dual reference mode, there are a few
restrictions, such as a maximum value of n × f of 150 kHz.
In the internal reference mode, the instrument's reference frequency is derived from
its internal oscillator and the oscillator signal is used to drive the experiment.
In the external reference mode, the experiment includes some device, for example an
optical chopper, which generates a reference frequency that is applied to one of the
lock-in amplifier's external reference inputs. The instrument's reference channel
"locks" to this signal and uses it to measure the applied input signal.
If the instrument is operated in internal reference mode, measuring a signal which is
phase-locked to the internal oscillator, with the reference phase correctly adjusted,
then it will generate a stable non-zero X channel output and a zero Y channel output.
If, however, the signal is derived from a separate oscillator, then the X channel and
Y channel outputs will show variations at a frequency equal to the difference
between the signal and internal oscillator frequencies. If the latter is now set to be
equal to the former then in principle the variation in the outputs will cease, but in
practice this will not happen because of slow changes in the relative phase of the two
oscillators.
In the virtual reference mode, unique to SIGNAL RECOVERY lock-in amplifiers,
the Y channel output is used to make continuous adjustments to the internal
oscillator frequency and phase to achieve phase-lock with the applied signal, such
that the X channel output is maximized and the Y channel output zeroed.
If the instrument is correctly adjusted, particularly ensuring that the full-scale
sensitivity control is maintained at a suitable setting in relation to changes in the
signal level, then the virtual reference mode is capable of making signal recovery
measurements which are not possible with most other lock-in amplifiers.
The model 7124 is composed of two line-powered units, the 7124 RCU (remote
Connection Unit) and the 7124 Main Console. These two units are interconnected
using two, twin fiber optic cable assemblies.
The complete system uses digital signal processing (DSP) techniques implemented in
field-programmable gate arrays (FPGA), a microprocessor and very low-noise analog
circuitry to achieve its specifications. Block diagrams of the 7124 RCU and the 7124
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