Model 7124
Precision Lock-in Amplifier
Introduction
Since their invention back in the 1960's, lock-in amplifiers have been
used whenever the need arises to measure the amplitude and/or phase of a
signal of known frequency in the presence of noise. Unlike other AC
measuring instruments they have the ability to give accurate results even
when the noise is much larger than the signal - in favorable conditions
even up to a million times larger.
Early instruments used analog technology, with manual controls and
switches, and with output readings being taken from large panel meters.
Later, microprocessors were added to give more user-friendly operation,
digital output displays, and to support computer control. More recently
the analog phase sensitive detectors forming the heart of the instrument
have been replaced by DSP (digital signal processing) designs, further
improving performance.
But the addition of this digital technology has had one unfortunate
side effect, which is that the instrument itself can act as a source of
digital clock and switching noise, which is typically coupled back into
the experiment via the signal or internal oscillator connectors. This
noise is of course rejected by the lock-in and generally does not impair
its performance, but the power it dissipates in the sample or device under
test can cause serious problems. This is particularly the case in low
temperature physics experiments.
SIGNAL RECOVERY is therefore proud to
introduce the model 7124 precision lock-in amplifier, which is
particularly suited to such work. It uses a unique analog fiber optic link
to interconnect a remote connection unit (RCU), to which the experiment is
connected, and a main instrument console. In normal operation there are no
digital clock signals within the RCU, and so it can emit no switching
noise.
This architecture gives an instrument with all the advantages of the
latest DSP technology for signal detection, and a powerful processor for
easy user operation, as well as the low noise performance that until now
has only been available in instruments of all-analog design.
Signal and Reference Connections
In normal use the 7124's signal and reference connections are made at
the RCU. The signal input can be switched to operate in single ended or
differential voltage mode, or in current mode with a choice of two
transimpedance settings. It can also be used to switch between two
single-ended voltage signals, for simple sequential measurement, under
computer control, of two inputs. In cases where further preamplification
is needed then one of the SIGNAL RECOVERY
remote preamplifiers can be used, with its output connected to the 7124's
single ended voltage input. This flexible choice of input modes allows the
best possible connection to be made to the experiment. The RCU also has
both general purpose analog and TTL logic reference inputs, as well as the
output for a precision DDS oscillator that generates a sine wave signal of
adjustable frequency and amplitude.
The RCU is connected to the main instrument console via a 16ft (5 m)
fiber cable bundle that carries the applied signal and oscillator outputs
in analog form, and control signals to the RCU and the reference signal
from it, in digital format. However, the RCU contains no digital clocks or
oscillators that are running continuously, and so unlike all other lock-in
amplifiers that use any form of logic control, there is no digital switch
noise present at the signal connectors.
Main Console
 The
7124's main console is a compact, benchtop unit with a color display, and
keys for operating the instrument controls, accessing different menus, and
easy entry of numeric values. It receives signals from the RCU module and
processes them using powerful DSP algorithms running in a dedicated field
programmable gate array (FPGA), supported by a ColdFire processor. The
operating frequency range is from 0.5 Hz to 150 kHz, with a main
ADC sampling rate and analog output DAC update rate of 1 MHz, which
is ideal for use with the available time constants, which go down to 10 µs.
Manual operation is straightforward and based on a similar menu
structure to that used on the model 7280, using the color TFT
display panel in conjunction with the keys grouped around it and the
numeric keypad to adjust the instrument's controls, with the selected
outputs being shown both on the display and being available as analog
signals from four rear-panel connectors.
The Main Display is used in normal operation and shows four
user-selected instrument controls on the left-hand side and four
user-selected outputs, output offset status, and the present reference
frequency, on the right. The output display selections include digital and
bar-graph displays in a variety of formats. Error information, such as
input and output overload, and reference unlock indication, is clearly
shown along the top edge of the display, while soft keys along the bottom
edge are used for selecting controls and to initiate numerical keypad data
entry.
Extended Operating Modes
The instrument includes the extended operating modes made popular by
other SIGNAL RECOVERY lock-in amplifiers,
such as the models 7265 and 7280.
In normal Single Reference mode, harmonic analysis can be
performed on harmonics up to 127 x F, while in Dual
Harmonic mode the signals at two harmonics of the reference signal
can be simultaneously measured. The instrument can therefore be used to
measure a fundamental frequency and one harmonic of it at the same time.
Dual Reference mode permits measurement of two signals at
two unrelated frequencies to be performed simultaneously. For example, in
an optical experiment the signals passing through two different paths can
be independently measured if they are modulated at two different
modulation frequencies.
 The
Spectral Display allows the spectrum of the signals
present at the input to be calculated and displayed, which can help when
choosing the reference frequency
The instrument also includes a "tandem" demodulation
mode which allows an amplitude-modulated signal to be first demodulated at
a carrier frequency, with the output from this demodulation being
processed by a second demodulator running at a lower frequency.
The Synchronous Oscillator output is an analog sinusoidal
signal equivalent to that being used to drive the in-phase demodulator,
and available in both internal and external reference modes. Hence, for
example, if the instrument is set to 2F reference mode and a 1 kHz
reference is applied, then this output will be a 2 kHz sine wave.
Inputs and Outputs
The 7124RCU unit has inputs to the signal and reference channel, and
an oscillator output. It also has an 8-way digital output port for
peripheral control. The main console provides another oscillator output,
auxiliary signal channel input, four auxiliary ADC inputs, and four DAC
outputs. The DAC outputs can be set to function as analog outputs for the
signal measurement (e.g. X, Y, Magnitude, Phase) and/or general purpose
programmable analog outputs.
Computer Control
External control of the unit is via USB, RS232 or Ethernet interfaces,
using simple mnemonic-type ASCII commands.
Software support is available in the form of a LabVIEW
driver supporting all instrument functions, and the Acquire™
data acquisition software. The driver and a demonstration version of the
software, DemoAcquire, are available for download from this site.
Click here for detailed
specifications
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a trademark of AMETEK, Inc.