What is a Lock-in
Amplifier?...
Essentially the lock-in
amplifier measures an AC voltage (or current) and gives an output in
the form of a DC voltage proportional to the value of the AC signal being
measured. It is called an "amplifier" because the DC
level at the output is usually greater than the AC level at the input and
is termed "lock-in" because it locks to and measures the
particular frequency of interest ignoring all other signals at the input.
The heart of the lock-in
amplifier is a phase sensitive detector, sometimes known as the
demodulator. It is this part of the instrument which demodulates the
frequency of interest and it should be noted that its output is also a
function of the relative phase angle between the input signal and the
associated reference signal. It follows therefore, that the lock-in
amplifier can also be used to measure the relative phase relationship of
two signals of the same frequency.
The signal path of a lock-in
amplifier is gain adjusted to achieve a specific input sensitivity when
the output of the instrument is displaying full scale. However, the input
circuits are designed to handle signals that are many hundreds of times
larger than these calibrated input maxima thus giving the instrument the
capability to make reliable measurements even when the signal of interest
is dwarfed by other unwanted stimuli. This performance feature is called
"dynamic reserve" and is a measure of the ability of a lock-in
amplifier to recover signals that are buried in noise.
Over the last twenty years or
so the lock-in amplifier has developed from the fairly simple instrument
of the late sixties to the present units with performance specifications
and features that have surpassed all expectations. Lock-in amplifiers have
found applications in many fields and are, for example, routinely used to
recover small optical signals in spectroscopy and other light measurement
experiments. They are also invaluable for detecting and measuring minute
AC electrical signals such as may be found in superconducting, cryogenic,
electronic and many other measurement situations.
Digital electronics have been
used in lock-in amplifiers for many years but the Digital Signal
Processing (DSP) lock-in amplifier represents a further step in
development and is made possible by the latest advances in device
technology. DSP lock-in amplifiers provide a number of performance
advantages over their analog counterparts, for example a very high dynamic
reserve without the need for conditioning electronics and drift-free
output stages.
The use of DSP also opens up
the possibilities for new features such as dual reference operation, where
the lock-in amplifier simultaneously measures two different frequency
components of the input signal, and Virtual ReferenceTM
detection, that is signal recovery without a reference at all. All this
becomes possible without an increase in hardware cost.
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