What Is A Lock-In Amplifier - Ametek 7124 Instruction Manual

Chapter 1, INTRODUCTION

1.2 What is a Lock-in Amplifier?

1-2
Appendix F
Gives an alphabetical listing of the computer commands for easy reference.
New users are recommended to unpack the instrument and carry out the procedure in
chapter 2 to check that it is working satisfactorily. They should then make
themselves familiar with the information in chapters 3, 4 and 5, even if they intend
that the unit will eventually be used under computer control. Only when they are
fully conversant with operation from the front panel should they then turn to chapter
6 for information on how to use the instrument remotely. Once the structure of the
computer commands is familiar, appendix F will prove convenient as it provides a
complete alphabetical listing of these commands in a single easy-to-use section.
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forming the heart of the instrument have been replaced by DSP (digital signal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.
The model 7124 precision lock-in amplifier is designed to address this problem. It
uses a unique analog fiber optic link to interconnect a remote connection unit (the
7124 RCU), to which the experiment is connected, and a main instrument console. In
normal operation there are no digital clock signals within the 7124 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.
The model 7124 is therefore particularly suited to work in low temperature physics,
but will be equally useful in many other fields of scientific research, such as optics,
electrochemistry, materials science, fundamental physics and electrical engineering.
In these and other experiments it can function as a:-
 AC Signal Recovery Instrument
 Vector Voltmeter
 Phase Meter
 Spectrum Analyzer
 Transient Recorder
 Precision Oscillator
 Frequency Meter
 Noise Measurement Unit