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Technical Description
3.1 Introduction
3.2 Operating Modes
3.2.01 Introduction
3.2.02 Single Reference / Dual Reference
3.2.03 Tandem Demodulation
The model 7124 lock-in amplifier is a sophisticated instrument with many
capabilities beyond those found in other lock-in amplifiers. This chapter discusses
the various operating modes provided and then describes the design of the instrument
by considering it as a series of functional blocks. In addition to describing how each
block operates, the sections also include information on the effect of the various
controls.
The model 7124 incorporates a number of different operating modes which are
referred to in the following technical description, so in order to help the reader's
understanding they are defined here.
Conventionally, a lock-in amplifier makes measurements such as signal magnitude,
phase, etc. on the applied signal at a single reference frequency. In the model 7124
this is referred to as the single reference mode.
The dual reference mode incorporated in the model 7270 allows the instrument to
make simultaneous measurements at two different reference frequencies, an ability
that previously required two lock-in amplifiers. This flexibility incurs a few
restrictions, most notably that in most cases both signals be passed through the same
input signal channel, which implies either that both signals are derived from the
same detector (for example two chopped light beams falling onto a single
photodiode) or that they can be summed prior to measurement, either externally or
by using the differential input mode of the instrument. Nevertheless, the mode will
prove invaluable in many experiments. Note that the restriction that that one
reference frequency be from the internal oscillator and one from an external source
which used to apply is removed for instruments with the latest firmware, allowing
dual reference mode operation with two external reference signals. However, in this
case one of the references is limited to a maximum of 3 kHz, and because there is
only one fiber optic cable to transmit the reference from the RCU to the main
console, a maximum of one external reference signal can be connected to the 7124
RCU.
A further development of the dual reference mode is Tandem Demodulation. In this
mode, the input to the second set of demodulators is taken not from the main ADC as
is the case with normal dual reference mode, but from the filtered X-channel output
of the first set of demodulators. Hence, for example, the mode can be used to
measure the modulation amplitude of an amplitude-modulated "carrier" frequency.
The carrier frequency is at the internal reference frequency, and so this can be
detected by the first stage demodulators. If the output time constant of this first stage
is short enough, then the X output will represent a signal at the modulation
frequency. The second set of demodulators, this time operating at the modulation
frequency, then measure the amplitude and/or phase of this modulation.
3
Chapter
3-1
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