Introduction; Operating Modes; Single Reference / Dual Reference; Single Harmonic / Dual Harmonic - Ametek 7280 Instruction Manual

Technical Description

3.1 Introduction

3.2 Operating Modes

3.2.01 Introduction

3.2.02 Single Reference / Dual Reference

3.2.03 Single Harmonic / Dual Harmonic

The model 7280 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 7280 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 7280
this is referred to as the single reference mode.
The dual reference mode incorporated in the model 7280 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, such as the requirement that one of the reference signals be external and
the other be derived from the internal oscillator, the limitation of the maximum
operating frequency to 20 kHz (unless the unit is equipped with the -/99 or -/98
extended frequency options) and the requirement that both signals be passed through
the same input signal channel. This last restriction 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.
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 7280 allows this multiple to be set
to any value between 2 (i.e. the second harmonic) and 32, as well as unity, which is
the normal mode. The only restriction is that the product n × f cannot exceed 2 MHz.
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 20 kHz (800 kHz with the option -
/99 or 2 MHz with the option -/98).
3
Chapter
3-1