Introduction - Ametek 7270 Instruction Manual

Front Panel Operation

5.1 Introduction

This chapter describes how to operate the model 7270 using the front panel controls,
and discusses its capabilities when used in this way. Chapter 6 provides similar
information for when the unit is operated remotely using one of the computer
interfaces.
It is assumed that readers are already familiar with the use of the front panel
keys, but if not then they should refer to the detailed description of their
operation given in chapter 4.
The model 7270 uses a flexible, menu-based, control structure that allows many
instrument controls to be adjusted from the front panel with only a few key presses.
Furthermore this design makes it very easy to introduce new features or improve
existing ones without the restrictions which would result from a fixed front panel
layout.
The instrument may be operated in one of four modes, as follows:-
Single Reference
This is the normal operating mode of the unit, where it functions as a conventional
dual phase lock-in amplifier. It includes both internal and external reference modes
and provides detection either at the reference frequency or one harmonic of it.
Virtual Reference
Virtual reference mode is an extension of internal single reference mode operation,
where the Y channel output is used to make continuous adjustments to the internal
oscillator frequency and phase. The effect is to achieve phase-lock with the applied
signal such that the X channel output is maximized and the Y channel output is
zeroed. Virtual reference mode operation is only possible with signals at frequencies
between 100 Hz and 250 kHz.
Dual Reference
In dual reference mode the model 7270 can make simultaneous measurements at two
different reference frequencies. If one is an external and the other the internal
oscillator, then both can be of any frequency from 0.001 Hz to 250 kHz.
Alternatively, if two external reference frequencies are used then one of them is
restricted to a maximum frequency of 3.0 kHz.
Tandem Demodulation
Some experiments, such as pump-probe optical work, generate an amplitude
modulated carrier signal. The carrier signal is at one, typically higher, frequency, and
the modulation at another, lower, frequency. The required measurement is the
amplitude and/or phase of the modulation.
Conventionally, lock-in detection of this signal would require two instruments. The
first would be run at the carrier frequency with its output filters set to a sufficiently
short time constant to allow the amplitude modulation to pass. Hence the X-channel
analog output would be the modulation that was to be measured; to achieve this the
signal would then be passed to the signal input of a second lock-in amplifier running
at the modulation frequency.
The built-in Tandem Demodulation mode, which is essentially the same as dual
5
Chapter
and
5-1