Internal / External Reference Mode; Virtual Reference Mode; Principles Of Operation; Block Diagram - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.2.04 Internal / External Reference Mode

3.2.05 Virtual Reference Mode

3.3 Principles of Operation

3.3.01 Block Diagram

3-2
In the internal reference mode, the instrument's reference frequency is derived from
its internal oscillator and the oscillator signal is used to drive the experiment.
In the external reference mode, the experiment includes some device, for example an
optical chopper, which generates a reference frequency that is applied to one of the
lock-in amplifier's external reference inputs. The instrument's reference channel
"locks" to this signal and uses it to measure the applied input signal.
If the instrument is operated in internal reference mode, measuring a signal which is
phase-locked to the internal oscillator, with the reference phase correctly adjusted,
then it will generate a stable non-zero X channel output and a zero Y channel output.
If, however, the signal is derived from a separate oscillator, then the X channel and
Y channel outputs will show variations at a frequency equal to the difference between
the signal and internal oscillator frequencies. If the latter is now set to be equal to the
former then in principle the variation in the outputs will cease, but in practice this
will not happen because of slow changes in the relative phase of the two oscillators.
In the virtual reference mode, believed to be unique to SIGNAL RECOVERY lock-
in amplifiers, the Y channel output is used to make continuous adjustments to the
internal oscillator frequency and phase to achieve phase-lock with the applied signal,
such that the X channel output is maximized and the Y channel output zeroed.
If the instrument is correctly adjusted, particularly ensuring that the full-scale
sensitivity control is maintained at a suitable setting in relation to changes in the
signal level, then the virtual reference mode is capable of making signal recovery
measurements which are not possible with most other lock-in amplifiers.
The model 7280 uses digital signal processing (DSP) techniques implemented in
field-programmable gate arrays (FPGA), a microprocessor and very low-noise analog
circuitry to achieve its specifications. A block diagram of the instrument is shown in
figure 3-1. The sections that follow describe how each functional block operates and
the effect it has on the instrument's performance.