Internal Oscillator - General; Internal Oscillator - Update Rate; Internal Oscillator - Frequency & Amplitude Sweeps - Ametek 7280 Instruction Manual

In basic lock-in amplifier applications the purpose of the experiment is to measure
the amplitude of a signal which is of fixed frequency and whose phase with respect to
the reference input does not vary. This is the scalar measurement, often implemented
with a chopped optical beam. Many other lock-in amplifier applications are of the
signed scalar type, in which the purpose of the experiment is to measure the
amplitude and sign of a signal which is of fixed frequency and whose phase with
respect to the reference input does not vary apart from reversals of phase
corresponding to changes in the sign of the signal. A well-known example of this
situation is the case of a resistive bridge, one arm of which contains the sample to be
measured. Other examples occur in derivative spectroscopy, where a small
modulation is applied to the angle of the grating (in optical spectroscopy) or to the
applied magnetic field (in magnetic resonance spectroscopy). Double beam
spectroscopy is a further common example.
In this signed scalar measurement the phase-shifter must be set, after removal of any
zero errors, to maximize the X channel or the Y channel output of the demodulator.
This is the only method that will give correct operation as the output signal passes
through zero, and is also the best method to be used in an unsigned scalar
measurement where any significant amount of noise is present.

3.3.09 Internal Oscillator - General

The model 7280, in common with many other lock-in amplifiers, incorporates an
internal oscillator which may be used to drive an experiment. However, unlike most
other instruments, the oscillator in the model 7280 is digitally synthesized with the
result that the output frequency is extremely accurate and stable. The oscillator
operates over the same frequency range as the lock-in amplifier, that is 500 mHz to
2.0 MHz, and is implemented using a dedicated direct digital synthesis circuit.

3.3.10 Internal Oscillator - Update Rate

The direct digital synthesis (DDS) technique generates a waveform at the DAC
output which is not a pure sinusoid, but rather a stepped approximation to one. This
is then filtered by the buffer stage, which follows the DAC, to reduce the harmonic
distortion to an acceptable level. The update rate is 22.5 MHz.
3.3.11 Internal Oscillator - Frequency & Amplitude Sweeps
The internal oscillator output may be swept in both frequency and amplitude. In both
cases the sweeps take the form of a series of steps between starting and finishing
values. Frequency sweeps may use equal increment step sizes, giving a linear change
of frequency with time as the sweep proceeds, or may use step sizes proportional to
the present frequency, which produces a logarithmic sweep. The amplitude sweep
function offers only linear sweeps.
A special form of the frequency sweep function is used to acquire lock when the
instrument is operating in the virtual reference mode. When this "seek" sweep is
activated, the oscillator starts at a user-specified frequency, which should be just
below that of the applied signal, and increments until the calculated magnitude output
is greater than 50%. At this point the sweep then stops and the virtual reference mode
achieves lock, by continuously adjusting the internal oscillator frequency to maintain
the Y channel output at zero.
Chapter 3, TECHNICAL DESCRIPTION
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