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Chapter 3, TECHNICAL DESCRIPTION
3.3.13 Internal Oscillator - General
3.3.14 Internal Oscillator - Update Rate
3.3.15 Internal Oscillator - Frequency & Amplitude Sweeps
3-10
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.
The model 7124, in common with many other lock-in amplifiers, incorporates an
internal oscillator, which may be used to drive the experiment. However, unlike
older instruments, the oscillator in the model 7124 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
150.0 kHz. The oscillator signal is available at either the main console OSC OUT
connector or, after transmission via an analog fiber optic link, at the same connector
on the 7124 RCU.
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 2.0 MHz.
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.
It is important to note that this type of phase-locked loop, unlike a conventional
edge-triggered type using a clean reference, does not automatically re-acquire lock
after it has been lost. Lock can be lost as a result of a signal channel transient or a
phase reversal of the signal, in which case it may be necessary to repeat the lock
acquisition procedure. However, if the measurement system is set up with sufficient
precautions, particularly ensuring that the full-scale sensitivity is maintained at a
suitable setting in relation to the signal level, then the virtual reference mode is
capable of making signal recovery measurements which are not possible with other
lock-in amplifiers.
When virtual reference mode is in use, the signal at the OSC OUT connector (on the
main console or the 7124 RCU) is a sinusoid which is phase-locked to the signal.
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