Chapter 3, TECHNICAL DESCRIPTION
3-8
external references.
External Reference Mode
In external reference mode the reference is taken from the front panel external
reference input, except in dual external reference mode, when the second reference is
applied to the rear panel TRIG IN input.
Internal Reference Mode
With internal reference operation the reference circuit is free-running at the selected
reference frequency and is not dependent on a phase-locked loop (PLL), as is the
case in most other lock-in amplifiers. Consequently, the phase noise is extremely
low, and because no time is required for a PLL to acquire lock, reference acquisition
is immediate.
Both the signal channel and the reference channel contain calibration parameters that
are dependent on the reference frequency. These include corrections to the anti-alias
filter and to the analog circuits in the reference channel. In external reference
operation the processor uses a reference frequency meter to monitor the reference
frequency and updates these parameters when a change of about 2 percent has been
detected.
In all cases, it is possible to configure the rear panel TRIG OUT connector to output
a TTL logic signal at the present reference frequency.
3.3.09 Phase-Shifter
Each demodulator has a digital reference phase-shifter, allowing the phase values
being sent to the in-phase and quadrature multipliers to be adjusted to the required
value. If the reference input is a sinusoid applied to the front panel REF IN
connector, the reference phase is defined as the phase of the X demodulation
function with respect to the reference input.
This means that when the reference phase is zero and the signal input to the
demodulator is a full-scale sinusoid in phase with the reference input sinusoid, the X
channel output of the demodulator is a full-scale positive value and the Y channel
output is zero.
The general-purpose setting of the external reference channel input detects positive-
going crossings of the mean value of the applied reference voltage. Therefore when
the reference input is not sinusoidal, its effective phase is the phase of a sinusoid
with a positive-going zero crossing at the same point in time, and accordingly the
reference phase is defined with respect to this waveform. Similarly, the effective
phase of a reference input when the channel is configured for TTL-logic level signals
is that of a sinusoid with a positive-going zero crossing at the same point in time.
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
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