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Chapter 3, TECHNICAL DESCRIPTION
3.3.05 Anti-Aliasing Filter
3-6
change in the AC Gain.
The user is prevented from setting an illegal AC Gain value, i.e. one that would
result in overload on a full-scale input signal. Similarly, if the user selects a full-scale
sensitivity that causes the present AC Gain value to be illegal, the AC Gain will
change to the nearest legal value.
In practice, this system is very easy to operate. However, the user may prefer to make
use of the AUTOMATIC AC Gain feature that gives very good results in most cases.
When this is active the AC Gain is automatically controlled by the instrument, which
determines the optimum setting based on the full-scale sensitivity currently being
used.
At any given setting, the ratio
DR
0.7
represents the factor by which the largest acceptable sinusoidal interference input
exceeds the full-scale sensitivity and is called the Dynamic Reserve of the lock-in
amplifier at that setting. (The factor 0.7 is a peak-to-rms conversion). The dynamic
reserve is often expressed in decibels, for which
DR(in
dB)
Applying this formula to the model 7124 at the maximum value of INPUT LIMIT
(2.0 V) and the smallest available value of FULL-SCALE SENSITIVITY (10 nV),
gives a maximum available dynamic reserve of about 1 × 10
this magnitude are available from any DSP lock-in amplifier but are based only on
arithmetical identities and do not give any indication of how the instrument actually
performs. In fact, all current DSP lock-in amplifiers become too noisy and inaccurate
for most purposes at reserves of greater than about 100 dB.
For the benefit of users who prefer to have the AC Gain value expressed in decibels,
the model 7124 displays the current value of Dynamic Reserve (DR) in this form, on
the input full-scale sensitivity control, for values up to 100 dB. Above 100 dB the
legend changes to "DR>100".
Prior to transmission via the fiber optic link the signal passes through an anti-aliasing
filter to remove unwanted frequencies which would cause a spurious output from the
main ADC within the Main Console as a result of the sampling process.
Consider the situation when the lock-in amplifier is measuring a sinusoidal signal of
frequency f
Hz, which is sampled by the main ADC at a sampling frequency
signal
f
Hz. In order to ensure correct operation of the instrument the output values
sampling
representing the f
frequency must be uniquely generated by the signal to be
signal
measured, and not by any other process.
However, if the input to the ADC has, in addition, an unwanted sinusoidal signal
with frequency f
Hz, where f
1
will appear in the output as a sampled-data sinusoid with frequency less than half the
sampling frequency, f
alias
indistinguishable from the output generated when a genuine signal at frequency f
is sampled. Hence if the frequency of the unwanted signal were such that the alias
Input
Limit
Full
-
Scale
Sensitivit
y
20
log(DR(as
a
ratio
is greater than half the sampling frequency, then this
1
= |f
- nf
|, where n is an integer. This alias signal is
1
sampling
))
8
or 160 dB. Figures of
alias
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