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3.3.03 Line Frequency Rejection Filter
Following the signal input amplifier there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.
The filter uses two cascaded rejection stages with "notch" characteristics, allowing it
to be set to reject signals at frequencies equal to either of, or both of, the
fundamental and second harmonic of the line frequency.
Instruments are normally supplied with the line frequency filter set to 60 Hz with the
filter turned off. If the local line frequency is 50 Hz then the filter frequency should
be set to this value using the control on the Configuration menu (see section 2.3).
3.3.04 AC Gain and Dynamic Reserve
The signal channel contains a number of analog filters and amplifiers whose overall
gain is defined by the AC Gain parameter, which is specified in terms of decibels
(dB). For each value of AC Gain there is a corresponding value of the INPUT LIMIT
parameter, which is the maximum instantaneous (peak) voltage or current that can be
applied to the input without causing input overload, as shown in table 3-1 below.
It is a basic property of the digital signal processing (DSP) lock-in amplifier that the
best demodulator performance is obtained by presenting as large a signal as possible
to the main analog-to-digital converter (ADC). Therefore, in principle, the AC Gain
value should be made as large as possible without causing the signal channel
amplifier or converter to overload. This constraint is not too critical however and the
use of a value one or two steps below the optimum value makes little difference.
Note that as the AC Gain value is changed, the demodulator gain (described later in
section 3.3.17) is also adjusted in order to maintain the selected full-scale sensitivity.
The full-scale sensitivity is set by a combination of AC Gain and demodulator gain.
Since the demodulator gain is entirely digital, changes in full-scale sensitivity which
do not change the AC Gain do not cause any of the errors which might arise from a
Chapter 3, TECHNICAL DESCRIPTION
AC Gain (dB)
INPUT LIMIT (mV)
0
6
12
18
24
30
36
42
48
54
60
66
72
78
84
90
Table 3-1, Input Limit vs. AC Gain
2000
1000
500
250
125
62
31
16
8
4
2
1
0.5
0.25
0.125
0.062
3-5
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