2.3 C
HARACTERISTICAL
C
URVES
OF
A
L
OCK
-I
N
A
MPLIFIER
For filtering different information out of a received signal the corresponding frequency
ranges ought not to overlap. The following experiment shows how small the difference
between the frequencies should be that they can be analyzed independently:
A sinusoidal signal with a frequency of 100 kHz and an amplitude of 20 mV
rms
is fed in to
the input of the lock-in amplifier. By sweeping the lock-in's reference frequency
ω
ref
around 100 kHz the frequency spectrum of the input signal is analyzed. In Figure 3, the
output of the lock-in amplifier is plotted in dependence on ω
ref
for different values of
damping (Roll-Off = 6 dB/octave, 12 dB/octave and 24 dB/octave). The time constant τ
has been kept constant.
For a damping of 6 dB/oct, the measured amplitude at 95 kHz drops to approximately
1 % regarding to the maximal value at 100 kHz. For 24 dB/oct, the amplitude at 95 kHz
is practically not measurable. So the curves show that the lock-in amplifier uses a very
narrow frequency band for signal detection.
12 (58)
Figure 3 Output signal of a lock-in amplifier for a single frequency input signal of
100 kHz and 20 mV
rms
amplitude at the input. The time constants have been kept
constant.
Signal: 100 kHz, 20 mV Amplitude
Roll Offs:
6 dB
12 dB
24 dB
100 k
95 k
90 k
85 k
10
2
10
3
10
4
10
5
10
6
105 k
110 k
115 k
Frequency [Hz]
A
m
p
lit
ud
e
[n
V
]
Summary of Contents for eLockIn 203
Page 52: ...7 APPENDICES 7 1 SCHEMATIC DIAGRAM OF THE SIGNAL PATHS 52 58...
Page 53: ...7 2 DIAGRAM OF THE LOCKIN AMPLIFICATION PATHS Diagram of the preamplifier switches 53 58...
Page 54: ...7 3 FRONTPANEL DIMENSIONS 54 58...
Page 55: ...7 4 INSTRUMENT SPECIFIC TEST SHEETS insert with 10 pages of data 55 58...
Page 56: ...7 5 PREAMPLIFIER CONNECTOR PINOUT 56 58...
Page 58: ...7 7 TRACEABILITY CHART 58 58...
