be amplified at the resonance frequencies of this tube can be detected.
The maxima of this frequency spectrum can be compared with the calculated maxima finding a
nice agreement between experiment and theory.
In this example, the width of the found
resonance peaks is much wider than
the detection bandwidth. This can be
verified by changing the detection
bandwidth (100 ms time constant vs. 1
s time constant). As both time
constants deliver the same result, the
measurement is not distorted by the
detection bandwidth.
Too small time constants (10 ms) on
the other hand result in a larger noise
level.
In addition to this, the effect of an dc
offset on the input signal can be
discussed at this example. The
microphone delivers a signal with a
certain DC offset. With short time
constants, this signal at f = 0 Hz is not
supressed enough and thus one finds a
strong signal increase towards small
frequencies.
This signal increase gets even larger if
the total signal becomes smaller
because the exciation amplitude is
dropped from 700 mV down to 70 mV
(Fig. 52)
Manual Anfatec PCI-Lockin Amplifier AMU2.4 – Rev. 1.10 dated 30/09/20
Page 54 (70)
Figure 44: Acoustic resonance in dependence of the time constant
(Ampl. 0.7 V)
frequency [Hz]
1E3
2E3
3E3
4E3
5E3
6E3
0,5
1
1,5
2
2,5
3
3,5
10 ms
100 ms
1 s
Figure 45: Acoustic resonance in dependence of the time
constant (Ampl. 70 mV)
frequency [Hz]
1E3
2E3
3E3
4E3
5E3
6E3
0,05
0,1
0,15
0,2
0,25
0,3
0,35
10 ms
100 ms
1 s
