detected spectral noise density than the resonance circuit itself. Still, the noise enahncement in the
resonance can be visualized with the lockin amplifier.
5.5 F
OURIERANALYSE
OF
A
SQUARE
SIGNAL
The output of a function enertaor is connected to the input of the lockin amplifier and the signal is
terminated with 50 Ohm. The generator provides a square wave form with 10 kHz and 1 V
rms
. The
lockin amplifier devides this wave form into its original Fourier components, which are all odd
higher harmonics of the base frequency of the square wave form:
The amplitudes of the single
Fourier components
are the peak amplitudes detected all odd
higher harmonics of the generator frequency.
2k - 1 = odd multiples
1
(
2
k
−
1
)
⋅
Amplitude of the k
th
fourier component of a square signal
Adding all these sine wave with their respective
amplitudes up, the square like wave form with a
single frequency is regenerated.
Manual Anfatec PCI-Lockin Amplifier AMU2.4 – Rev. 1.10 dated 30/09/20
Page 29 (70)
Figure 25: Circuit diagram; qucs/usb_square.sch
Figure 24: USBLockin connected with external
generator
