Gentec-EO T-Rad-USB (LEMO) - Manual (Page 10)

T-RAD-LEMO-USB Instruction Manual Version 2.0 June 2012 10
2.3 Frequency Domain Description of a Lock In Amplifier.
Figure 6 shows the frequency contents of a 25Hz square wave with a 60Hz interference signal
and broadband noise at about 1% of the signal.
Figure 6 Corrupted Input Signal
Since the input is a square wave, there are only odd harmonics. The fundamental is seen at
25Hz, the 3rd harmonic is seen at 75Hz, and the 60Hz interference is seen. Remember when we
said that even if the input signal is not a pure sine wave, the lock will extract the pure sine wave
component of the signal? Figure 6 shows why this is so. The reference signal is a pure sine wave
at the same frequency as the input signal so only the fundamental sine wave component of the
input signal is extracted. Of course this statement depends on the purity of the reference sine
wave. In an analog system, a pure sine wave cannot be generated due to distortion and phase
noise as well as frequency jitter. All of these combine to decrease the accuracy of the instrument.
Since the Gentec-EO Lock In Amplifier is a digital device, the reference sine wave is pure to a
much greater extent and these errors are minimized.
Figure 7 shows the spectrum of the signal after it has been passed through a band pass filter set
to 25Hz.
Figure 7 Band Pass Filtered Corrupted Input Signal
Note that the relative amplitude of the 60Hz and 75Hz signals have been reduced by the
band pass filter. The noise floor has also taken on the shape of the filter. Figure 8 shows
the spectrum of the multiplied signal.