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6
1 Lock-in Amplifier Basics
1.1 Introduction
The Lock-in amplifier is a device used to detect very small signals, which are always obscured by noise sources many
thousands of times lager. The lock-in amplifier can extract these small signals from noises and measure their values
accurately.
The lock-in amplifier is a weak signal detection method based on the coherence. Lock-in amplifiers use a key technique
known as phase-sensitive detection (PSD) to single out the required component of the signal. This component has the
same frequency with the reference signal and has a fixed phase differences with the reference signal. Noise signals at
other frequencies are rejected and do not affect the measurement.
The basic processing of small signals is amplification. Traditional amplifiers will amplify both the noise signals and the
required signals. If there is no band limiting or filtering, amplification will decrease the signal to noise ratio (SNR).
Therefore, filtering is needed to purify the signal and increase the SNR in order to measure the weak signal accurately.
PSD can be seen as a band-pass filter with a very narrow bandwidth. The basic modules of PSD include a multiplier
module and a low-pass filter (LPF) module, as shown in Fig.1. Sometimes PSD is described as a multiplier module
without a LPF.
Fig.1 PSD diagram
In Fig.1, SI(t) is the input signal plus noise in the time region, SR(t) is the reference signal, which has a fixed frequency
with the test signal.
Fig.2 Single-phase amplifier diagram
In Fig.2, the input signal SI(t) is defined as: S
I
(t) = A
I
sin(
ω
t +
φ
) + B(t) , where
ω
is the frequency of input signal, A
I
sin(
ω
t +
φ
) is the test signal, B(t) is the total noise.
The reference signal S
R
(t) is defined as: S
R
(t) = A
R
sin(
ω
t +
δ
).
These two signals enter the PSD module for multiplication simultaneously, and the output of the PSD is defined as:
