LOCKIN BASICS
T
HE
GENERAL
IDEA
OF
L
OCK
I
N
A LockIn amplifier is a phase sensitive bandpass filter with a centre frequency f and a bandwidth
t
.
It has two output signals in parallel: either the amplitude R and the phase
j
, or the real part X and
the imaginary part Y.
•
The centre frequency is the frequency at which the LockIn is searching for its signal.
•
The bandwidth gives the frequency range around this centre frequency, from which the output
signal is derived.
•
The output signal R is the measured amplitude of the analysed signal at the centre frequency f.
•
The phase
j
is the relative phase shift between the analysed signal and an internal reference signal
with the same frequency.
For analysing the input signal, the LockIn needs an internal frequency reference with a certain
amplitude and phase. (see Fig. 8).
The bandwidth of a LockIn can be visualised by a simple setup. A generator gives a constant
frequency f
in
signal to the input and sweeping the centre frequency f around the input frequency f
in
.
Figure 9 shows that the amplitude at the input frequency is always measured with the same value.
For other frequencies, the measured amplitude is dependent on the distance between the actual
centre frequency and the input signal frequency.
A schematic diagram with the basic components of the lock-in amplifier is shown in Figure 2 on
page 9. First of all the received signal is amplified and digitized. Divided into two separate
channels, the signal is multiplied by the mentioned reference signal (with frequency
ω
ref
) and the
90° phase-shifted reference signal respectively. The reference signal is generated with the lock-in's
digital oscillator. An additional implemented phase displacement
Ψ
enables to compensate phase
differences caused by the measuring equipment. After multiplication, the resulting signals are low-
pass filtered and provide now information about the real part
X
and the imaginary part
Y
of the
analyzed signal relating to the phase position of the reference signal. Out of them the amplitude
R
as
well as the relative phase shift
φ
are calculated.
Manual Anfatec PCI-Lockin Amplifier AMU5.0 – Rev. 1.01
Page 10 (34)
Figure 4: Schematic diagram with the basic components of the lock-in amplifier.
