Saluki SE1201 User manual

SE1201 Digital Lock-in Amplifier
User Manual
Saluki Technology Inc.

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The document applies to following models:
SE1201 DSP Lock-in Amplifier (50 mHz to 120 kHz)
Standard Pack:
1×Main Machine
1×Power Cord
1×USB Cable
2×BNC Connection Cable
1×Spare Fuse (2A, 250VAC)
1×U Disk or CD (for User Manual and PC Software)

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Preface
Thank you for choosing Saluki Technology Products.
We devote ourselves to meeting your demands, providing you high-quality measuring instrument and the best
after-sales service. We persist with “superior quality and considerate service”, and are committed to offering
satisfactory products and service for our clients.
Document No.
SE1201-03-01
Version
Rev01 2021.08
Document Authorization
The information contained in this document is subject to change without notice. The power to interpret the contents
of and terms used in this document rests with Saluki.
Saluki Tech owns the copyright of this document which should not be modified or tampered by any organization or
individual or reproduced or transmitted for the purpose of making profit without its prior permission, otherwise
Saluki will reserve the right to investigate and affix legal liability of infringement.
Product Quality Assurance
The warranty period of the product is 36 months from the date of delivery. The instrument manufacturer will repair
or replace damaged parts according to the actual situation within the warranty period.
Product Quality Certificate
The product meets the indicator requirements of the document at the time of delivery. Calibration and measurement
are completed by the measuring organization with qualifications specified by the state, and relevant data are
provided for reference.
Quality/Settings Management
Research, development, manufacturing and testing of the product comply with the requirements of the quality and
environmental management system.
Contacts
Service Tel:
+886. 909 602 109
Website:
www.salukitec.com
Email:
[e-mail]
Address:
No. 367 Fuxing N Road, Taipei 105, Taiwan

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Content
1 Lock-in Amplifier Basics........................................................................................................................................ 6
1.1 Introduction.................................................................................................................................................... 6
1.2 Functional Diagram........................................................................................................................................8
1.3 Reference Channel......................................................................................................................................... 8
1.4 Phase Sensitive Detectors...............................................................................................................................9
1.5 Time Constants and DC Gain.......................................................................................................................10
1.6 DC Outputs and Scaling...............................................................................................................................11
1.7 Dynamic Reserve......................................................................................................................................... 13
1.8 Signal Input Amplifier and Filters................................................................................................................14
1.9 Input Connections.........................................................................................................................................16
1.10 Intrinsic Noise Sources.................................................................................................................................16
1.11 External Noise Sources................................................................................................................................ 18
1.12 Harmonic Detection..................................................................................................................................... 20
2 Interfaces................................................................................................................................................................21
2.1 Front Panel................................................................................................................................................... 21
2.2 Rear Panel.....................................................................................................................................................21
2.3 Main Display................................................................................................................................................ 22
3 Menus.....................................................................................................................................................................27
3.1 [INPUT/FILTERS].......................................................................................................................................27
3.2 [REF/PHASE].............................................................................................................................................. 28
3.3 [GAIN/TC]................................................................................................................................................... 30
3.4 [DISPLAY].................................................................................................................................................. 31
3.5 [CHANNEL OUTPUT]................................................................................................................................ 32
3.6 [UTILITY]....................................................................................................................................................33
4 Remote Programming............................................................................................................................................38
4.1 Command Syntax......................................................................................................................................... 38
4.2 Detailed Command List................................................................................................................................38
5 Computer Operation.............................................................................................................................................. 43
5.1 Install Software.............................................................................................................................................43
5.2 How to Use the Software..............................................................................................................................45
5.3 Usage Examples........................................................................................................................................... 54
6 Performance Tests..................................................................................................................................................60
6.1 Self-Test....................................................................................................................................................... 61
6.2 DC Offset..................................................................................................................................................... 61

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6.3 Common Mode Rejection............................................................................................................................ 62
6.4 Amplitude Accuracy and Flatness................................................................................................................62
6.5 Amplitude Linearity..................................................................................................................................... 64
6.6 Frequency Accuracy.....................................................................................................................................65
6.7 Sine Output Amplitude Accuracy and Flatness........................................................................................... 65
6.8 DC Outputs and Inputs.................................................................................................................................66
6.9 Input Noise................................................................................................................................................... 67
7 Operation Examples...............................................................................................................................................70
7.1 Simple Signal Measurements....................................................................................................................... 70
7.2 Harmonics Measurements............................................................................................................................ 73
7.3 Optical Spectral Measurements....................................................................................................................76
7.4 Serial Communication..................................................................................................................................78

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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: SI(t) = AIsin(ωt + φ) + B(t) , where ωis the frequency of input signal, AI
sin(ωt + φ) is the test signal, B(t) is the total noise.
The reference signal SR(t) is defined as: SR(t) = ARsin(ωt + δ).
These two signals enter the PSD module for multiplication simultaneously, and the output of the PSD is defined as:

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)sin()()2cos(
2
1
)cos(
2
1
)sin()()sin()sin()()(




tAtBtAAAA
tAtBttAAtStSS
RRIRI
RRIRIpsd
In the time region, the output of the PSD consists three parts:
The first part is a DC signal. If AI, A and the phase difference (φ − δ) between the input signal and the reference
signal are constants, this part is a DC signal.
The second part is the frequency-doubled reference AC signal.
The third part is the result of multiplication of the noise and the reference. Because the sine signal is periodic and
there is no relevance between the noise signal and the reference signal. The integral of this part is zero.
In the frequency region, we can redraw these three parts:
The first part is at 0Hz, which is known as the DC component of one signal.
The second part is at 2fref Hz.
The third part is a random signal at all the frequencies, such as white noise. The frequency spectrum of white noise
does not change after any frequency drifts.
To sum up, the LPF output is defines as:
)cos(
2
1
  RIOutput AAS
Although we can determine the amplitude the amplitude of the input signal through adjusting the phase difference
)(  
, the accuracy is unsteady and insecure. In order to solve this problem, the dual phase lock-in amplifier was
invented, see Fig.3.
Fig.3 Dual Phase Lock-in Amplifier diagram
Now, we define the phase difference
 
, the LPF0 output
0Output
SX 
and LPF1 output
1Output
SY 
. Then
we calculate the amplitude R which is independent of θ:
R
I
A
YX
A
R)(2
2
22 

The phase difference is defined as:
)/(tan 1XY

 
.

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1.2 Functional Diagram
The functional block diagram of the SE1201 DSP Lock-In Amplifier is shown in Fig.4. On the whole, the SE1201
includes signal conditioners, reference signal generators, an algorithm module and a system control module and so on.
Fig.4 Functional Block Diagram of SE1201
1.3 Reference Channel
The reference channel is used to provide control signal which is associated with the detected signal. The SE1201 reference
input can trigger on an analog signal such as a sine wave or a TTL logic signal. The input is AC coupled and the input
impedance is 1MΩ.
Generally, both the sine wave and the TTL signal can be used as the reference signal. However, the generator's sine output
has a small and varying amplitude. Meanwhile, many function generators provide a stable TTL SYNC output which can
be used as the reference. Therefore, for frequencies below 1Hz, a TTL reference signal is required.
The SE1201 lock-in amplifier has two reference signal modes: internal reference mode and external reference mode.
In internal reference mode, the internal oscillator generates a digitally synthesized sine wave which is used to multiply
with the input signal. The phase-locked-loop (PLL) is not used since the lock-in reference provides the excitation. The
phase noise will not affect the internal reference signal. The phase noise is extremely low. This mode can work normally
from 50mHz to 120kHz.

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In external reference mode, an external sine wave or TTL logic signal can be used as the external reference signal. PLL
will be used in this mode, but it will generate a little phase jitter which may cause measurement errors.
The phase jitter means that average phase shift is zero but the instantaneous phase shift has a few milli-degrees of noise.
The phase jitter makes the reference signal plus noise at different frequencies. According to the coherence principle of
PSD, the output is not a single frequency, but a distribution of frequencies about the true reference frequency.
In fact, phase noise in the SE1201 is very low and generally has no effect. In applications that requiring no phase jitter, the
internal reference mode should be chose. Since there is no PLL in internal mode. The internal oscillator and the reference
sine waves are directly linked and there is no jitter in the measured phase.
1.4 Phase Sensitive Detectors
The PSD in the SE1201 acts as a digital multiplier as is shown in Fig.5. The input signal amplified and filtered is
converted to digital signal by a 24-bits A/D converter and then goes into the PSD. The reference sine wave is computed to
24 bit of accuracy, and the accuracy of the whole PSD is 48 bit.
The PSD module in lock-in amplifier is mainly used to implement the coherent modulation of the input signal and
reference signal. Generally, there are two kinds of phase-sensitive detectors (PSD's): digital PSD's and analog PSD's.
Traditional PSD's use an analog multiplier to multiply the input signal with the reference signal. There are many problems
associated with these, including harmonic rejection, output offsets, limited dynamic reserve and gain error. It will limit the
accuracy of PSD's and bring in various noises.
The digital PSD multiplies the digitized signal with a digitally computed reference sine wave. Because the reference sine
wave is computed to 24 bit of accuracy, the harmonics have -120 dB roll off. That is to say, the harmonics do not affect
the products of the PSD.
Fig.5 PSD diagram
Because the PSD based on analog method has temperature drift, there are always some deviation between the output and
actual result that is the uncertain system error. While the PSD based on digital method has a precise amplitude and never
change, so it will not generate any system errors. This eliminates a major source of gain error in a linear analog lock-in.
Considering that the inputs of analog multiplier are analog quantity, the reference signal will be affected by temperature
drift. This will cause errors in the reference and greater errors in the results of coherent modulation.
The dynamic reserve of an analog PSD's is limited to about 60 dB, because there are always many background noises.
When there is a large noise signal present, 1000 times or 60dB greater than the full-scale signal, the analog PSD measures
the signal with an error. Because the lock-in amplifier is mainly used to detect weak signals, when the amplitude of
background noise is similar to or larger than the signal amplitude, the results of coherent modulation will be wrong.

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To the digital PSD's, the dynamic reserve is limited by the quality of the A/D conversion. Once the input signal is
digitized, no further errors are introduced. Practically, the dynamic reserve of SE1201 can exceed 120dB.
The performance of a lock-in amplifier is largely determined by the performance of its PSD's. Almost in all respects, the
digital PSD outperforms the analog one. Besides, the digital PSD is also more convenient to modify.
1.5 Time Constants and DC Gain
The output signal of the PSD contains many signals of various frequency, such as the sum or difference between the input
signal frequency and the reference frequency. Only the signal whose frequency is exactly equal to the reference frequency
will result in a DC output.
The low pass filter (LPF) at the PSD output removes all the AC signals that unwanted, including the 2F (sum of the
signal and the reference) and the noise signals. This filter is what makes the lock-in such a narrow band detector.
Time Constants
The bandwidth setting of the low pass filter is the same as the conventional low pass filter. They are both determined by
the time constants. The calculation of the time constant is defined as:
f
TC
 2
1

Here f is the -3dB frequency of the low-pass filter. For example, to a one-order low pass filter of RC type, a 1s time
constant means its -3dB point occurs at 0.16Hz.
In fact, where there is an input noise, there is an output noise. By increasing the time constant, the output becomes more
stable and measurement becomes more reliable. The time constant reflects not only the stability of the system and the
accuracy, but also the respond time of the output.
The time constant also determines the equivalent noise bandwidth (ENBW). The ENBW isn't the filter -3dB pole, it is the
effective bandwidth for Gaussian noise.
Digital Filters vs Analog Filters
Analog filters have many limitations in performance. The temperature drift and non-linearity are two important problems
that limit the rolloff performance of an analog filter. A two-stage analog filter provides about a maximum rolloff of 12
dB/oct at high frequency points.
Space and expense are also limitations. Each filter needs to have many different time constant settings. These different
settings require different components and switches to select. Each setting is costly and space consuming. A large quantity
of analog devices also bring quite complexity to the device debugging.
Considering these limitations, we choose a 47 bits digital filter to accomplish narrow band filtering. The DC amplitude is
exactly 0 dB and the equivalent value of Q exceeds 145dB.
Synchronous Filters