How to Measure Small Signals Buried in Noise Using LabVIEW and Lock-In Amplifier Techniques

Publish Date: Nov 07, 2014 | 11 Ratings | 4.09 out of 5

Table of Contents

  1. Overview
  2. What is Lock-In Amplifier?
  3. How does the set-up work?
  4. What data acquisition hardware is required?
  5. How does it work in LabVIEW?
  6. Does it really work?
  7. Conclusion

1. Overview

How do you extract signals buried 100 dB in the noise? How do you eliminate background noise in a measurement? Consider a case in which you need to extract a 5 mV sine wave from a white noise signal with 5 V amplitude. Are those measurements even possible? The answer is yes! In this paper, I explain how you can use the new LabVIEW Lock-In Amplifier Start-Up Kit with NI data acquisition boards to cut development time and deliver applications 30 times faster than traditional methods.

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2. What is Lock-In Amplifier?

Lock-in amplifier is a technique used to make precise measurements of AC signals partially or completely buried in noise. You remove the noise by performing a modified fast Fourier transform (FFT) on the input signal at the frequency carried by the reference signal. A lock-in amplifier acts as a narrow band-pass filter (as narrow as 1 mHz) around the reference signal frequency eliminating undesired noise. It is equivalent to performing up to 1,000,000,000 points of FFTs every 10 ms, and keeping only the amplitude of the signal of interest.

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3. How does the set-up work?

Although you can apply the lock-in technique in several fields, the following example presents a light measurement application that you can expend to spectroscopy, nuclear, atomic, molecular, and optical applications. In Figure 1, you can see flashlight one (orange beam) modulated at 10 kHz pointing to a photodetector. Sun light acts as unwanted noise in the system. The photodetector converts incoming light from the sun and flashlight one into an electrical signal. The electrical signal connects from the photodetector to channel 0 that is reading 6 V (mostly unwanted noise from the sun). You need to modulate the light electrically or with a chopper. The same modulation method generates the reference signal used by the lock-in amplifier. In this example, the reference signal is coming from the on/off switch of flashlight one that is connected to channel 1 -- reading from 0 (flash light off) to 5 V (flash light on). The NI PCI-4472 digitizes the data. The NI Lock-In Amplifier Start-Up Kit processes this data and the flashlight one signal (5 mV) extracts from the sun light. By adding the second flashlight (green beam) modulated at 1k Hz on channel 2, you can use the same electrical signal from the photodetector (channel 0) to extract flashlight one and flashlight two signal information.

 

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4. What data acquisition hardware is required?

One of the great advantages of this new approach to measure small signals is the flexibility you gain by using off-the-shelf multipurpose data acquisition hardware. When selecting your acquisition device, you need to keep in mind the following four crucial characteristics:

  • Dynamic range -- Higher dynamic range speeds up measurements because it reduces the need to change gains and wait for settling times. For high dynamic range, use a 24-bit device, such as the NI 4472, with up to 160 dB. To amplify small signals, use the NI 4451 with several gain stages and up to 120 dB dynamic range.
  • Simultaneous sampling -- In order to perform precise lock-in measurements, the acquisition devices must have simultaneous sampling instead of multiplexing to account for settling. Devices, such as the NI dynamic signal acquisition boards (NI 4472, NI 4452, NI 4542) and the NI simultaneous-sampling multifunction DAQ board (NI 6115) have one ADC per channel and small phase-mismatch between channels.
  • Sampling rate -- The sampling rate should be at least twice the maximum frequency of the reference signal. In other words, the maximum frequency of the input signal should be less than or equal to half of the sampling rate. For references from 45 kHz up to 500 kHz, use NI 6115; for references from 3 Hz to 45 kHz, use NI 4472; for references from 45 kHz to 100 kHz, use NI 4x5x devices.
  • Antialiasing: To ensure limitation of the frequency content of the input signal is limited, you can add a low-pass filter, or antialiasing filter, before the sampler and the ADC. An antialias filter passes low frequencies but attenuates the high frequencies.

Device

Dynamic Range

Simultaneous Sampling

Sampling Rate

Antialiasing

NI 4472

24 bit -- up to 160 dB

Yes

102.4 kS/s

Yes -- automatic set to 1/2 of the sampling rate

NI 4451, NI 4452, NI 4551, NI 4552

16 bit -- up to 120 dB

Yes

204.8 kS/s

Yes -- automatic set to 1/2 of the sampling rate

NI 6115

12 bit -- 70 dB

Yes

10 MS/s

Yes -- at 50 kHz and 500 kHz


 

These are multichannel devices that can use some channels as a lock-in amplifier while other channels are reading additional signals, such as temperature, vibration, performing FFTs, power spectrum, and more. Using the NI PCI-4472 in the example, you can take advantage of eight analog input signals to make several combinations of reference and signal inputs. The LabVIEW Lock-IN Amplifier start-up kit expands the flexibility of the system to meet your application needs.

Possibilities you can choose from include:

  • One reference channel and up to seven signal channels
  • Up to four pairs of reference and signals
  • Seven reference signals and one signal input channel

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5. How does it work in LabVIEW?

The new Lock-in start-up kit consists of three VIs:

  1. LockinPLL.vi -- a software implementation of phase locked loop (PLL)
  2. LockinDemodulatorSettings.vi -- calculates the settings for the demodulator
  3. LockinDemodulator.vi -- extracts a frequency component from the input signal

1. LockinPLL.vi -- it is a software implementation of a phase locked loop (PLL) algorithm whose main purpose is to measure the frequency and phase of the reference signal. The reference signal can have a frequency ranging from mHz to MHz and can be either a sine or a square wave. The PLL algorithm operates on blocks of reference signal data to extract the required information. The larger the block size, the better the estimate. This is a generic implementation that you can use in several applications to very accurately measure frequency. In addition, it leverages years of development from LabVIEW, producing very reliable and stable results.

Contained in the "Reference Info" cluster data, the main output of this VI is:

  • fr -- measured reference frequency
  • Phase -- measured reference phase with respect to the beginning of the data block
  • Order -- coerced value (if necessary) of the Order control
  • Updated -- true if reference data has been updated in this call
  • Block Size -- size of the data block passed to the VI via the Signal (in) control
  • fs -- sampling frequency passed to the VI through the fs control

2. LockinDemodulatorSettings.vi -- This VI calculates some of the internally used settings for the mixer and low-pass filter blocks in the demodulator VI as well as the actual low-pass filter time constant and settling time. Parameters, such as Time constant (TC), rolloff (Rolloff), and type (Type), of the low-pass filter are important inputs for this VI. The main output of this VI is the "LP Filter Settings," which returns the lock-in engine private filter settings.

3. LockinDemodulator.vi -- This VI extracts a frequency component from the input signal that uses the reference signal to specify the frequency and phase. The main output of this VI is the "Data (out)", a 2D array containing X and Y components of the input signal at the frequency of the reference signal for all filter roll-off values (one per column). Note that we calculate all filter stages necessary to obtain different roll-offs in parallel, so you can pick the output of any stage without paying a "resettling time" penalty associated with stand-alone equipment.

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6. Does it really work?

One of the keynote presentations of NIWeek 2002 used the same flashlight set-up discussed earlier in this article. In this experiment, we measured the intensity of the light emitted by a small flashlight. The raw analog input signal from the photodetector (without the lock-in technique) was close to 10 V (it had a nice 120 Hz sine wave and high DC offset because of environmental light noise).

With the new LabVIEW Lock-In Amplifier Start-Up Kit, we locked onto the flashlight reference signal at 2.345738 kH and discovered the 1.2 microV signal that was buried 160 dB in the input signal noise. To demonstrate further to the 1,500 NIWeek audience, we removed the flashlight and watched the signal intensity drop to less than 100 nV (noise floor of the board).

We have tested the NI lock-in amplifier with reference signal frequencies up 350 kHz, however the only limiting factor is processing power. Our test machine was a 1.7 GHz -- quite slow for today's standards.

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7. Conclusion

With the new LabVIEW Lock-In Amplifier Start-Up Kit, you can use off-the-shelve acquisition hardware and NI LabVIEW to measure signals buried in 160 dB noise or eliminate background noise that in the past you could only complete with stand-alone equipment. In fact, the NIWeek keynote presentation showed a NI 4472 board extracting 1.2 micro-volts from a >9 V noise signal. Using the free start-up kit, you achieve more flexibility and better performance at a lower cost.

 

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