20-04-2012, 03:53 PM
How to Measure Small Signals Buried in Noise Using LabVIEW and Lock-In Amplifier Techniques
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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.
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.
How does it work in LabVIEW?
The new Lock-in start-up kit consists of three VIs:
LockinPLL.vi -- a software implementation of phase locked loop (PLL)
LockinDemodulatorSettings.vi -- calculates the settings for the demodulator
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.
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.