High Channel Count Applications with the Lock-In Amplifier
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Overview
Most stand-alone lock-in amplifiers operate only on a single (reference, signal) pair. That is true even if they offer two signal inputs since either only one of the input signals can be looked at a time, or an addition or subtraction operation is performed between the two before processing takes place. By comparison, an innumerable amount of different lock-in amplifier configurations are available when performing the lock-in demodulation in software. In this document, we will be focusing on a lock-in amplifier application with multiple signals and a single reference.
Test
The test acquired data from multiple NI-4472s and then in software applied the lock-in amplifier algorithm to the signals with a single channel designated as the reference. The NI-4472 was chosen because it is a high channel count, simultaneously sampling device with a large dynamic range which is important for signals buried deep in noise. The lock-in amplifier calculates dual phase amplitudes at a selectable harmonic frequency.
Tests were run in two different setups. The first setup consisted of a PXI-8176 with a PXI-1000B chassis. The second used a Dell Optiplex GX260 with a P4 2.4 GHZ processor and 512 MB of RAM connected to the same PXI-1000B chassis over a MXI-3 connection.
See Also:
High Channel Count Lock-In Amplifier with NI-4472
Results
*The number of channels refers to the total number of channels acquired from. Therefore, a 32 channel system consists of 31 signals and one reference.
Optimization
The multi-channel VI used to do the demodulation will work on up to 127 channels. For applications with a smaller channel count where the rates above are not satisfactory, you can optimize the code by performing multiple single channel demodulations in series. Using this method, we were able to sustain a sampling rate of 23 kHz on 31channels. This is almost a 3x improvement over the single multi-channel VI!
Other Configurations with the Lock-In Amplifier
- Multiple Signal - Reference Pairs: With a single NI-4472, four pairs of signals and references can be used with the lock-in amplifier VIs. This is equivalent to having four standard stand-alone lock-in amplifiers.
- Multiple References: Same principal as multiple signals, but reversed. Multiple references can be used to detect small signals at different frequencies within the noisy signal.
- Mulitple Harmonics: The harmonic at which to calculate the signal amplitude is software selectable. Consequently, you can use mulitple instances of the phase and frequency extracting VI to measure the signal amplitude at multiple harmonics of the reference simultaneously.
Related Links:
How to Measure Small Signals Buried in Noise Using LabVIEW and Lock-In Amplifier Techniques
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