Realization of Lock-in Amplifier in LabVIEW

[ivoc]

Hello, I need to realize  lock-in detector using LabVIEW. I downloaded the NI Lock-in Amplifier Start-up Kit.

I have studied it, but I have got the following questions.

For example, my aquisition rate is 100 kHz, data block size 1000, so duration of block of data is 10 ms. Then, this data is filtered with a low-pass filter. I can set the Time constant 1 s. I dont understand how I can use a filter with so low cut-off frequency - there is no such frequencies in my data block (the lowest frequency is determined by its length - 10 ms => 100 Hz). And another question: as I understand, one data block corresponds to one point for X or Y component, so is there a kind of averaging over the data block?

Thank you for advance for answers.

Alex.

[F._Schubert]

Input low-pass: this is normally used to filter the power line frequency 50(60)Hz and the harmonics 100(120)Hz. If you aquire continously your data blocks and don't reset the filter, you can get rid of those frequencies.

Output integrator: A lockin amplifier always has an integrator (averaging, low-pass) as the last function block. Normally, you will get a 'time-constant' for the specs of that block.

Felix

[ivoc]

Hello, Felix!

Thank you for your rapid answer. But the question is not quite clear for me. It concerns the low-pass filter at the output integrator stage. I use the LockinDemodulator.vi supplied with NI Lockin Startup Kit. And this VI returns two values - X and Y components (not arrays, single values) but the input is an array - block of data. So, X and Y are averaged values over block of data. But I can choose the size of this block arbitrary, and it can be greater than the Time Constant, so as a result I will have X and Y components that change with frequency determined by the data block size, not by the Time constant.

Is it possible to obtain the algorithm of demodulation realized in NI Lockin Startup Kit (the VI is closed by password) or it is a commercial know-how?

Thank you in advance, Alex.

[F._Schubert]

I don't have the toolkit, so I can't help you with the specifics. But I guess that if you use a too large block as input, the averaging will be done on a subset of this data as specified by the time constant.

To understand the LIA, you can start with wikipedia. Also follow the external links, as they lead to some good app notes by lockin manufacturers.

Felix

[Coq_Rouge]

I many cases a full lock-in setup is not needed. But only the phase sensetive detector part. This may simplify your design a lot