Understanding a Lock-In Amplifier

[kelOb]

Hi,

i'm just wondering if someone can help me get the full idea of a Lock-In Amplifier. I am trying to isolate a signal of footsteps buried in noise and think this may be the way to go.

So far this is my understanding:

I multiply my signal with another signal, pass it through a psd, use a low pass filter and retrieve teh DC of the lower signal i wanted? Is this right? I'm not too sure how I should go about this with LabVIEW though. I am using a cRIO. Should i setup a sine wave with amplitude of x and frequency of y, and multiply that by my signal comin from cRIO, then put it thorugh an FFT & PSD VI? I've looked up a few papers and exmaples but I'm missing something and not too sure where I'm getting lost. Any help would be greatly appreciated!

Regards,

Kelly 🙂

[Supreeth.K · Accepted solution]

Hi keLOb,

There is a good community example that describes how to implement a lockin amplifier using labview along with providing the source code which can be found here:

https://decibel.ni.com/content/docs/DOC-1762

The example has been implemented on a cRIO by programming the FPGA, so this should definitely guide you through the right path. You can go through the example and post questions if you are unsure about any specific section of the code.

Thanks and Regards,

[kelOb]

 Hi Supreeth.K

many thanks for your reply it is greatly appreciated. That should point me in the right direction!

Kindest regards,

Kelly

[nidhi88]

Hi Supreeth.K,

I am performing experiments on atomic magnetometry. I want to use a lock-in amplifier, can you please suggest me if Lock-in amplifier through Labview FPGA will be appropriate or I should go for high-end Lockins available in the market like SR 830 which costs a lot(USD 5000).

Thanks in anticipation.

Regards,

Nidhi Singh

[dmsilev]

Hi Nidhi,

I think the answer to that depends on two things: (1) What fraction of the needed FPGA/etc hardware do you already own and (2) What are the magnitudes of the signals you expect to be measuring.

For (1), if you're starting from scratch and need to buy the FPGA, the ADC module, controller, etc., I suspect you're going to end up fairly close to the price of an SR830 or similar. If you have some or all of the necessary parts in-hand, then that changes.

(2) is important, because if you're looking at microvolt or even nanovolt signals, a lockin like the 830 comes with a good broadband preamplifier stage which boosts the signal up to the point that the ADC has something to work with. A good voltage preamplifier with a noise figure in the few nV per root Hz range isn't all that cheap, and for nanovolt signals, it's basically a necessity for making the measurement work. On the other hand, if you're expecting a 1 V signal (or even a 1 mV signal), then a preamp isn't needed.

[nidhi88]

Hi Supreeth,

Thank you for the inputs, it was really helpful.

[eddycur88]

Hi all,

Continuing this issue regarding USB-4431 as lock-in amplifier.

I have another question regarding hardware.

Due to the need to move it from one place to another we prefer solution based on notebook and thus USB DAQ-card.

I found this oneUSB-4431, having 4 channels (seems to be working in pseudo-differential input mode, if I am not mistaken), 24-bit resolution, up to 102.4 kS/s sampling rate, delta-sigma ADC and simultaneous sampling mode and +-10V input range - so looking similar to PCI-4474/4472. But it has smaller dynamic range (100dB instead of 110dB), smaller timebase accuracy, and sample-rate resolution and it seems that it doesn't hav antialiasing configurable filters, etc.

I need it for light-measuring applications with signal levels >10uV and I intend to use low-noise configurable AC/DC pre-amplifier (from Femto company) having gain and bandwidth control in order to apmlifier signals before DAQ-card.

Can I use this USB-4431 for this purpose?

It seems to be slightly less good comparing to recommended internal PCI cards like NI 4472(4471), but maybe it is enough good for my application?

If not, which USB DAQ board can you recommend for this purpose?

Thank you in advance,

RG

[_kb]

Redirect

RG, please try to keep questions to a single thread, rather than copying across several of them.  It looks like your question may be answered now, but if you need any further assistance, please continue your discussion under the new thread you created.