Digital Reference for Lock-In Amplifier

[dfd]

Hello,

We have a simultaneous DAQ board and are planning to use Labview's handy lock-in software (<link no longer exists>) with a reference and modulated signal. However, if we also plan to lock-in to a frequency that is slightly off the reference frequency, we were wondering which option might be better:

1) digitally frequency shift the reference frequency by changing the 'dt' of the waveform

2) digitially generate a sinusoidal reference signal at our required reference frequency and somehow phase-lock it to the modulated signal

Any suggestions would be greatly appreciated,

Donna

[johnsold]

Donna,

How are the frequencies related? How much is "slightly off?" In general frequencies cannot be phase locked unless they are harmonics.  You can generate non-harmonic frequencies with phase locked loops using clever counting techniques.  The frequencies will be accurately calculated from the reference but the output will not phase locked.

What are you trying to do with the slightly off frequency signal?  How much interference is present? What is the signal to noise ratio?  Maybe there is a different way to accomplish what you want to do.

Lynn

[dfd]

Hi Lynn,

Like the flashlight example, we are chopping an experimental signal at 1 kHz with a shutter. The chopped signal and the reference signal( which controls the shutter at 1 kHz) are passed to the DAQ and then lock-in. When we want to lock-in on that 1 kHz frequency we are fine. However, what if we wanted to lock-in at  1 kHz +/- 5 Hz to detect sidebands from a 5 Hz experimental signal? Do we need some sort of auto-tuning vi that will minimize the y/x ratio?

[johnsold]

No.  Sidebands are still related to the reference frequency. They are the effect of modulating the reference frequency signal.

What you need is to be sure that the lock-in effective bandwidth is large enough to pass the sidebands. I do not have the lock-in software so I cannot give you specific details.  In general the lock-in bandwidth is set by the bandwidth of the low-pass filter in the system.

Lynn

[F._Schubert]

As Lynn said, check the low-pass (integrator) which is the last step/block of the lockin.

In a simplified model, your signal with f=1kHz is modulated with the same f. You then get the same signal (including sidebands) at f-f=DC, f+f=2f=2kHz and higher. So after the demodulation (multiplication), you have the DC signal and your sideband at 5Hz. If the integrator/low-pass does cut off at a higher frequency, you can filter the 5 Hz signal.

To understand/verfy this, it's pretty easy to use some simulated signal (better SNR than the experiment and no other error sources) and perform an FFT at the different steps.

Felix

[dfd]

Thanks to both of you for your help - will try.

-Donna

[EZT]

Hello,

We are trying to do something really similar and was wondering if this problem was solved (I just sent Donna a private message asking about her result).  Also, is there another way to do this?  I was going to start a new thread as well, but wanted to post here first as this is directly related to what we want to do.

More specifically, we have a infrared signal that is modulated at 280KHz.  This signal is interfered with the same wavelength infrared beam that is modulated at 260 Hz, which then creates a sideband with a better signal to noise compared to just when locked-in to the 280 KHz signal.  So, our signal is just one input, but our reference is ONE frequency, but comes from two mixed signals.  Here are some pictures of the frequency spectrum showing where we want to lock-in and the setup of the experiment (see attachements).  We actually want to lock-in at a sideband of the 3rd harmonic (3*280KHz+260Hz). 

I imagine we can input the two references into a daq device, but how should we modify the Lock-in amplifier VI that NI provides ("lock-in amplifier example") to mix the two reference to make the harmonic sideband reference?  We would greatly appreciate any help!  The necessary hardware to do this operation cost $30k!!!!  Thank you

[Karl-G]

I would suggest making the new thread that you mentioned, as well as listing out the hardware you are using in your application. The new post with links to this thread will be the best way to restart the discussion on this topic.

[EZT]

Thanks for your response Karl.  I have already created a new thread and added stuff about our hardware (we don't have much right now as far as NI hardware, but the experimental setup exist and is functioning):  http://forums.ni.com/t5/LabVIEW/Lock-in-amplifier-VI-to-sideband/m-p/2761588/highlight/false#M814043

Please check it out and let me know if you have any feedback.  Thanks for your time.

-Eric

[Sveta4]

Hi,

sorry that i not answering your questions but decided to use this forum for another problem that relates to lock in amplifier.

I wrote a simualtion of  lock in amplifier using Matlab and now i want to test it for different SNR. I added to the signal white gaussian noise. In all the spec that i read, always written that the lock in amplifier can handlle -100dB of noise.

When I add a noise of SNR=-30dB the lock in stopes working, when i look at the spectrum of the signal+noise i see that the componenet that i am looking for is covered by noise so no wonder that the lock in doesn't work.

what i don't understand how is that in spec always written that lock in works at -100dB? i think that i am missing something.

attached spec of lock in amplifier : http://infrared.als.lbl.gov/content/PDF/equipment/SR810830_specs.pdf

look at the part of dynamic reserve.

Thanks for any help.