1. Home
  2. Electronics Design
  3. Noise and SNR calculation with a Lock-in Amp

Noise and SNR calculation with a Lock-in Amp

I am trying to make low noise optical measurements using a Lock-in after getting the signal with a PD circuit (one of the Hobbs' circuits). However, I have trouble on calculating my SNR and noise in units of 1/sqrt(Hz).

1- First (without the Lock-in) I am taking the data using a Tectronix oscilloscope. Using 128 sample averaging, I detect a noise level around 800uV. Should I divide it by the sqrt(bandwidth) of the PD circuit to have the noise in 1/sqrt(Hz)?

2- Again without Lock-in, looking at FFT of the oscilloscope, can I make a SNR calculation? (There is approximately 60 dB difference between the white noise and my frequency in FFT screen)

3- Later, in order to make some improvement on the noise performance, I use a Lock-in. For Lock-in output (which is a DC for a sine input), what should I do for SNR calculation and how should I calculate the noise? Should I use the BW of the Lock-in instead of the PD circuit?

Any help is appreciated. Thanks a lot.

Huseyin.

Reply to
=?iso-8859-9?q?H=FCseyin_Rahmi
Loading thread data ...

In order to calculate the noise voltage spectral density, you have to know the bandwidth accurately, and it has to be the right bandwidth. If your noise canceller circuit has a noise bandwidth of, say, 5 MHz, then you want to use that. You won't be able to do good noise measurements with a scope.

I'd recommend just using a quiet light source (e.g. a flashlight) to make the noise canceller generate really really pure shot noise, and use that as your noise standard. That's actually the best way to measure the noise bandwidth of the whole system.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

That only works if the noise is uniform with frequency. The 1/F part of the noise doesn't work like that.

Does the scope have an anti-alias filter and how many bits is its digitizer good to? Are you using all the bits? You will usually see an RMS noise that is 1/sqrt(12)RMS times the LSB of the digitizer.

Most lockins have a narrower bandwidth than most PDs. The bandwidth of the lockin sets the system bandwidth.

All sorts of nonsense measurements can be made when you use a lockin. Beware of the bandwidth of the lockin vs the bandwidth over which you measure. Also read the manual carefully if the source frequency is changing.

Reply to
MooseFET

Also be careful about getting the system gain.

Reply to
MooseFET

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.