Finding frequencies

[tger003]

Hi,

Im relatively new to labview and i am having some issues.

Im doing a project where i have an photodiode and led, the led is flashed on and off at a particular frequency and i am trying to get the signal from the photodiode.

What i want to do is ignore all other signal frequencies (ie. ambient light) and only get the signal information from the led

Does anyone have any ideas on how to go about this?

Thanks

[gwd]

Sounds like you want a lock-in amplifier, a special case of phase-sensitive detection.

There WAS a lock-in amplifier vi on the NI website although that link seems to be missing now. You may be able to find it with a more careful search or elsewhere on the web.

[JamesC]

Hi Tger003,

You should find the Lock-in Amplifier you need located Here.

Regards
JamesC
NIUK and Ireland

[tger003]

Ok thanks i had downloaded that but im not quite sure how to use it,

will it work with the Usb-6009 that im using?

Also how would i set the led as the reference signal?

(Sorry about the completely newb questions but labview is so much different to anything ive used before)

[gwd]

>  will it work with the Usb-6009 that im using?

That's a definite maybe. It all depends on the frequency the LED is flashing, what other interfering frequencies are present, etc. etc.

To use the LIA you need to be able to sample BOTH the reference AND the signal at a rate high enough to get some tens of points per cycle. So hundreds of hertz are probably OK but several kHz certainly isn't.

Phase sensitive detection is best when (1) interfering signals are strong or close in frequency or (2) phase information is useful. If this is not the case you might be better off just using Averaged DC-RMS.vi to separate DC from AC.

(This might work great if you have incandescent light but very badly if it's fluorescent.)