signal divide

[abs]

I am using a optical chopper (100 Hz) and want to collect signal(from photodiode through a analog channel. This is easy! Then I need to get signal when chopper on and off separately and get the difference between chop. on and chop. off signals. I can double the chopper sync. out so that it gives 200 Hz (synchronized to chopper but doubled)and use this 200 Hz to identify chop. on and chop. off. How can I separate chop. on and chop. off signals with external trigger (200 Hz)and then I'll be able to do the subtraction and other maths. -Thanks

[sergesl]

Hello ABS,

could you provide more information regarding the National Instruments' software and hardware that you are using for your acquisition? In addition, what do you call chopper sync?

[abs]

Hi Serges,
I am using AD card 6024E and LabView 7
Let me explain my application little bit more. I am using a optical chopper and looks at a signal with a photodiode. Chopper frequency is 100 Hz and then photodiode gives me a signal that looks like a square wave (high and low values with 100 Hz frequency). I want to get the difference between high and low values to get the spectral details. Since I can't distinguish high and low values with 100 Hz trigger, I am going to use 2X100 Hz=200 Hz(this can be taken from Chopper sync. out) so every time a rising edge occur it identify High (chopper On) or low (chopper Off)signals. Then I need to get the difference between these two parts (of the same signal), high-low. That means, for every full wave of 100 Hz signal I get high-low signal.

I am going to use two analog channels
arbitrarily
CH0 Signal from photodiode
CH1 100 Hz (from optical chopper)
CH2 200 Hz (by doubling the above signal)
Sending both 100 Hz and 200 Hz triggers ensure that rising edges of both channels are accounted for.

I think this gives some details what I want to achieve and the attached word file indicates the behavior of signal and triggers
Thanks
Tissa

[sergesl]

Hello ABS,

From what you are saying, you would like to perform frequency or period measurement. If this is the case, you do not need to produce the 2X signal yourself. The counters on the DAQ devices can be used to measure frequency and periods and you can find examples to do this by going to the LabVIEW example finder in the help menu.
Below are attached three VIs that perform these types of measurement more accurately by using a much higher frequency signal to check the High or Low.
You can also have a look at the following tutorial for more information on how the period measurement is done using counters: http://zone.ni.com/devzone/conceptd.nsf/webmain/FFED601BAB01963F86256802007B8B45?opendocument.
Hope this helps.

[abs]

Hi,
I need to get the difference in amplitudes of high and low signals and I need to synchronize with 100 Hz and 200 Hz triggers to distinguish high and low signals (amplitudes).
Thanks
Tissa

[sergesl]

Hello ABS,

the word amplitude makes it clear now. To accomplish this, you can use the 200 Hz signal as the sample clock for an analog input channel. That way, every rising edge of the 200 Hz will cause you to sample the 100 Hz signal. You can then use a program in LabVIEW that substracts the current value from the previous value of the acquired samples (i.e. using shift registers). See attached image...