Need to Measure Light Freq. using photodiode and have it analysed by FFT, details below.

I a new to LV. Project is to measure light freq. using photodiode heald against a TV screen. Am required to have it analysed by FFT, I have read lots but still at dead end of which VI I can manipulate, tried Spectrum Analysers but not right. Need it to collect at rate of at least 100/sec. I am using a CB-68LPR I/O and PCI-MIO-16E-1. Photodiode is connected to an amplifier and filter before it is hooked into the connector board so signal is clean. I will be measuring freq. of ciliary beat where photodiode placed against a TV screen.

Julesy wrote:
> I a new to LV. Project is to measure light freq. using photodiode
> heald against a TV screen. Am required to have it analysed by FFT, I
> have read lots but still at dead end of which VI I can manipulate,
> tried Spectrum Analysers but not right. Need it to collect at rate of
> at least 100/sec. I am using a CB-68LPR I/O and PCI-MIO-16E-1.
> Photodiode is connected to an amplifier and filter before it is hooked
> into the connector board so signal is clean. I will be measuring
> freq. of ciliary beat where photodiode placed against a TV screen.
Hi Julesy,
are you shure what you want ti do?
TV screens don't show pictures, they show a point (3 points on a colour
TV) with changing intensity passing by very fast.
For instance, if you h
ave the PAL Standard, each picture is built with 625
lines, in 20 ms, interlaced. This means, 64 ?s line with the leading and
trailing overhead. The visible part of a line is aubout 52 ?s long.
If you attach your fototransistor to the screen, and its diameter may
cover a circular area of maybe 3 mm and the visual part of the line would
be 520 mm, then: And if this covers four of the screen lines, then you
will receive pulses of 0,3 ?s length if the line passes the center of the
fotodiode. Not passing the transistor in the center would give back an
even shorter pulse.
And if you get all this working (which I don't think) then you would
measure 2 groups of pulses every 10 ms with 2 pulses each (remember
interlacing).
Even if all the DAQ would work, is your fototransistor fast enough. Think
about miller capacities. Its not so simple at it seems first to solve this
measurement problem.
Please let me know what you intend to do. I was an TV- enginear many years
ago, and I don't remember ev
erything relevant anymore, but there are some
hardware- related things to be aware if if you want to measure TV-
signals.
Bye,
Rainer

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