Home Forum XMEGA Xminilab problems with 40kHz signals?
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 Subject :problems with 40kHz signals?.. 2015-09-01 14:48:21 
Joined: 2015-08-27 09:44:42
Posts: 5

I'm trying to detect the carrier frequency of an IR transmitter, which should be in the range of 38 to 40 kHz.  When I connect an IR diode to CH1 of the xminilab, I clearly see the various pulses encoding data, but if I try to zoom in to see the carrier frequency, I instead just see solid blocks, like in the first attachment, sshot-block.png.

As a test, I connected the AWG output to CH1 and set the AWG to a 40kHz square wave.  Now I do see the wave, but it is not square.

I'm guessing these are unrelated problems.  The first might be a problem with the photodiode.  Any ideas about the second issue?


 sshot-block.png [41 KB] :: No carrier wave visible
 sshot-awg-40kHz-square.png [69 KB] :: poor square wave
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 Subject :Re:problems with 40kHz signals?.. 2015-09-03 14:36:30 
Joined: 2015-08-27 09:44:42
Posts: 5
I think the answer is that the scope input has some capacitance, although it's not clear to me where that is coming from in the schematic.
If I put a resistor in parallel with the photodiode, then I can see the carrier frequency, although it is smoothed out.
I guess the same smoothing must be happening to the square wave from the AWG.
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 Subject :Re:problems with 40kHz signals?.. 2015-09-03 14:38:55 
Joined: 2012-04-02 15:42:39
Posts: 463
Location: Sarasota, FL

I don't know much about the IR.

About the 40kHz signal not being square, it is because the device only has a bandwidth of 200kHz.

A squarewave is equivalent to the sum of a sine wave at that same frequency, plus an infinite series of odd-multiple frequency sine waves with diminishing amplitudes (harmonics).

Since the device is limited to 200kHz, not all of those harmonics are being sampled.

The signal from the AWG is bandwidth limited as well (44kHz).

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Gabriel Anzziani
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