Electronics > Projects, Designs, and Technical Stuff
Square wave filtering question!
demispark:
Hello everyone,
I'm currently a senior EE student about to start my senior project. I've been thinking of some ideas and believe I have a promising one however I have a few questions that remain unanswered. I would like to develop a positioning system using inferred light. I will have a source using IR LED's and receivers using phototransistors.
Is it possible to filter a particular frequency square wave from other frequency square waves? I can then process these signals individually.
My idea is that if i want to use multiple light sources, they should be modulated at different frequencies. Is it possible to decipher which source the signal is coming from with the phototransistor? Is there a filter that can pick out a particular square wave at a certain frequency?
Perhaps I am going about this the wrong way. Any feedback is appreciated!
bob91343:
Recognize that a square wave is composed of many frequencies. If you superimpose two square waves of different frequencies you get a hodge podge of elements. About the only thing that might work is to filter specific components that are unique to each signal.
If you have square waves at 1 kHz and 1.1 kHz then you only get a 3 kHz component when the first one is present, and only get a 3.3 kHz when the second one is present. So a narrow filter could tell when a specific wave is present, as long as the spectrum of each is unique.
If you have more than two signals you must be careful in selecting the frequency. Of course you can look for the fundamental also.
T3sl4co1l:
Sure, use synchronized (coherent) squarewaves and a mixer. This correlates the signal with a reference, rejecting anything out of phase on average -- which is any other frequency than the wave and its harmonics.
So you'll still have selectivity at (odd) harmonics, the width of those bands being the passband of the "averaging" filter.
In the limit as averaging is done over all time, the bandwidth is zero, and non-harmonic frequencies are orthogonal (fully excluded).
Sine waves have the particular orthogonality feature that they are unique (down to sign), i.e. they have no harmonics. All other waveforms have harmonics, and may have nonzero mixing products there. Hence why we're so interested in sinusoidal signals for RF purposes.
Over finite time scales, the averaging is just a filtering process, so you need to take some account of what interfering signals may be present, and how much is tolerable relative to your signal strength (in other words the SNR).
Tim
hamster_nz:
I thought about this for a while too once, and came to the conclusion that best luck might be had when the frequencies were relatively prime (e.g 700Hz, 900Hz 1100Hz). What information are you attempting to get? (e.g. time of flight, brightness, phase, a data stream...)
Have you considered using a pattern, maybe:
"FLASH ALL CHANNEL" "FLASH CHANNEL 1" "FLASH CHANNEL 2" "FLASH CHANNEL 3"
with a few milliseconds between channels
Marco:
--- Quote from: demispark on August 21, 2020, 03:28:11 am ---Is it possible to filter a particular frequency square wave from other frequency square waves? I can then process these signals individually.
--- End quote ---
You can modulate the burst from each transmitter with a different frequency so you can seperate them out ... you can even do it with square waves, just use power of 2 frequencies and the Hadamard transformer.
But doing all that while keeping the signal clean enough to measure edge movement of the squarewave with picosecond accuracy, forget about it.
Navigation
[0] Message Index
[#] Next page
Go to full version