> Perhaps you are seeing a small frequency offset between RX and TX. I > recommend experimenting a little more in simulation: > > * What happens you pass the same square wave through that low pass > filter you mentioned. Whats it look like? >
When I pass the square wave through the low pass filter it does become a sine wave, so that means it's normal. Does the low pass filter implement a Fourier transform? If it does, that could explain why if ends up looking like a sine. In the comments in the fr_firdes.cc file, it says "// a little algebra gets this into the more familiar sin(x)/x form". This looks like the Fourier transform of the rect(t) function, but I can't really tell what is happening from the code. > * What happens when you apply a small frequency shift to the square > wave? I recommend using the channel model block for this. > > It took a while to figure what the channel block was used for because I never seen it while looking through the blocks in grc. The taps parameter was used for a filter it seems, but I don't really understand the taps very well. I left it at the default 1.0 + 1.0j hope that is ok for this purpose. When I changed the frequency shift parameter in the channel model block, the signal on the scope sink became "shaky". If I increase the shift a little more, the signal changes to something similar to the picture I attached in the previous post. I think I get a different signal when I shift frequencies because the signal hovers around 50u whenever I'm not near the the source's frequency. I would like to know what units are used in the channel model block's frequency shift parameter because when I use 150m the Fourier transform sink shows something like a 30kHz shift. I also realized that the low pass filter should be set to have a cutoff frequency greater than 1kHz. If I am observing a fluctuation in the RX and TX frequency offset, then is there a way to accommodate for this. Since the offset changes at random intervals, I don't see a way to shift in the opposite direction by the correct amount. Now that I know the low pass filter makes a signal look like a sine wave, is it possible to revert the signal back to the original form? Thanks, Frederick
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