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