John:
An abrupt change in the time domain signal produces wide band energy
(hams call it "key clicks"). The fourier transform of a dirac delta is
a flat line for a constant energy across all frequencies. Now the dirac
delta is a mathematical object but in a sampled work an abrupt change
from one sample to the next cannot be easily distinguished from one and
the "wide band energy" is aliased and wrapped and is a mess.
Bob
John Gilmore wrote:
What you're describing is currently tough with the basic tx since it's
effectively "always on" once you fire it up. If you underrun, the
FPGA will continue transmitting the same value to the DAC (which has
the upconverter in it), thus even if you're "not transmitting", it's
highly likely that you really *are* still transmitting.
...
* modify the FPGA code such that when the TX fifo is empty, you
ramp the value fed to the DAC down to zero over say 8 clocks.
Why would it take 8 clocks? Surely the hardware could do the hard
part (jam it to zero immediately) and the software could do the rest
(if it needs to taper off over 8 clocks, end with the right samples).
Hmm, in fact, why is this a problem at all? Why can't the software
ensure that the final sample before underrun is a zero? The hardware
will then keep repeating this final sample forever.
(I suppose this would be possible if you aren't using any of the
upconverters -- but if the FPGA or the DAC is upconverting, then you
have to jam a real zero, not an upconverted zero? Is this even
possible?)
John
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