On 12/30/2011 7:59 PM, n4is wrote:
> That is not the case on weak signal bands, and in special 6m, digital modes
> are above 50.250 MHz, there is a clear reason for that, digital is not a
> narrow mode, there are phase noises that can trash the band.
Digital signals of various types have no more phase noise than any other
modulation. It depends on the quality of both the TX and the power amp,
if there is one. It's also possible for sound card used to generate the
digital signal to produce some IMD. Sound cards are usually a lot
cleaner if run at least 6dB below clip, and like any audio interface,
it's important to make sure that nothing is getting overdriven in the
rig (like the audio from the computer to the rig). These are all good
operating practice. If you don't pay attention to this stuff, you're a
lid.
As to sticking to a frequency -- users of JT65 and PSK modes have always
stuck to very limited portions of a band. W4TV observed that a JT65 user
will set his dial frequency to 1838 kHz, which puts the JT65 signal
between 1838.25 kHz and 1840.5 kHz.
One thing that folks who don't use these modes may not realize is that
JT65 uses a high degree of error correction and averaging to provide an
improvement in signal to noise ratio of 20 dB or more as compared to a
CW signal. Think about that for difficult paths on 160M. JT65 and
several other advanced digital modes were developed by Nobel Laureate
Joe Taylor, K1JT, and use techniques like those used to recover very
weak signals from distant space probes. They are pretty slick.
73, Jim K9YC
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