[amsat-bb] going digital

[Phil Karn]

On 07/22/2014 07:27 AM, JoAnne Maenpaa wrote:

> When thinking of projects to enter into the digital communication
> world plan on building or buying that digital interface to connect
> your radio to the soundcard.

Soundcard interfaces to existing voice radios were a good start, but
it's time to move past them. I've designed several modulation and coding
schemes to fit existing radios, and their inherently limited bandwidth
is a major pain. For ARISSat-1 I agreed to cram everything through a SSB
filter, and I'm not doing that again.

Not only does the narrow bandwidth often limit the data rate to much
less than what the link budget could actually support, but it enormously
aggravates the Doppler problem. It doesn't help that there are no
industry-wide standards for SSB phase response or computer tuning, or
that every time you retune one (which is often) there is a poorly
characterized phase jump.

Doppler is much easier to handle at high data rates because it's the
ratio of the symbol rate to the Doppler that matters, not the absolute
amount of Doppler.

Now that we have a variety of "pure digital" radio front ends to choose
from, it's time to set aside the voice radio + soundcard model. Not only
do the SDRs support wider bandwidths but they tend to be considerably
smaller, lighter and easier to work with in software. Some, like the
Funcube dongle, are even considerably cheaper than conventional radios.

--Phil