It is really interesting to see that arguments going on, mostly over
Firewire vs. USB and how USB would be a better choice, probably as a result
of the greater prevalence of USB. After all, with USB 2.0 having speeds
comparable to IEEE 1394a (400Mbps Firewire) and USB 3.0 being much faster,
doesn't using USB make a lot more sense?
And then there is the question of Ethernet. People have been talking about
the efficacy of Ethernet relative to IEEE 1394 and USB as well.
This is actually an amazingly complex question all around but there are some
aspects that are quite specific to amateur radio and are relatively easy to
understand, mostly having to do with the nature of RF.
In all three cases; i.e. USB, IEEE 1394a, and twisted-pair Ethernet
transmission; we are sending serial digital data at relatively high rates of
speed over wire between devices. Since all three accomplish this task in a
different manner, HOW this is done has a direct impact on the success,
especially in the presence of strong RF fields that usually accompany
amateur radio transmissions.
When you place two devices in an RF field connected by a conductor, you
*will* induce RF currents in the conductors. That is a given. It is how
antennas work. In this case what we want to do is to reduce the RF currents
to a minimum so as to have the least impact on the data communications
between the devices. There are several things we can do to reduce the effect
of the RF field on the data being transported by the wires. They are:
1. Reduce the RF currents on the wire.
2. Structure the transmission of data so that induced RF currents are not
mistaken for data (interference) at the receiver (the data receiver in this
case, not the amateur device intended to receive signals out of the air).
The most common way to deal with these things is to transfer data over a
differential (balanced) transmission line. Since the desired signal current
in each wire of the pair is equal and opposite (differential) and the the
undesired induced signal is the same in both wires, it is possible to reject
the unwanted common-mode signal.
Further, you want to come up with some way to break the path for the common
mode currents. You want to raise the impedance to common-mode currents
substantially at the RF frequencies involved on both the data and the the
ground or shield between the devices. One of the simplest ways to do that is
to break the physical connection and make sure that there is very little
coupling at RF. One of the best ways to accomplish this is with a
properly-designed transformer.
So if we look at the three technologies and compare how they fare in this
situation, you come up with the following matrix:
Ground Differential Transformer
Isolation decoupling
USB no no no
IEEE 1394 no yes no
Ethernet yes yes yes
So, as you can see, from just an RF-point-of-view USB is the *least*
desirable of the three interconnects.
So don't be rushing off to support USB without a full understanding of the
issues involved.
--
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
br...@lloyd.com
+1.767.617.1365 (Dominica)
+1.931.492.6776 (USA)
(+1.931.4.WB6RQN)
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