[TenTec] RFI from USB-to-serial adaptor cable

[Rick Denney]

Folks,

As it turns out, I'm getting significant RFI from the USB-to-RS232
cable I'm using to run my RX-320. The cable clearly has some circuitry
in it, though it must be hiding in the molded plugs. It required a
devie driver that was the commonly available driver for such devices.
It works just fine, but when I plug it into the USB port, "band noise"
increases by about 2-3 S units. I can't hear anything but noise in the
RFI. It seems to affect 20 meters more than, say, 80 meters, but it's
always there.

I have tried it with all the other devices from the computer connected
or disconnected, but the noise vanishes when I pull the USB plug, and
reappears three seconds after plugging it back in. That three-second
delay tells me it's a noisy processor in that cable, not something
like common-mode RFI on the shield (and it is a well-shielded cable
with a clear jacket).

Options seem to be: Find a PCI-based RS-232 board and don't convert it
from USB, find an adaptor cable that doesn't create the noise, or find
a way to shield the adaptor cable in use. I haven't tried ferrite
beads or the like, but I'm sure others have experienced this same
issue there is an obvious answer.

(I built the serial cable for connecting the Omni V myself, and
soldered the shield to the shell of the DB9 and DB25 plugs. I also
used metallic plug cases and well-shielded cable. There is no noise I
can detect emanating from that cable.)

Advice craved and appreciated.

Rick, KR9D


---
Richard W. Denney, Jr. PE|Iteris, Inc.
Associate Vice President |107 Carpenter Dr. Ste 230 |    703.925.3819
rwd at iteris.com           |Sterling, VA 20164        |Fax 703.471.1757