From: Roy Lewallen on Sat, 04 Jun 2005 14:06:41 -0700


wrote:

A basic definition that is industry-wide, government-wide,
has "dbm" as decibels of "0 dbm" related to a power level of
1.0 milliWatts in a "50 Ohm system." That has become so
widespread that specification writers don't always include
those words. It is implicit when referring to RF components.

. . .

That's common in the RF industry, but many others also use dBm -- for
example, it's often used in video systems where the standard impedance
is 75 ohms, and others where the standard is 600 ohms. In all those
applications, dBm is universally defined and understood to mean dB
relative to 1 mW, regardless of the impedance.

Quite true, Roy. :-)

I restricted myself to "50 Ohms" for a couple of reasons:
As far as "pure" RF components go, 50 Ohms is the Z
characteristic; I didn't want to complicate my explanation.

The TV Cable industry is HUGE and they use 75 Ohms. However,
so many radio amateurs run around snarking on "TV" that it
could have raised a lot of unneccessary babbling in here. :-)
TV cable is digital in some locations (ours is in the SF
Valley of L.A.) and has more TV channels crammed into the
same VHF-UHF space than old analog TV. Don't know the
modulation of my digital TV cable signals, whether it is
wider or narrower than analog channels...but my TV service
crams over 400 channels into the same bandwidth. TV sure
isn't the narrow-band stuff that many hams are used to.

"dbv" (sometimes "dbu" but rarely) refers "0" as 1.0 microVolt,
almost any characteristic. Not seen much in specifications,
though.

The "VU" (for Volume Unit) is an old, old one in the audio
and telephone industry with "0 VU" being 1.0 mW into 600
Ohms impedance. Not only that, the "VU" industry standard
used to call out the indicating meter's ballistic (needle or
meter motor) characteristics! :-)

"dbc" is an often-used term on component specifications but
is still a relative term of db in regards to the Carrier of
a signal where the noise is called out. ["C" for Carrier]

"dba" is a legal term in the USA standing for "Doing
Business As" in local governments that require business
licenses. :-)

My apologies if some ISPs show two more postings of my
message. When I posted via Google on Saturday early
afternoon, Google was interrupting itself with lots of
users (?) or something that kept prompting "server error."
I've since removed redundant posts on Google, but some
other ISPs may be storing the multiples. Stuff happens.