In message , Jeff Liebermann
writes
On Sun, 3 Aug 2014 08:31:59 +0100, Ian Jackson
wrote:

In message , Jeff writes
Jerry does not seem to take on board that this is an international
group, not a group of ex-US cable technicians who slovenly misuse dBm
to mean dBmV, when the rest of the world uses dBm correctly to mean dB
relative to a milliwatt.

I think only a small minority of US cable TV people would misuse dBm
when they really mean dBmV. I worked in the industry all my life, 20
years of which were for an American company - and I never found this to
be a common practice.

A few do. The problem is not the cable distribution system levels,
but rather the accessories attached to it. The RF levels on the cable
will probably be in dBmV @ 75 ohm forever. OTA signals will probably
remain as dBuV @ 75 or 300 ohms.

Continental Europe usually uses dBuV for CATV, MATV and OTA. The UK has
essentially always been dBmV for CATV, although we're quite happy to add
or subtract the appropriate 60dB when required (or around 48dB for dBm).
[I, for one, can't see any point in using large numbers when small
numbers will do just as well.]

However, the Wi-Fi signal levels
from the cable modem/router/wireless box is in dBm. The MoCA bridge
signal levels are also in dBm[1]. This is not really a problem
because each technology usually involves different cable techs,
engineers, and support groups.

At one time, I worked on a project to distribute 2.4GHz Wi-Fi via the
inside RG-6/u coax system. It rapidly increased in complexity and
didn't work very well. It was quickly and mercifully killed. We
initially has some confusion due to the different units, but the
project manager simply demanded that we specify the full units of
measure and equivalent units where appropriate, which largely
eliminated the problem. That left the problem of people using 50 ohm
test equipment on 75 ohm systems without a 50-75 ohm impedance
matching pad.

Impedance matching L pads:
http://www.microwaves101.com/encyclopedias/380-l-pads

50/75 dB conversion formulas and cheat sheet:
http://www.maximintegrated.com/en/app-notes/index.mvp/id/808
http://pdfserv.maximintegrated.com/en/an/AN808.pdf (106 KBytes)

50/75 ohm matching pads (and vice versa) can sometimes be more of a
hindrance than a help (the 'horses-for-courses' principle needs to be
sensibly applied).

1] Note that dBm and dBmW are independent of the impedance since xx
milliwatts produce the same amount of smoke no matter what the
impedance. However dBmV and dBuV require that the impedance be
specified.

In practice, dBm (or the rare dBmW) generally implies 50 ohms to the RF
guy and 600 ohms to the audio/phone guy, whereas dBmV and dBuV generally
implies 75 ohms. However, as usual, it usually depends on the context,
and it's a case of "if in doubt, tell it as it is".

One oddity I found (many years ago) was a piece of cable TV equipment
had one RF parameter was specified in dBm, while all the others were in
the usual dBmV. This was not a mistake, and the reason given was that
the dBm signal was CW, while the rest were vision and sound IF and RF
signals. I hope I may be forgiven for not understanding or agreeing with
the reasoning for this decision, as not only was it unnecessary, but it
could also have been considered a typo, and caused confusion.



--
Ian