[TenTec] dbm and VU units of audio measurement

[Adam Farson]

Hi Paul,

You made an interesting point. A brief Google search did not reveal any
specific reference to the historical evolution of power metrology at Bell
Labs.

I suspect that in the days before traceable (and thus legally enforceable)
RF power measurement, Bell Labs specified 0 dBm as 1 mW in 600 ohms - the
well-known "milliwatt" of wireline telecom. The dial-up test tone source in
telephone exchanges was 0 dBm at 1 kHz (now 1004 Hz to avoid aliasing in PCM
systems).

It obviously did not take the metrologists long to realise that 1 mW is an
absolute power level, irrespective of load resistance. Thus, as accurate RF
power-measuring instrumentation came on the scene, it was calibrated in dBm
(or dBW for higher power levels). This was particularly true of low-power
microwave power meters such as the HP 430 series, although I have seen an
"SP" Bird 43 scaled in dBm and dBW.

Many European radio regulatory bodies specify maximum permitted transmitter
output in dBW. Thus, the UK "legal limit" is +26 dBW (400W) PEP.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

-----Original Message-----
From: tentec-bounces at contesting.com
[mailto:tentec-bounces at contesting.com]On Behalf Of Paul Christensen,
Esq.
Sent: Wednesday, July 30, 2003 12:27
To: tentec at contesting.com
Subject: Re: [TenTec] dbm and VU units of audio measurement


> Per ITU and ISO standards, which have legal and regulatory binding force
in
> all member countries, "dBm" is more than a mere label - it is the legal
> definition of dB with reference to an absolute power level of one
milliwatt.

Today, dBm represents one milliwatt of power irrespective of individual
voltage, current or impedance parameters.  But I believe that today's
international definition of dBm is different than it was when it was used in
the early Bell Labs development era during the 1930s.  I suspect that dBm
evolved from a standard consisting of 1mW across 600-ohms (and hence the
corresponding 0.775 Vrms), to a uniform measurement of power regardless of
the voltage, current and impedance involved.  But...it's just a hunch on my
part based on my readings through the years.

-Paul, W9AC

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