Thanks very much, Paul, for adding some background to my previous
posting(s). I was beginning to think that I was preaching to an empty church
from the lack of responses! <:}
Now, if we can just get hams in general to stop thinking about "voltage dBs"
and "power dBs" the battle will have been won!
Your mention of 50 ohms as a near optimum output Z reminds me that at audio
frequencies we no longer can regard the Zo of the line as purely real. In
fact, coincidently RG-213 has Zo = 50-j51 ohms at 50 Hz. Even at r-f, common
lines have complex Zo but for most applications the reactive part is small
enough to ignore. But not so at low audio frequencies.
It is an interesting fact also that under the proper conditions, it is
possible to terminate a line such that the reflection coefficient rho can
take on a value as large as 2.41! This unlikely but possible outcome
requires a line with high attenuation and a corresponding complex
characteristic impedance.
I am curious to see if your material on the optimum Zo values for coax lines
matches mine - perhaps others might be interested in knowing why we use 50
ohm or 75 ohm lines instead of some other values.
Haven't heard from you for awhile - trust things are going well.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>
----- Original Message -----
From: "Paul Christensen, Esq." <w9ac@arrl.net>
To: <tentec@contesting.com>
Sent: Saturday, July 26, 2003 8:48 PM
Subject: Re: [TenTec] 5 mV = -67 dBm
> > So, here is a case where an industry found it desirable to describe
signal
> > voltage in decibel terms without requiring that the voltage be measured
> > across the same resistance as the implied reference: 600 ohms.
>
> That's right George. With the proliferation of transformerless audio
> designs during the late '60s and '70s, it no longer became necessary to
> "power match" the source with the load. For decades, dBm (as previously
> noted is the equivalent of 1 mW across 600 resistive ohms which nets 0.775
> volts AC RMS) was used since the majority of line-level audio circuits
> demanded a 600-ohm termination. Due to the highly reactive nature of the
> transformer, it was (and is) necessary to properly terminate the secondary
> of the transformer for optimal transient response, but also...matching the
> designation load with the transformer's source impedance was critical in
> older power-matching circuits.
>
> When solid-state electronics began replacing transformers (e.g., discrete
> and LSI op-amps), the source impedance of audio circuits plummeted to near
> zero. As a practical matter, even the best audio op-amps require a
nominal
> resistance at their output.
>
> Approximately twenty years ago, Richard Cabot of Audio Precision
Corporation
> drafted a white paper on the ideal output impedance of an audio circuit.
> Interestingly, it is not near zero ohms. Rather, under long audio paths,
> the audio cabling begins to take on transmission line qualities. And if I
> recall correctly, the optimum output impedance for most line level
> applications is...near 50-ohms. There is a direct correspondence
between
> the optimum audio output impedance and how we derived at 50-ohm
transmission
> lines. Another topic of discussion which is OT to this reflector is
> exactly why do we have 50, 52, 70 and 75-ohm RF transmissions lines.
AT&T
> never opted for 50-ohms on their landline audio circuits and for some
reason
> settled on 150-ohms as evidenced by their 600:150-ohm "repeat" coils.
How
> many of us remember the Western Electric 111C transformer? I imagine
there
> is probably some history associated with the 150-ohm twisted-pair
circuit...
>
> Today, we can pretty much load the output of a solid-state device without
> much worry. Provided that the load impedance is at least ~ 10 times
greater
> than actual source impedance, dBu is a relevant standard....we no longer
> care that it is 0.775 volts across 600 ohms...we only care that 0.775
volts
> is capable of being placed across the destination load. Most audio studio
> equipment is NEVER terminated to the source's low output impedance.
Nearly
> everyone bridges the load with ~ 10K-ohms or greater. In a
transfomerless
> environment, we no longer care about transient ringing and overshoot when
a
> complex wave is impressed upon a reactance as is the case with
transformers.
> Even the best Jensen transformers must be terminated for optimum transient
> response.
>
> Circling back....dBm had its roots with 600-ohm circuits. Today dBm is
used
> universally to represent 1mW across any load. As an example, commercial
> lasers are almost always specified in dBm, irrespective of the actual
> terminating impedance.
>
> -Paul, W9AC
>
>
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