Mark, I think that you may still have a problem there.
The dBv reference is defined to be one volt. Thus 5 mV relative to one volt
expressed in dBv would be
dB = 20 log (5x10^-3)/1 = -46 dB
Since we used one volt for the reference, we would say that 5 millivolts
corresponds to -46 dBv, provided that it was measured across the same
resistance value as the one volt reference.
Recall that any relationship expressed in dB is inherently a power ratio -
watts over watts. We can use volts or amps instead of watts provided that
both the numerator and denominator voltages or currents are measured with
the same resistance.
Since watts = volts-squared divided by resistance in ohms, we can deal
directly with volts or amps and forget about the squaring by using 20 log
V2/V1 as our equation, instead of the more familiar 10 log P2/P1. But, this
is valid only if the same resistance value is present for both voltages.
Your value of -67 dBv corresponds to the voltage V in
-67 dB = 20 log (V/1)
where again the reference is one volt measured across some reference
resistance..
The result is V = 446.7 microvolts, measured across the same resistance as
the one volt reference.
All the above again subject to the fumble-finger exception.
Tricky stuff . . .
I haven't seen your Pegasus manual, but with microphones it is typical to
rate them loosely with a dB number for the output where it is "understood"
that 0 dB is some voltage output that would be put out by some reference mic
if it were exposed to some sound pressure level SPL at some specified
distance and frequency, etc.
A mic with -67 dB output is pretty low output. A high output mic, such as
the old D104 high-impedance crystal mic was rated up around -52 dB or so.
The output voltage is very much a matter not only of the sound pressure
level but the resistance into which the mic is connected.
Most mics these days are "low-impedance" around 200-600 ohms and rigs are
designed for such mics in terms of their input circuitry. A typical Icom
radio - sorry, don't know about the Pegasus, but probably close - expects an
input of around 10 millivolts at the mic connector or 100 millivolts at the
accessory back-panel connector.
Hope this helps a little with the eternal dB problems! <:}
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: "Mark Erbaugh" <mark@microenh.com>
To: <tentec@contesting.com>
Sent: Friday, July 25, 2003 7:26 PM
Subject: Re: [TenTec] 5 mV = -67 dBm
> Thanks for all the replies. I've sorted it out. 5 mV is -67 dBv. The dBm
> was my partly my mistake and partly in the Pegasus manual. The manual says
> that the mic input level is -67 dB (no suffix). I was looking in the ARRL
> handbook for assistance and saw dBm, so that's what I assumed.
>
>
> 73,
> Mark
>
> ----- Original Message -----
> From: "Mark Erbaugh" <mark@microenh.com>
> To: <tentec@contesting.com>
> Sent: Friday, July 25, 2003 5:40 PM
> Subject: [TenTec] 5 mV = -67 dBm
>
>
> > According to the specs on my Pegasus, the microphone input is 5mV or -67
> > dBm. Could someone explain the relationship between mV and dBm?
> >
> > I just acquired a closeout Radio Shack mixer. The line output is 2V.
Would
> > it be possible to use a simple resistor voltage divider to bring the
> voltage
> > down to a level suitable for mic input to the Pegasus? If so, what would
> be
> > the recommended values? Or is there a better circuit?
> >
> > 73,
> > Mark
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
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