George,
Yes, I think you are right. If the mic input were rated 5 mV, it should
be -46 dBv. But I did verify that the Pegasus manual says 5 mv or -67 dB. I
wonder what reference point TT is using?
Mark
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: <tentec@contesting.com>
Sent: Friday, July 25, 2003 9:16 PM
Subject: Re: [TenTec] 5 mV = -67 dBm
> 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
> > >
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
>
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