[Yaesu] conversion scale from dbu to microvolts

[Barry Johnson]

Hi Steve,

Let me say it again ... a dB is a dB is a dB ... but you have to give 
it a reference appropriate for a given application.  Recall that a 
wrt power levels, the dB = 10 * log(P1/P2) and wrt voltage (or 
amplitude levels or whatever) levels it is obviously dB = 20 * 
log(V1/V2) ... Power is proportional to voltage squared.

A dB is a statement of the relative ratio of two values and nothing 
more UNTIL you give it a reference.  As you mentioned, this 
"referenced dB" is denoted with a suffix to denote the reference 
system, e.g., dBmV.

You can also get into another fun use of dB by considering the 
S-unit.  It is defined (typically) as 50 microvolt (a few used 100 
uV) RF at the receiver input produces an S-9 on the meter and a 
CHANGE of one S-unit is taken to be 6 dB (5 dB by some and often with 
the 100 uV group).  Often the calibration instructions provide the 
input in terms of dBmV.

Again, there isn't a power dB or a voltage dB or an acoustic dB or 
..., the dB is simply a dB for all until you reference it to 
something like 0 dB = 1V/Pa @ 1KHz as the reference for the 
sensitivity of a microphone stated to be -25 dBV/Pa.

In closing, the dB is one of the greatest mathematical  definitions I 
can think of in that it is used in essentially every engineering and 
scientific field.

73,
Barry W4WB



At 12:19 PM 7/29/2006, you wrote:
>At 09:44 AM 7/29/2006 -0500, you wrote:
> >Hi Roger,
> >
> >Hmmm.... last time I knew, dBs are dBs ... but you do have to specify
> >the reference level.  If you look in the table below in the
> >International reference values, one will find the appropriate
> >reference for his situation, i.e., the 50 ohm case.
> >
> >Tnx es 73,
> >Barry W4WB
>
>However, rember that a volt dB is not the same value as a power dB. That
>is, when power is doubled, the voltage has increased only by a factor or 1.41.
>
>So when you say you've doubled your power, the power increased by 3 dB.
>When you say 3dB=uV, you do mean you've doubled your voltage, such as from
>1 uV to 2 uV. BUT the power has increased more than simply double.
>
>Most non-communications-type signal generators are only calibrated in
>output voltage, and in power in dBm (decibels referenced to 1 milliwatt).
>Comm-type signal generators, however, are typically calibrated in output
>voltage and dBV, or decibles referenced to some voltage value, typically 1
>microvolt but sometimes 1 millivolt or even 1 volt.
>
>So when the calibration instructions say to set the output of the generator
>to, say, 6 dBuV, they mean 4 microvolts (1 uV doubled then doubled again).
>
>One reason (perhaps the major reason, for ham equipment) that dBuV is used
>is because long ago, ham receiver designers standardized 1 S unit as being
>an increase of 6 dB of the input voltage. So 6 dBuV, or  would be S1; 12
>dBuV is S2; 24 dBuV is S3; etc.
>
>Having said that, I just looked at page 32 of my Kenwood TS130S Service
>Nanual (don't have a Yaesu manual handy), which has a Reference chart in
>the lower righthand corner from 'Japanese "SSG"' to 'American "SG"'. And
>that chart sets 0 dBuV to be a voltage of HALF the above, or 0.5
>microvolts. Then it gives 6dBuV as 1 uV, 12dBuV as 2 uV, 24dBuV as 8 uV;
>etc. until it lists 120 dBuV as being 0.5 VOLTS.
>
>This seems counter-intuitive, since when you say "zero decibels-microvolt",
>or 0 dBuV, this would normally be interpreted as 0 dB with respect to 1
>microvolt. That statement means, simply, that you have set 1 microvolt to
>be 0 dBuV. But the Kenwood chart sets 0 dBuV at 0.5 uV; I don't know why,
>not being specifically schooled in communications theory.
>
>I believe the reason the receiver manufacturers came up with this system is
>because it allows them to calibrate receivers without regard to the input
>impedance, whether it's 50 ohms, 75 ohms, 300 ohms-balanced, etc. Audio
>output voltage is specified the same way; thus, you find output
>transformers specified as having a specific output voltage, such as 70.7
>volts.
>
>Hope the above isn't all wet  ;o\
>
>Steve, K0XP
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