[Yaesu] conversion scale from dbu to microvolts

Sat Jul 29 13:19:31 EDT 2006

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