[Elecraft] K3 dbV in 2.38

[Jim Brown]

On Thu, 18 Sep 2008 05:36:40 -0700 (PDT), alsopb wrote:

>First question.  What is meant by stable value of AFV?

>Here's what I see for a bunch of AFV edited values one after another with no
>change in K3 settings.
>1579,1688,1598,1624,1696,1613, 2168,1758,1425.
>The range is 6db.  

Sure. It's audio, and you're looking at the reading of a voltmeter looking at 
that audio. ANY voltmeter will have a varying reading on program audio (that 
is, music, someone talking, even band noise and QRN), because audio voltage 
varies with time over a range of 40-60 dB. Many audio voltmeters, including 
the one in the K3 that generates the AFV and dBV readings include an 
integration circuit that averages the voltage over a selected time period (1 
second or one half second in the K3). This integration drastically reduces 
the variation as someone talks or static crashes crash. That's why your 
readings vary by only 6 dB rather than 30-50 dB. As Eric and Lyle have 
observed, these voltmeters are designed for use with test signals that are 
constant (like a signal genertor). 

As noted, it's quite nice to have this function. The only thing wrong is 
improperly calling it dBV. dBV MEANS an AC voltage reading where 0dBV = 1 
volt RMS. dBm means 1 milliwatt in whatever impedance is being used. If you 
don't believe that, look at the manual for any HP product that generates or 
measures voltage. Voltages expressed in dB with respect to some reference are 
widely used in audio. Often, some reference voltage other than the standard 
(for example, the output of a generator, or the peak value of a system's 
response) can be selected as a reference, and all subsequent readings 
referenced to that voltage. In that scenario, the notation dBr is often used. 

I suggest that the log voltmeter (that is, reading in dB) begin with one of 
the international references, either 0.78V (0dBu) or 1 V (0dBV), and then 
change to dBr when some other reference has been selected. Now, you have an 
actual calibrated audio voltmeter, rather than only a relative voltage 
indicator. 

73,

Jim Brown K9YC
Audio Systems Group, Inc.