[Elecraft] K2 S-Meter Sensitivity

[Mike Czuhajewski]

Don Brown posting--

> >I have a Tektronix 191 leveled RF signal generator and a HP step
> >attenuator that goes down to 100 db. I set the RF level to 0.5 Volts and
> >the attenuator to 40 db giving 50 UV. With this connected to the K2 I
> >adjust the Cal S HI so the meter reads S9. This will usually come out
+or-
> >a little from the manual set point.

W1HUE reply--

> I hope you have kept in mind that the TK 191 calibration markings are
> Peak-to-Peak voltage, not RMS...

I hope you also kept in mind that the formula for figuring dB is ten times
the log (base 10) of a power ratio and twenty times the log of a voltage
ratio. (The latter assumes that the same impedance is used for both
voltages.)  An online reference is
http://www.its.bldrdoc.gov/fs-1037/dir-010/_1468.htm.

With voltage 1 being 0.5 volts, or 500,000 uV, and voltage 2 being 50 uV
(and both in a 50 ohm system), that comes out to a difference of 80 dB and
not 40.  Reducing 0.5V by 40 dB drops it to only 0.005v or 5000 uV.

Another online definition of the decibel can be found at

http://www.lecroy.com/tm/Library/LABs/LAB412/

This one is even handier in this case, since if you scroll down far enough
you'll find a chart that gives decibel equivalents for a variety of voltage
ratios. One of those is 40 dB for a 1/100 voltage ratio. Starting out with
0.5 volts and adding 40 dB would take it down by a factor of 100, to 0.005V
or 5000 uV.

To reduce a signal of 0.5 volts RMS to 50 uV, you would have to crank in 80
dB (which is still within the range of your HP attenuator).

73 DE WA8MCQ
(not an Elecraft owner but starting to feel the urge!)