[Elecraft] XG-1 MDS Calculation

[Bill Tippett]

         I need a logic check on the XG-1's MDS calculation.  ARRL says:

5.1.1 The purpose of the CW Minimum Discernible Signal (MDS) Test
is to determine the level of signal input to the receiver that will produce
an audio output that is 3 dB above the noise floor.

http://www.arrl.org/members-only/prodrev/testproc.pdf (page 38)

...but the XG-1 Manual (page 5) does not appear to adjust for the 3 dB
MDS definition or the actual noise bandwidth of the K2's 700 Hz Xfil:

**************************************************************************
Signal-to-Noise And MDS Calculations
Using the results from the previous page, you can calculate the
signal-to-noise to noise ratio (S+N/N) at 1 microvolt, and estimate
the MDS (minimum discernable signal) as follows:

A. Divide S+N by N; call the resulting ratio R.
B. Take the base-10 logarithm of R ("log" key on most calculators).
C. Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in dB.
D. If the S+N/N is greater than 10 dB, then the MDS is approximately
equal to the result from (C) subtracted from -107 dBm.

Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).

A. R = 1.0/.03 = 33
B. log(30) = 1.52
C. 20 x 1.48 = about 30 dB (this meets the requirement for step D)
D. MDS = -107 dBm - 30 dB = -137 dBm

http://www.elecraft.com/manual/XG1%20Manual%20rev%20C3_web.pdf
********************************************************************************

Is the procedure on page 5 actually estimating Noise Floor and not
MDS?  If so, can I estimate MDS simply by reducing the Noise Floor
measurement by 3 dB and adjusting for the actual filter noise BW?
I get very consistent results with ARRL's after compensating for my
700 Hz Xfil actually being 340 Hz (1.7 dB noise BW effect versus the
standard ARRL 500 Hz BW) plus 3 dB for the MDS definition.  Using my
actual measurements and adjusting for both MDS and noise BW, I got:

MDS = -107 - 34 + 1.7 + 3 = -136.7 dBm at 7.040 versus
ARRL's ETR results of -136.5 at 3.520 and -137.6 at 14.020.

http://www.arrl.org/members-only/prodrev/pdf/lab/k2.pdf (page 17)

         Anyone have any thoughts?  I also know that I should be using
an RMS-responding meter but I'm just eyeball-averaging using a
cheap average-responding DMM.

                                         73,  Bill  W4ZV