[Elecraft] XG-1 MDS Calculation

[Robert Friess]

The definition of MDS is a signal that is equal in power to the noise
present in the channel.  When the s+n/n ratio is 3 dB this condition is met
with the signal and the noise each making an equal contribution to the total
power.  In the case of a measurement made with the XG1, the s+n/n ratio is
much greater than 3 dB and the contribution of noise to the total power
measured in the channel is negligible, therefore the  MDS is simply equal
to -107dBm - s+n/n.

73,
Bob, N6CM

----- Original Message ----- 
From: "Bill Tippett" <btippett at alum.mit.edu>
To: <elecraft at mailman.qth.net>
Sent: Monday, May 17, 2004 12:12 PM
Subject: [Elecraft] XG-1 MDS Calculation


>          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
>
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