[TenTec] Orion's Noise Floor & MDS / Elecraft XG-1

[Richard Detweiler]

Hi Bill and all,

May I suggest running the test again with the 1Khz filter ( or the Inrad 
#352 also in the 1Khz position ).

This might help improve the numbers by eliminating the 500 Hz/ 250Hz Filter 
pre-amp .


73's
Rich
K5SF


>From: Bill Tippett <btippett at alum.mit.edu>
>Reply-To: tentec at contesting.com
>To: tentec at contesting.com
>Subject: [TenTec] Orion's Noise Floor & MDS / Elecraft XG-1
>Date: Mon, 17 May 2004 11:09:55 -0400
>
>Hi Don,
>
>KD6HQ wrote:
> >Thank you for suggesting the XG-1 oscillator.  If the specs are correct 
>the
>built in attenuator of the Orion will bring it down another 18 db which
>should test the circuits well enough.
>
>         You motivated me to drag out my XG-1 and try the MDS approximation 
>that
>Elecraft recommends on pages 3 and 4 of their XG-1 documentation here:
>
>http://www.elecraft.com/manual/XG1%20Manual%20rev%20C3_web.pdf
>
>Instead of requiring a signal at the actual Noise Floor (~138 dBm), 
>Elecraft
>makes an approximation using the 1 uV (107 dBm) output.  To briefly 
>summarize,
>they turn off AGC, and measure the RX audio output voltage for the 1 uV 
>XG-1
>test signal (S+N) and then no test signal (N only).  Then they compute the
>difference in dB [20 log (S+N / N)] and subtract this from the -107 dBm 
>output
>of the XG-1, which yields an approximation of MDS.  The problems I see are
>that you should really use an RMS voltmeter instead of an 
>average-responding
>meter for this measurement, and then you should subtract another 3 dB from
>Elecraft's result since MDS is normally defined as the point which will 
>give
>you 3 dB S+N / N, instead of 0 dB S+N / N (which is what I believe 
>Elecraft's
>approximation yields).
>
>         With Orion, you cannot truly turn AGC "Off" since even "Off" on 
>the
>front panel gives you the following fixed settings:
>
>Hang 00.00 S
>Decay 1000 dB/S
>Threshold 00.37 uV
>
>As most of us have discovered, setting Threshold very low causes the DSP to
>"boost" the internal receiver noise and destroy S/N performance.  Instead,
>I used the following settings for Prog AGC when making the measurements:
>
>Hang 00.00 S
>Decay 2000 dB/S (maximum)
>Threshold - various, see below
>
>I got the following results with 500 Hz BW, 7.040 MHz, Preamp On:
>
>Threshold       Noise Floor (unadjusted for 3 dB MDS definition):
>
>00.37   115
>3.04            122
>10.08   133
>21.28   138
>31.91   138
>47.87   138
>63.83   138
>95.74   139
>191.48  139
>
>To adjust for MDS = 3 dB S+N / N, 3 dB should be subtracted from the above
>results.  This would yield about -135 dB MDS for 7 MHz which agrees nicely
>with ARRL's measurement of -135.4 at 7.020 MHz.  I am frankly surprised my
>measurements are as close as they are since I am only using an inexpensive
>DMM that is average-responding (not true-RMS) and I'm eyeball-averaging
>the numbers as they dance around.
>
>         The bottom line to all this is something most of us have 
>discovered
>long ago...namely to maximize S/N performance, do not be afraid to adjust
>Threshold well above what is recommended in the Ten-Tec manual.  The other
>point is that you certainly can use the XG-1's -107 dBm (1 uV) output 
>together
>with Orion's internal -18 dBm attenuator to produce a -125 dBm signal.  If
>you turn Orion's Preamp Off, -125 dBm is very close to the -125.1 dBm MDS
>that ARRL measured at 7.02 MHz for Preamp Off in their Expanded Test 
>Report.
>
>                                         73,  Bill  W4ZV
>
>
>
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