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

[Bill Tippett <btippett@alum.mit.edu>]

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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