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Date: Fri, 18 Oct 2013 22:35:27 -0230 (NDT)
From: jcraig@mun.ca
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: T/A OPDS DK7FC
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Hi Jay, Group,

Lately, I've been busy trying to rx below 100 kHz with an old Marconi XH100
receiver. One of these ebay chinese DDS boards was used to provide the LO,
but it was not stable.   The 10.7 MHz OCXO BFO was tapped to clock to an
AD9851, but to my disgust, it went dead when the mode was changed from USB
to AM which means no more BBC4 and other LW AM stations.

The next step was to remove the 125 MHz osccillator from the dds board and
poke it in the OXCO.  It's much more stable, but still drifts ~1Hz
or so with WWVB on a QRSS30 Argo screen.

This setup will be used tonight looking for  XRS again on 74.5 kHz.
The OP32 on 137.555 kHz will be back soon!

Cheers
Joe VO1NA

On Fri, 18 Oct 2013, jrusgrove@comcast.net wrote:

> T/A OPDS enthusiasts
>
> It appears that I mistakenly ran with a known setup issue last night ... =
so
> my reported SNR numbers should not be considered accurate. The issue is n=
ow
> resolved and the setup is running. Hope to catch both Stefan and Markus
> tonight ... maybe VO1NA can be encouraged to join in as well. Have only
> received Joe's OPDS32 on one ocassion several weeks back.
>
> Jay W1VD  WD2XNS  WE2XGR/2  WG2XRS/2
>
> ----- Original Message -----
>  From: Markus Vester
>  To: rsgb_lf_group@blacksheep.org
>  Sent: Friday, October 18, 2013 11:41 AM
>  Subject: Re: LF: T/A OPDS DK7FC
>
>
>  Stefan, Jay, John,
>
>  the "96%" number shows the temporal overlap between the receive period (=
ie
> the duration of the currently used FFT, about 35 minutes) and the identif=
ied
> Op sequence (33 minutes). With 10 minutes spacing between subsequent FFTs,
> the sequence does not always fit completely into one of the slots. so it
> says that 96% of the sequence was included in the evaluation. However the
> missing part has only neglegible impact on the detection sensitivity.
>
>  Ideally Opera can be treated as an AM signal, with a central carrier and
> modulation sidebands around it. The "2mHz" bandwidth figure refers to the
> bandwidth of the carrier, which should be as small as possible. Opds appl=
ies
> some smoothing to the power spectrum and then tries to measure the -10 dB
> bandwidth of the central peak. Stable and phase coherent signals
> consistently show less than 3 mHz bandwidth. An intermediate width up to
> about 30 mHz typically indicates a coherent signal but with a slight ther=
mal
> drift. Even higher bandwidth (~ 100 mHz) are mostly due to incoherent
> keying, ie random phase dashes caused by stopping the TX oscillator or
> divider during gaps.
>
>  Opds internally uses an "autofocus" concept similar to synchroneous
> demodulation, where the central spectral peak is used as a phase referenc=
e.
> Narrower carriers produce better demodulated SNR. For fading or incoherent
> signals, the phase has to be tracked faster or even on a dash-by-dash bas=
is,
> which is much less efficient.
>
>  The "dBOp" column is showing SNR according to Jos=E9's Opera scale, whic=
h is
> approximately based on average power. It shows 4 dB more negative values
> than the standard WSPR scale, ie. carrier power in 2.5 kHz. A marginally
> decode with WSPR-15 would need -38 dB, and an Opera signal with same PEP
> would then show as -42 dBOp. For a coherent signal, the Opds-32 threshold
> should be around -50 dBOp, which in theory is 8 dB better than WSPR-15 and
> 11 dB better than standard Opera-32.
>
>  Please be aware that the SNR figures shown in opds results can sometimes
> be inaccurate. With an incoherent signal, often not all of the carrier po=
wer
> is captured during the bandwidth measurement, producing a low or invalid =
SNR
> reading. The SNR measurement also doesn't work well for strong signals
> (> -20 dBOp), eg for DK7FC who should really be plus several dB here.
>
>  So why was Stefan's TA signal not stronger last night? My guess is that =
TA
> propagation just didn't extend into central Europe: While UK and duch
> stations received Bob well on 74 kHz, little or nothing at all apperared =
on
> Hartmut's and my 74 kHz grabbers.
>
>  Best 73,
>  Markus (DF6NM)
>
>  PS The weather has improved here, so I have put out the TX antenna for a
> possible joint TA session tonight.
>
>
>
>  From: Stefan Sch=E4fer
>  Sent: Friday, October 18, 2013 2:36 PM
>  To: rsgb_lf_group@blacksheep.org
>  Subject: Re: LF: T/A OPDS DK7FC
>
>
>  Hi John, Jay, Markus,
>
>  OK. Well, it seems to work. But i can't value the results. Why is it only
> 96% and what is the meaning of 2 mHz here?
>  Seems the S/N is rather low. Condx must have been poor.
>  Were there other reports from US stns?
>
>  73, Stefan/DK7FC
>
>  Am 18.10.2013 13:05, schrieb John Andrews:
>  Stefan, Markus, Jay,
>
>  Results from last night:
>
>  2013-10-18 05:09:25 DK7FC   5981km 137560.016Hz   2mHz -42.8dBOp  96%
> 20.6dB
>  2013-10-18 04:29:25 DK7FC   5981km 137560.016Hz   2mHz -44.8dBOp  96%
> 17.8dB
>  2013-10-18 03:49:23 DK7FC   5981km 137560.016Hz   2mHz -44.0dBOp  96%
> 20.3dB
>  2013-10-18 03:09:23 DK7FC   5981km 137560.017Hz   2mHz -43.7dBOp  96%
> 19.0dB
>  2013-10-18 02:29:23 DK7FC   5981km 137560.016Hz   3mHz -47.5dBOp  96%
> 17.9dB
>  2013-10-18 01:49:23 DK7FC   5981km 137560.016Hz   3mHz -46.4dBOp  80%
> 16.8dB
>
>  John, W1TAG
>
>
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