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, [email protected] 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 now
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: [email protected]
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 identified
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 applies
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 thermal
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 reference.
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 basis,
which is much less efficient.
The "dBOp" column is showing SNR according to Jos�'s Opera scale, which 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 power
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�fer
Sent: Friday, October 18, 2013 2:36 PM
To: [email protected]
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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