[Skimmertalk] Dynamic Range of SDR

[dd5xx at web.de]

There was no caller yet -HI- I just had started my CQ call.

73
Saki, DD5XX
 
 

Gesendet: Dienstag, 28. November 2017 um 19:53 Uhr
Von: "Bob Wilson, N6TV" <n6tv at arrl.net>
An: "DD5XX (Saki)" <dd5xx at web.de>, "SkimmerTalk Reflector" <skimmertalk at contesting.com>
Betreff: Re: [Skimmertalk] Dynamic Range of SDR

This happens occasionally when Skimmer mixes up the callsign of the CQing station (W3LPL) and the caller (DD5XX).  If W3LPL had to send your callsign twice in a row before you came back to him, Skimmer could think the CQing station was DD5XX instead of W3LPL, generating an incorrect S/N report by his local skimmer.  It's likely that's is what happened in this case.
 
Since the S/N ratio of CQing stations is logged in the RBN raw data files, it is possible to view the max. reported S/N for every skimmer, by band, during the CQ WW contest.  I'll see if I can pull a report together unless someone else has already done this.  Maybe Tomi can add it to Tableu faster than I can write a new report.
 
There are some S/N ratios of 99 dB listed in the files.  Nothing over 100.  Not sure what to make of these.
 
73,

Bob, N6TV 
On Tue, Nov 28, 2017 at 2:12 AM, <dd5xx at web.de[mailto:dd5xx at web.de]> wrote:I had a weird S/N report during CQWWCW contest on 40m. I was running 100W with a very low performance ANT in the center of a city between huge buildings. Of course I know the S/N reports I got from US stations, usual lower than 10dB in best times <20dB. W3LPL reported 73dB, I was shocked -HI- of course this never can be a real S/N report but who knows what happened at that particular time when he spotted me. I never saw such spots before, I even wasn't aware that it can go up so high. However, it was funny to see this on the RBN.
 
73
Saki, DD5XX
 

Gesendet: Montag, 27. November 2017 um 23:44 Uhr
Von: "Lee Sawkins" <ve7cc at shaw.ca[mailto:ve7cc at shaw.ca]>
An: skimmertalk at contesting.com[mailto:skimmertalk at contesting.com]
Betreff: [Skimmertalk] Dynamic Range of SDR

It appears to me there is a simple way to determine the dynamic range of any SDR. The lowest s/n you will see reported is zero. So is the dynamic range not simply the highest reported s/n reported by your SDR, as long as it doesn't go crazy when hit by huge signals.
During the CQ WW I was single band 160 and had my SDR offline during times I was operating. I turned it on when I was not on 160.
I was turning around on 20 while 160 was dead and heard VE7NY. He was operating from VE7SZ's QTH. His signal was extremely strong as I am 5 miles away, 600 ft higher in elevation and directly in line towards Europe. I was curious what the reported s/n was. Surprisingly, my Red Pitaya 14 bit was not reporting him on 20 on the frequency he was using. He was being reported on his second harmonic on 10 with a s/n of 14 dB or so and occasionally on slight offsets from his 20 meter frequency at low s/n. None of these "bad" frequencies should have made it through any dx cluster's bad spot filtering. Looking at the HDSDR using the same Red Pitaya he was S9+25 dB. I removed the power from the pre-amp I was using, leaving the input and output connected. The signal dropped to S5 and the Red Pitaya then reported him on the right frequency with a s/n of 74 dB, which is the highest report I have ever seen. I never run my skimmer like this normally. I just did this as a test. No
rmally the highest s/n I see reported is in the mid 60s. I believe this is the dynamic range of my setup. Am I wrong? Of course there may be different numbers on different bands, but I believe that at least for 20 meters I am in the ball park believing my dynamic range is about 65 dB. Theoretically a 14 bit ADC should have a dynamic range of 85.8 dB plus whatever can be gained by decimation. http://www.qsl.net/z33t/dynamic_range_eng.htmlWhat[http://www.qsl.net/z33t/dynamic_range_eng.htmlWhat] I believe happens with VE7SZ's main signal is that it gets clipped by the ADC and is ignored. I do not see any problems created by this. If I decrease the gain of my system to stop the clipping, I am going to lose all the weak signals. Lee VE7CC
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