[Skimmertalk] Dynamic Range of SDR

[dd5xx at web.de]

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>
An: 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 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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