[Skimmertalk] Dynamic Range of SDR

[Lee Sawkins]

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.  Normally 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