Marco, I am interested in your comment that band noise is -129dBm in 3kHz BW.
I listen to online SDRs located in Europe. The S meters on these receivers show that the band noise on the lower to middle HF band is rarely lower than -100dBm in a 3kHz BW - the lowest I've seen is around -105dBm to -110dBm. I'm presuming the S meter calibration is accurate. Antenna type etc. and local conditions also influence antenna noise level but I can't imagine that will account for a +30dB difference in noise level. Is the -129dBm noise level you see confirmed by Perseus observations (it seems to have an accurate S meter)? The reason I raise the matter is that it seems to me that modern day receivers have more than enough sensitivity for most of the HF bands- even SDRs like the Perseus/QS1R Interested in your comments. . Henry. Marco IK1ODO -2 wrote: > Hello Waine, > > your receiver ADC gets all the signals that are present at his input. > That may be the antenna, or the output of a preselection filter if > there is one between antenna and ADC. > You want to have a receiver sensitive enough to hear the band noise; > that requires a noise figure in the order of 10 dB in HF. That means > approx. -129 dBm of input power in 3 kHz BW. > On the other side, the ADC has to handle the strongest signals that > reach the input. If you listen to 40m by the evening in EU, you get > up to +10 dBm peak (1 V pk, measured...) due to BC signals close to > the amateur bands. > So the required dynamic range is of the order of 139 dB in 3 kHz. > Since it is currently unreachable, you need a preselector to limit > the signals that the ADC sees, and an attenuator to work on low bands > (where the band noise is higher and a loss of sensitivity is acceptable). > To have 139 dB of dynamic range one needs a 100 MS/s ADC with 16 bits > of ENOB, or 94 dB dynamic range ... current 16 bit ADCs have far > less, at the point that there is a very little difference in dynamic > range between 14 and 16 bit converters. > > 73 - Marco IK1ODO / AI4YF > >
