[hpsdr] Hermes now rxing

[Glenn Elmore]

I would say this is both normal and the desired behavior. It's what a 
commercial spectrum analyzer does. The reported level is referenced to 
the antenna/input connector. What happens inside the receiver, other 
than the resolution/display bandwidth, shouldn't affect this.  When 
internal attenuation or preamplification is added or removed, the 
receiver fixes the reporting to make those changes invisible. The 
exception to this is that the displayed S Meter reading is reporting the 
total power within the selected bandwidth. If it's mostly a carrier with 
high S/N then you get the carrier's level. If it is mostly/all noise 
then you get the noise power in that bandwidth.  Attenuation or 
preamplification within the receiver just changes the sensitivity (noise 
figure) but doesn't/shouldn't change what is coming in or how it is 
reported.
Having the reporting done this way not only removes the receiver from 
the measurement but lets one calculate, for example, system noise figure 
by subtracting 10*log(bw) from the displayed reading on noise and 
comparing it with KTB in 1 Hz - -174 dBm.  This is useful in comparing 
QRN levels to a "quiet" world on HF (which I never see any more!)

Glenn n6gn

> The noise floor with the S-ATT at zero is very low, probably around 
> -135db
> on 20M right now.  As I raise the S-ATT the noise
> floor rises to -110db and the signals disappear.  I would have thought 
> just
> the signals would decrease as I raised the S-ATT!?
> 
> Is this normal behavior?
> 
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