[hpsdr] Hermes now rxing

[Scott Traurig]

Perhaps it is more important to realize that the step attenuator is in
front of the first receiver amplifier. Because of that the receiver noise
figure is going to be increased (worsened) by the amount of step
attenuation selected.

73,

Scott/w-u-2-o


On Tue, May 5, 2015 at 6:44 PM, Glenn Elmore <n6gn at sonic.net> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
> 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?
>>
>>  *****************************
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at openhpsdr.org
> Subscription help:
> http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
> HPSDR web page: http://openhpsdr.org
> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20150505/b39de115/attachment-0003.htm>