[hpsdr] [HPSDR] Hermes now rxing

[MDS Info]

Scott,

This is counter-intuitive to me.  I suspect this is wrong, sorry.  So, I
took my Wavetek attenuator and hooked it to the rx input on the Hermes, 20M
ssb.
As the DAT31 is right there before any amplification, the Wavetek is doing
the same job.

With the S-ATT set at zero, as I increase the input attenuation (Wavetek),
only the signal decreases.  Of course, part of the noise floor is band
noise, so some
lowering of the ultimate noise floor was shown.

The external Wavetek works exactly as I suspected the internal DAT31 should,
only decrease the incoming signal (plus noise).

I am concluding that there is some problem with my Hermes.

When I tune to an unused portion of the band, the noise floor shows -117db.
As I increase the attenuation to 30db and beyond, the bottom reaches -120 no
more.

73's

Glen K4KV


  -----Original Message-----
  From: Hpsdr [mailto:hpsdr-bounces at lists.openhpsdr.org]On Behalf Of Scott
Traurig
  Sent: Tuesday, May 05, 2015 10:45 PM
  To: Glenn Elmore
  Cc: hpsdr
  Subject: Re: [hpsdr] Hermes now rxing


  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?


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