[R-390] sdr and r390

[jgedde at optonline.net]

Jose,

It's not a fair test to use the SDRPlay as a spectrum analyzer.  Without
going crazy into the math, noise and bandwidth are closely related.  Firstly
noise floor over several MHz is a far cry from the noise floor over 16 kHz
(max).  Secondly, I would guesstimate the R390a's noise floor bests that of
the SDRplay by 20 dB or more from the get go.  My opinion is, and take it
for what you wish, that noise floor is just bragging rights.  Outside of a
laboratory environment, it's academic.  Don't get me wrong, I want my R390A
to hit -143 dB too, but I'll never see any benefit apart from bragging
rights.

Since you posted, I've hooked up my R390A to my SDRPLay2 and it yielded
similar results (using SDRUno).  Also keep in mind the SDRPlay uses AGC
which will have the effect of raising its gain its noise floor to support
its dynamic range...  -100 dB is quite respectable - not quite CD quality
but better than a cassette tape with dbx or Dolby C back in the day.  In
short, you're not missing any signals in all likelihood.

A real measurement might consist of shorting the balanced antenna inputs on
the R390A and seeing what the SDRPLay says the noise floor is then.  Then
try turning off all AGC in the SDRPLay and setting the IF gain to say mid
span.  Again, SDRPlay is not a spectrum analyzer and it reports dB - but
relative to what?  It's an uncalibrated measurement unless you reference it
to something...  Think of dB as a comparison between two numbers...  Without
knowing one, the other is meaningless.

I hooked my R390A to a 2 uV source with 30% modulation using high grade
shielded cabling.  I clearly heard the signal.  Then I ran it in MGC mode,
disconnected the sig gen, connected the antenna and was rewarded with
deafening noise.  Lesson learned?  Even 2 uV sensitivity is academic with
real world noise nowadays.

73's
John

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of jm
Sent: Monday, April 22, 2019 5:03 PM
To: hs0zed at gmail.com; R-390 Forum <r-390 at mailman.qth.net>
Subject: [R-390] sdr and r390

Hi Martin,

One of the sdr programs that I use is SDRuno. When you move down the "grass"
the dbm scale moves accordingly. So if the noise floor is at -100dbm, it is
maintained at the same level in the new position.

 I
Jose EA8DCP



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