Re: [Discuss-gnuradio] DC component

[Marcus D. Leech]

> I have noticed that there are some fixed frequency spurious signals in my
> N210.  These spurious signals are probably associated with the harmonics of
> the clock.  If your DC component is at some nice even frequency like 2GHz, I
> would suspect a spurious signal to be the cause.
>
> Evan
Spurious signals are a virtually-inevitable aspect of modern 
receivers/transmitters.  Many of us are used to radios that are
  "purpose built", and probably don't realize that most such radios 
have their own problems with spurious signals ("spurs"),
  but that they get "tweaked" in the design phase to move those 
(inevitable) "spurs" outside the operational envelope of
  the particular application at hand.  Your radio have a CPU?  Move the 
fundamental of its clock frequency so that the
  fundamental and harmonics fall outside of your band of interest.  But 
in a radio whose "band of interest" lies anywhere
  from DC up to a few GHz, that's a very tall order.

The good news is that most of the time, these "spurs" are quite weak, 
and are generally overwhelmed by any actual signal coming
  in from the antenna at the the same frequencies.  For modern wideband 
modulation schemes, an in-band spur that's 30-40dB below
  your nominal incoming signal make essentially no difference to the 
receive SNR.  For narrowband weak signals that are coming in
  just above the noise floor, it might be a different story.

I've attached a plot of 50MHz of spectrum (thanks to the latest 
50Msps/sc8 support in UHD) around 1.420GHz, with the receiver input 
terminated
  in a 50 Ohm lab-grade termination.

You can clearly see spurious signals spaced every 5MHz, and a stronger 
one right at 1.40MHz.  The 5MHz may be from the ethernet clock,
  not sure, but the stronger spur at 1.4GHz is very likely due to an 
even harmonic of the 100MHz master clock.  Even though this "spur"
  at 1.4GHz is 40dB "out of the noise", in most applications the 
signals themselves will *dwarf* that spur.  The other spurs, across 50MHz
  of bandwidth are no more than 20dB "out of the noise".  They don't 
worry me that much, even for applications like radio astronomy,
  where the signals are really weak.  Placing a low-noise gain chain 
ahead of the receiver, with enough gain to "swamp" the receiver spurs
  is all I need to make these go away.

It's true that a $40K laboratory-calibrated receiver like an (Agilent, 
R&S, etc) spectrum analyser will likely have fewer "spurs".  But if you
  open one of those up, you'll notice a lot of individually shielded 
sub-assemblies--that's not just for show.  They'll also do tricks like
  spreading the clocks for the control/monitoring CPU, shifting clocks 
around, to make the "spurs" move away from the current band of
  interest.  And for the most part, things like laboratory spectrum 
analysers are "deaf as a post"--they aren't designed, generally, to be
  "off-air" receivers, so they tend to be less sensitive to their own 
"spurs".




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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