Re: [Discuss-gnuradio] SBX Daughterboard dead?

[Marcus D. Leech]

On 03/02/2012 07:34 PM, jsrdor wrote:
> Hello Again,
>
> I think i know the answer to this question but I still hope i am not right.
>
> Setup N210 with SBX
>
> One of my colleagues plugged in TX/RX an external discone antenna with an
> RG58 cable playing around in the 3 Ghz band. Somehow he managed to get down
> to 400 Mhz where db increases partly because of the antenna gain and the
> reduced attenuation of the cable. After some experimentation the SBX stopped
> receiving signals. I changed antenas (LP0965) cables etc removed and
> reinstalled the SBX but it wont give me any signals. Usrp appears to work
> alright - leds flashing etc but i dont have another daughterboard to test
> it.
>
> SBX is flashing as well but no signal in TX/RX nor RX2. When tuning
> different frequencies there are no errors etc, but i receive just noise. I
> think that the down left corner of the SBX (as you see it- to the left of
> RX2) is getting hotter than it used to.
>
> That leads to the question, is the Db dead? Any way to fix it? Is there a
> limit in the input db when connecting an external antenna? I would guess an
> attenuator would be needed?
>
> Thanks alot,
> R.I.P SBX
>
> Jason
There are several mechanisms that can cause a low-noise-amplifier to 
fail.  The SBX has an LNA as its first stage.

The most common are:

   o ESD  -- a static discharge, lightning discharge close enough, etc

   Sometimes, you can get static build-up on antenna elements when 
there are high winds with very dry snow.   Sometimes you can ameliorate
   this with an in-line gas-discharge surge arrestor.  Sometimes, a 
large-valued carbon resistor in shunt (grounded) can bleed-off any
   static charges before they reach dangerous levels.  This method has 
the disadvantage that it will negatively affect the noise figure of
   the LNA and reduce sensitivity somewhat.  Sometimes, somebody just 
touching the antenna at the wrong time, after having walked across
   the carpet, can cause significant gate damage on the GaAsFET 
transistors in an LNA.  It's a tradeoff between robustness, and ultimate
   sensitivity.

  o High EM levels.

     If you operate at a site with high-level transmitters, even ones 
that are out-of-band, you can get coupled EM levels "seen" at the LNA
     beyond its safe operating envelope.  Keep in mind that the SBX, as 
designed is "broad as a barn door", so the LNA isn't particularly
     frequency-dependant.  If the integrated energy from all sources 
within its "window" (probably from 100MHz up to 10Ghz or more) is
     more than about -10dBm, you run the risk of damaging the LNA.

When professionals deploy sensitive receivers at sites with high RF 
levels, they  usually place a narrowband low-loss filter in front of the
  LNA. This has several useful properties, including keeping the 
bandwidth-integrated RF levels below that which could cause damage,
  and also keeps the LNA within its linear operating envelope, even if 
levels aren't high enough to cause damage, they can be high
  enough to cause non-linear behaviour, leading to intermodulation 
products.  What this means is that deploying a broadband receiver at such
  a site is usually not done.

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