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Re: [Discuss-gnuradio] SBX Daughterboard dead?

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] SBX Daughterboard dead?
Date: Fri, 02 Mar 2012 20:24:18 -0500
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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

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,

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

  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

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