SBX Daughterboard dead?

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

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On 03/02/2012 07:34 PM, jsrdor wrote:

receiving signals. I changed antenas (LP0965) cables etc removed and
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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

Thank you for your reply. This is what happens when software developers
touch
devices such as usrps. Since i have no intention to fry the tvrx
daughterboard we are planning to use now , will the following setup be
enought?

WideBand Rx only 300-3Ghz antenna, grounded through its base

surge arrestor

usrp

As i am at least ignorant in rf , would i need an attenuator to reduce
the
incoming signal? Do i need a second LNA infront of the one thats on the
daughteboard?

Apart from that, since its more of a computer lab than rf lab, what
instrumentation would you consider as necessary in order to measure
currents
voltages and other signal parameters to avoid another
daughterboard-fried
fiasco?

Thank you for your patience.

Jason


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