Re: [Discuss-gnuradio] maximum input signal power for WBX

[Marcus D. Leech]

On 11/27/2010 11:02 PM, Steve Mcmahon wrote:
> Hello:
>
> What is the maximum recommended input signal power for the WBX daughterboard? 
> Where can I find this in the documentation or online at Ettus.com? My 
> understanding is that the LNA is very sensitive and it is easy to overload it 
> and damage it. I thought it was 0 dBm, but I have seen several numbers on 
> this mailist, such as -10 dBm and -15 dBm, so I'm making this post to get a 
> definitive answer. Thank you.
>
> Steve McMahon
>
>
>
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************************************************************************************
******** MY OWN PERSONAL OPINION AS A PART-TIME RF ENGINEER *********
************************************************************************************

The first stage in a WBX is an MGA-62563, which has a specced max Pin of
+20dBm and
  22dB gain, the next stage device has a specced max Pin of +13dBm. 
Which means that
  with the attenuator stage set to minimum, the max Pin in the first
stage would be about -9dBm,
  in  order not to seriously overload (and possibly damage) the second
stage, which is an
  MGA-82563, "fronted" by a digitally-controlled attenuator with a
minimum insertion loss
  of roughly 1.5dB (HMC472LP4).  This is all from the published
schematics in the
  "Downloads" section of the Ettus website.

The reality seems to be that the GaAs LNAs are *sometimes* more delicate
than their spec
  sheets would imply. If it were me, I'd never put more than -25dBm into
a receiver chain that
  had a GaAs LNA in it. Such a signal would generally be regarded as
"thundering strong",
  "earsplittingly loud", etc :-)

One needs to keep in mind that the daughtercards with active Rx chains
on them (that is
  all of them except LF_RX and BASIC_RX) are designed for *over the air*
use, which means
  that they generally expect signals to be arriving no stronger than
-30dBm, and usually a *lot*
  lower, like -60dBm or lower.  A good receiver generally has a
minimum-discernible-signal, for
  narrowband signals, below -115dBm.  Injecting signals into such a
sensitive receiver that are
  over 100dB stronger than that is asking for trouble.  Best case is
that you end up in non-linearity
  territory, and worse case is that you damage the first-stage LNA (or
possibly subsequent
  stages).  The maximum input power of the motherboard ADC is roughly
+10dBm, but I'd
  generally de-rate that by about 2-3dB.

Gate insulators on microwave GaAsFET transistors are *thin*.  Really,
really thin. Bias them
  the wrong way, that insulator goes "pffffffft".  Put too much power
into them, that insulator goes
  "pfffffffft".  Give it a shot of ESD, and that insulator, yup, goes
"pffffffft".  There's not a lot you
  can do to protect them, either.  Anything you put in front of an LNA
generally degrades the
  noise figure, often severely.  Limiter diodes?  That would badly screw
up your impedance matching,
  and kill your noise figure.  Gas discharge tubes?  Fine at low
frequencies, but the shunt capacitance
  will start giving you headaches at higher frequencies, and won't
protect you from somebody putting
  in +10dBm.  Input circuit with a DC-path to ground? Sometimes works,
but sometimes, your    
  "impulse" ends up getting stored in the shunt inductor, and when the
field collapses?  "Pfffffftttttt".
  Again, that won't protect against too much input power in "normal" ranges.

My understanding is that SiGe LNA parts are a little more forgiving, but
not by a whole lot, and they
  generally have poorer noise figures than their GaAs counter-parts
(although that is changing
  quickly).

Bottom line?  Sensitive microwave receivers are, well, *sensitive*.  If
it were my lab, I'd make
  certain that my techs knew never to inject more than -25dBm into *any*
LNA-based
  receiver, unless they were dead-certain that it could survive the
experience.

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