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 > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > ************************************************************************************ ******** 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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
