Thanks Marcus! The 0dBm limit is much easier to work with than the -15. I indeed can add a 3 or 6dB attenuator to ensure that this is the case. Thanks a lot for the clarification regarding U800 and U813.
I don't mind sharing the frequency band and the details of the receive chain. We operate at 434MHz, we use an LNA with a gain of 17dB and noise figure of 0.5dB, then a cable (up to 50m of LMR400), then a 6dBm limiter and a saw filter with about 3dB insertion loss. I guess that an attenuator will not have a dramatic influence on the noise figure. To Dan and others: The LNA is a high linearity LNA which automatically implies that it can generate a lot of power (about 1/4W); that's where my concern comes from. Sivan On Sun, Jul 9, 2017 at 11:55 AM, Marcus Müller via USRP-users < usrp-users@lists.ettus.com> wrote: > Hi Sivan, > > to add to what Dan already said: You're right, the -15 dBm limit is a bit > overzealous (though I really must stress it's better to be safe than sorry > on that side). > > We're actually in the process of relaxing the limits we're stating for > this; compare [1], where we already spec a maximum input power of 0dBm. Of > course, it's absolutely correct that the maximum input power is what we can > be sure that, even under maximum gain, will not lead to damage. > > Regarding U800/U813: these are ESD protection, not power limiter diodes! > > Now, at +0dBm power (and even more so at +3dBm), the signal will not be > distorted only on the very lowest gain settings. Consider adding a simple > attenuator; Friis' noise formulas contradict that (having attenuation (i.e. > reducing gain) should happen as late as possible in the signal chain to > minimize overall Noise Figure), but these assume amplifiers are still > linear, and you'd probably break that condition. > > If you could share the frequency bands you're working on (if preferable, > also in confidentiality directly with me), we can try to come up with a > NF-vs-gain and IIP3-vs-gain relationship that would help you choose the > optimal operating point. > > Best regards, > > Marcus > > [1] https://kb.ettus.com/B200/B210/B200mini/B205mini#RF_Specifications > > On 08.07.2017 10:03, Sivan Toledo via USRP-users wrote: > > Hi, I am trying to understand the input-port limit of the B2X0 series, > which is specified as -15dBm in the User's Manual (http://files.ettus.com/ > manual/page_usrp_b200.html). > > The issue is that if I use external front-end components (masthead LNA and > a saw filter), it is difficult to limit power to -15dBm (limiting to a 0 or > single-digit dBm is possible with common limiters). > > Is the -15dBm the limit that will cause overload and distortion even on > the lowest gain setting, or is it a safely limit above which the unit may > get damanged? > > Looking at the schematics of the B210, the input if fed to a switch that > can sustain almost 1W, then through something that looks like a limiter > (U800 and U813), then through another switch, and then to the inputs of the > AD9361, which can tolerate up to 2.5dBm. So it's hard to see why anything > up to 2.5dBm will damage the B2x0, and assuming that U800 and U813 do have > some useful limiting function, maybe much more is safe. > > Can you please clarify? I am considering using B2x0 for an application > that may subject them to about 3dBm, maybe 3.5dBm (we use an LNA, followed > by a 6dBm-max limiter, then a SAW filter with an insertion loss around > 3dB), and I want to make sure that this is safe. > > Thanks, Sivan Toledo > > > > _______________________________________________ > USRP-users mailing > listUSRP-users@lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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