Hi Sivan, ah in that case, you'll probably be fine; NF would be (assuming all the limiter, the filter and a potential attenuator have a 0dB Noise Figure):
$F_\text{total} = F_\text{LNA} + \frac{F_\text{AD936x}}{G_\text{LNA}G_\text{cable}G_\text{limiter}G_\text{filter}G_\text{attenuator}}$; The NF of the AD936x depends on the gain you use, and from rough memory varies between let's say 35 dB @0dB gain over 25 dB @25dB, to 15 dB @30dB to a final ca 5dB from 55dB onwards. Let's say we're in a strong-signal case and you operate the B2xx at 30dB gain, and guessing 9dB loss at 100m distance for the cable, and assuming the limiter is practically lossless, as well as a 6 dB attenuator $F_\text{total} = 10^{0.05} + \frac{10^{1.5}}{10^{1.7}\cdot 10^{-0.45} \cdot 1 \cdot 10^{-0.3} \cdot 10^{-0.6}}$; which amounts to $F_\text{total} = 10^{0.05} + {10^{1.5-1.7+0.45+0.3+0.6}}= 10^{0.05} + 10^{1.15}}\approx 11.5\text{ dB}$. Now, that doesn't read very impressive, but your mentioning of SAW filters might indicate that you're dealing with a narrowband signal, which might mean that with the oversampling-induced SNR gain you can effectively get a much nicer effective system temperature. Best regards, Marcus On 09.07.2017 11:37, Sivan Toledo wrote: > 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 <mailto: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 > <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 >> <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 list >> USRP-users@lists.ettus.com <mailto:USRP-users@lists.ettus.com> >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com> > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com <mailto:USRP-users@lists.ettus.com> > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com> > >
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