[USRP-users] Input power limit for B2x0 series

Marcus Müller marcus.mueller at ettus.com
Sun Jul 9 04:55:38 EDT 2017


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 list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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