[USRP-users] Input power limit for B2x0 series

Sivan Toledo stoledo at tau.ac.il
Sun Jul 9 05:37:41 EDT 2017


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 at 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
>
>
>
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