[USRP-users] Input power limit for B2x0 series

Dan CaJacob dan.cajacob at gmail.com
Sun Jul 9 15:46:36 EDT 2017


If your LNA is pumping out at 1/4W, just add an attenuator inline near your
radio to bring your signal power down to safe levels - it won't hurt you in
any appreciable way since you've got a nice LNA upfront already.

On Sun, Jul 9, 2017 at 7:03 AM Marcus Müller via USRP-users <
usrp-users at lists.ettus.com> wrote:

> 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 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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>
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-- 
Very Respectfully,

Dan CaJacob
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