[USRP-users] Input power limit for B2x0 series

Marcus Müller marcus.mueller at ettus.com
Sun Jul 9 07:02:53 EDT 2017


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 <mailto: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
>     <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
>>
>>
>>
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>
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