[USRP-users] Fwd: B2x0 maximum receive power before device damage occurrs.

Marcus D. Leech mleech at ripnet.com
Thu Jul 16 17:20:02 EDT 2015


On 07/16/2015 03:38 PM, Urban Hakansson via USRP-users wrote:
>
> Marcus
>
> It seems to me that -15 dBm is a specific value chosen for B2x0 and is 
> not a number  specced across the entire product line where LNAs may be 
> involved.
>
> Otherwise, why does the "Application Note Examples Provided with the 
> USRP Hardware Drive" 
> http://www.ettus.com/content/files/kb/application_note_uhd_examples.pdf state
> "care must be exercised to guarantee that the maximum input 
> specification of the particular daughterboard is not exceeded. This 
> level is -10 dBm for all daughterboards
> -
> other than BasicRX and LFRX"
>
> Why is not -15 dBm specced?    The loopback Kit at 
> http://www.ettus.com/product/details/LPBK-KIT contains 30 dB 
> attenuators. Assuming max TX power is + 20 dBm, this would make sure 
> max RX power is -10 dBm. But if the B2x0 can only handle -15 dBm the 
> max input power level would be exceeded by 5dB (almost 3.2 times the 
> specced max RX power)
>
> Here is an example of how higher received power levels could occurr. 
> Let's say we have an indoor communication system running openBTS 
> GSM850 MHz: A GSM MS is transmitting channel requests at +21 dBm. The 
> BS RX gain set to 30 dB (just below the huger IIP3 15 dB drop on the 
> B210 +8 dBm to -7dBm) to be able to detect far away mobiles throughout 
> building(through doors, walls, floors etc.).  Assume a MS is 10 meters 
> from BS, which is not unrealistic in an indoor environment. Due to 
> being that close to the BS I think I can assume free space loss model 
> at frequency .  At 10 meters (30 feet) the received power level would 
> be in the range 0 to +0.23 dBm, neglecting antenna gain/attenuation, 
> coaxial cable power loss etc.
>
> When I look at a data sheet I want to find out what the radio can 
> actually handle before damage will occur, not what power level one 
> could reasonbly be expect to receive. Are these max power ratings -10 
> dBm, or -15 dBm at least somewhat influenced by what Ettus thinks one 
> could reasonbly expect to receive,  rather than simply being based 
> what the radio hardware is actually capable of handling?
>
> Urban
-15dBm is a conservative value that Ettus has been recommending as a 
maximum power level for a long time across the entire product line, and they
  are deliberately conservative, due to long experience with much larger 
amounts of power being delivered to RX boards, with resultant catastrophic
  damage.

I'll let someone from R&D who was involved in actually designing the 
hard comment on *how* conservative that -15dBm value is.

Also, as a bit of an aside, I ran the path-loss calculations for 850 MHz 
over 10m, and the path loss is 51dB, meaning that +20dBm is presented at
   the RX antenna as -31dBm, assuming low-gain antennae on either end.


> ------------------------------------------------------------------------
> *From: *"Marcus D. Leech via USRP-users" <usrp-users at lists.ettus.com>
> *To: *usrp-users at lists.ettus.com
> *Sent: *Thursday, July 16, 2015 1:17:16 PM
> *Subject: *Re: [USRP-users] B2x0 maximum receive power before device 
> damage occurrs.
>
> On 07/16/2015 01:10 PM, Urban Hakansson via USRP-users wrote:
>
>     Hi,
>
>     RX power -15dBm max  according to
>     http://files.ettus.com/manual/page_usrp_b200.html
>
>     Why is this the max Rx Power limit? I assume this is the max RX
>     Power before device damage may occur.
>
>     I looked at the schematics at
>     http://files.ettus.com/schematics/b200/b200.pdf "B200 RF
>     Interface", page 8 of 8.
>
>     Following the RF path from the RX1 (J802) SMA port to the
>     AD9361-RX here are the components I have identified, followed by
>     the max input power ratings.
>
>     1) U807: SKY13335−381LF, a pHEMT GaAs FET I/C switch that can
>     handle up to *+31 dBm*
>     2) U800:  ?
>     3) U801: SKY13317−373LF, a GaAs pHEMT single-pole, triple-throw
>     (SP3T) antenna switch that can handle up to *+30 dBm* (1 Watts)
>     input power.
>     4) U802: BD3150L50100A00, a balanced to unbalanced transformer,
>     that can handle up to 2 Watts (*+33 dBm*)
>     5) AD9361-RX, and RFIC that can handle up to *+2.5 dBm.*
>
>     When I look at the major components in the RX RF path, I cannot
>     help but think the B2x0 should be able to handle up to *+2.5 dBm*?
>     Where does the -15 dBm max RX power limit come from? (There are a
>     bunch of resistors, capacitors, an-15dBm is specced across the
>     entire product line where LNAs may be involvedd inductors in
>     between all these components, all of which I assume can handle
>     whatever comes in.)
>
>     Which component(s) in the RF path is it that will experience
>     damage at this RX power level?
>
>     Thanks for your consideration,
>
>     Urban Hakansson
>     Senior Software Engineer
>     Tecore Networks
>     Phone: +1 410 872 6315
>     Fax: +1 410 872 6010
>     Email: uhakansson at tecore.com
>
>
> -15dBm is specced across the entire product line where LNAs may be 
> involved.  Some devices may be able to tolerate much more than that, 
> but keep in
>   mind the RX chains on these devices are designed to be plugged into 
> an antenna system, where a -15dBm power level would be considered 
> exceedingly
>   loud.
>
>
>
>
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