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Re: [Discuss-gnuradio] Connect two WBX daugterboards

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Connect two WBX daugterboards
Date: Fri, 15 Apr 2011 14:56:36 -0400
User-agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv: Gecko/20110303 Thunderbird/3.1.9

On 15/04/2011 2:45 PM, Brook Lin wrote:
Hi All,

I am using two USRP N210 with WBX. Can I connect two WBX with cable
directly? Or I need an attenuator between them. If so, what kind of
attenuator I need.

Thanks in advance,
This subject comes up a *lot*.

Generally, the RX portions of all the USRP-family daughter-cards (except the BASIC_RX and LF_RX) are designed to receive *off-air* signals directly from an antenna. Driving them directly from the TX side risks destroying the sensitive low-noise-amplifier (and possibly other) portions of the RX chains. This is true of *any* reasonably-sensitive receiver that is designed for off-air signals--not just a quirk of USRP-family daughter-cards.

In a "from the antenna" receive chain, signals of as low as -100dBm are usually easily discernible, and at -40dBm are usually of excellent quality, depending on the signal bandwidth, modulation involved, receiver-side sophistication, etc, etc.

What you want to do is to make sure that the RX chain receives a signal of no more than about -20dBm (10 microwatts--plenty!!). So I'd go with a 20 to 30dB attenuator if you're going to cable them together.

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