[USRP-users] USRP Input Protection

Marcus D. Leech mleech at ripnet.com
Thu Mar 3 09:55:28 EST 2016


On 03/03/2016 07:40 AM, Marcus Müller via USRP-users wrote:
> Dear Jan,
>
> Well, in that case, a bandpass that only lets through your signal of 
> interest surely sounds the most "high quality" solution, but it would 
> limit your flexibility, as you would have to change that filter 
> depending on what frequency you want to receive.
>
> Protection diodes for high bandwidths / Gigahertz frequencies and low 
> powers are inherently hard to get -- low powers imply thin 
> semiconductor interfaces, thin semiconductor interfaces imply small 
> separation between positive and negative charges, which implies high 
> capacitance, which implies low-pass characteristics if inserted 
> between signal line and ground.
>
> This quickly boils down to a matter of money: Of course you could 
> "just" use attenuators and a properly cooled LNA close to the antenna...
>
> Best regards,
> Marcus
There are two issues at play here--one of which is about protecting the 
USRP input, the other is about preserving dynamic range/signal-quality
   in the presence of very loud adjacent signals.

Even if one found an RF limiter diode that would start clamping at very 
low input powers, while that would protect the downstream device,
   there'd still be horrible signal quality issues during the time said 
diode is clamping.

This is why at, for example, repeater sites where there may be dozens of 
different TX and RX for different purposes, they use fairly
   aggressive analog filtering.  Without that, the physics of the 
situation simply won't allow linear operation of your RX chain, even 
ignoring
   potential damage.


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