|From:||Marcus D. Leech|
|Subject:||Re: [Discuss-gnuradio] Ettus N210 GMSK 9600|
|Date:||Wed, 23 Mar 2016 22:22:06 -0400|
|User-agent:||Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0|
On 03/23/2016 10:08 PM, Dan CaJacob wrote:
I'd actually said most of this to Tom privately, but forgot to copy the list.
It's instructive to consider what kind of "normal" signals one might expect to receive on a receiver intended to be connected to an antenna.
Consider your average +70dBm (10kW) FM station. 10km away from the TX, we expect about 90dB path loss, which means that the
theoretical power seen by your antenna would be about -20dBm. Perhaps a bit more, since the TX antenna won't be strictly isotropic.
At 50km away, that's 100dB of path loss, giving about -30dBm at your antenna terminals (ballpark, hand-waving). Now, a signal of
-30dBm is actually considered fairly strong, and your typical FM receiver will produce a very nice, nearly-noise-free output from
signals entering your antenna at levels of -30dBm.
The RX inputs of most SDRs (Ettus radio are no exception) are designed for the kinds of signal levels that are typical in "connected to
an antenna" environment. They are less well suited to "connected to a laboratory signal generator", without significant attenuation.
The RF signal levels floating around in a lab are typically much "louder" than you might find "on air", so if one is used to lab conditions,
it may not be obvious that a sensitive receiver designed for "from the air" reception would be unhappy in laboratory signal-level
conditions. So, keep the levels very modest indeed.
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