I did some experiments tonight, to check the sensitivity of the BASIC_RX, to try to decide how much gain I really need in front of it for one of my projects.
I have a signal at about -32dBm, and verified the pk-pk voltage on a good oscilloscope, using a 50-ohm load in front of the scope, to make certain that I was measuring what the input circuit of the BASIC_RX would see. Then I plugged the -32dBm signal into the BASIC_RX/USRP2, and observed that at my selected bandwidth of approximately 250KHz, the FFT display had the wideband noise signal at -20dB. I then unplugged the input from the BASIC_RX, which should cause it to "see" the 50-ohm resistor that's across the transformer input to the BASIC_RX (at least, according to the schematics). The result was that the output on the FFT display dropped by 28db, giving an input noise level of around -60dbm with no external signal. Now granted, that resistor on the BASIC_RX input circuit (why is it there?) will contribute roughly -174dBm/Hz of noise, but that is *tiny* in this context. Now, it's my understanding that the maximum power level capability of the A/D is about 10dBm, but one should perhaps back off from that by about 1dB or so. Doing the "math" on the 14-bit A/D resolution, you'd expect a noise floor around -75dBm or so, which means that roughly 15dB of "bottom end" on the A/D is "missing" when plugged into a BASIC_RX. Is this a correct analysis, or have I missed something somewhere in the analysis? Is there enough noise bumping around inside a USRP2 enclosure to 'stimulate' the lower few bits of the A/D without anything connected? -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
