Here's a snapshot of the 3rd-order IMD products out of a WBX http://www.sbrac.org/files/3rdorder.png
It shows the 3rd-order products at roughly -50dBc, which is extremely good, when compared against many of the commercial amateur-radio transmitters in surveys like this one: http://www.remeeus.eu/hamradio/pa1hr/productreview.htm Exact results would be frequency and daughter-card dependant, obviously. Note that the digital (baseband) gain is set at -13dB, above this, the IMD products start to creep up in strength, but the fundamental tones themselves don't get any stronger. The point at which the analog chain starts to become non-linear will be different on different daughtercards, at different frequencies. It's *not* safe to assume that a baseband magnitude of "1" represents the maximum still-linear signal power. Indeed, the full-scale output of the ADC on (for example) an N210 corresponds to roughly 0dBm. Various daughtercards have up-converters (quadrature mixers) with various conversion gains ranging from about 3dB to about 5dB. On the WBX, this drives a GVA-84+ amplifier, whose p1dB point is at about 20.5dBm, and whose gain is 21.5dB at 1GHz, so with full-scale output through the mixer, the GVA-84+ would be operating well past the P1dB point, and be operating in increasingly non-linear territory. The RFX2400 has a mixer with similar conversion gain to that on the WBX, but with two-stages of TX-side RF gain, with an RF3315 as the last stage. P1dB for the RF3315 is at about +23dBm. But with two stages of gain on the RF output side of the mixer, I would expect the RFX2400 to hit non-linear operating conditions sooner than on the WBX. So, to recap. A signal magnitude of 1.0 out of UHD doesn't mean "maximum RF output power while still maintaining excellent IMD performance". It simply means "full-scale output from the DAC". The extent to which full-scale output from the DAC drives the analog daughter-cards into non-linear operating territory is highly dependant on the specific daughtercard, the frequency of operation, and the TX gain settings on the daughter-card. Because of conversion gains in the mixers, and generous post-mixer gain, backing-off from 1.0 baseband magnitude doesn't necessarily mean any significant sacrifice in RF output power. Anyone who cares to can download the daughter-board schematics for themselves, download the appropriate data sheets from the manufacturers, and confirm all of this for themselves. -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
