Here’s a snapshot of the 3rd-order IMD products out of a WBX
It shows the 3rd-order products at roughly -50dBc, which is extremely
when compared against many of the commercial amateur-radio transmitters
in surveys like this one:
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
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
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
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.
Shirleys Bay Radio Astronomy Consortium