On 11/05/2020 06:42 PM, Lukas Haase via USRP-users wrote:
Hi,
I m studying:
https://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf
How exactly is "Gain" in the "RX Figures" defined? Can it be related to the
digital samples (e.g. in dBFS or RMS)?
If, for example, I apply a exactly Pin=-20dBm, f=916MHz CW (calibrated via VNA)
at the SMA input, I get -14.05dBFS (=rms value of IQ samples in 20*log10) when
I calculate the RMS values of the samples.
(Note: set_rx_gain(0) and I use 516MHz as center frequency to avoid DC
correction).
Also, the statement on the gain values is not quite clear: "Gain range for both RX
and TX is 0-31.5 (the maximum RX gain seen in the tests of 37.5dB is due to the
additional 6dB provided by the ADC in the USRP motherboard on top of the UBX's maximum
gain".
How does this relate to the value which is set with set_rx_gain()?
Does that mean that a "Gain" value of 10 dB on the plot corresponds to
set_rx_gain(4) ?
What confuses me is that I *can* call set_rx_gain() with values of 0...37.5,
and the output scales accordingly (i.e., the lowest value seems to be 0 and the
highest 37.5; not 31.5).
Would be great if someone could confirm.
Thanks,
Luke
The precise implications of the gain-setting function are highly
dependent on the particular daughterboard architecture. When you set
the gain, you're typically (certainly true on UBX) setting a step
attenuator, which is usually sandwiched between a pair of low-noise
amplifiers, then a mixer, which may have either gain or loss of a few dB.
The only way you can relate the dB values in your FFT display to
power-as-seen-at-the-RF-connector is using an external calibration source,
and you'll have to do this exercise over your entire expected
operating-parameter space. RF analog components will NOT provide exacty
the same gain/loss levels across their entire operating range.
The additional gain-range setting is due to the presence of a *baseband*
VGA on some motherboards ahead of the ADCs. UHD will distribute
gain settings over the available gain-setting elements (in both the
daughtercards and motherboard). By default, it will distribute gain to
favor noise figure over linearity. You can set individual gain
elements if you prefer:
https://files.ettus.com/manual/classuhd_1_1usrp_1_1multi__usrp.html#ad602e7681b796deddd9231f022ffef11
Now the names for the various gain elements will differ depending on the
motherboard and daughterboard in use, and they can be
shown using "uhd_usrp_probe".
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