Hi Lukas, For the UBX, the gain values of 0-31.5 are probably where you want to stay. I have not found a reason to use the additional 6dB of digital gain. The gain values you set should be reasonably accurate in a relative sense. If you want to increase/decrease by 10 dB on your FFT display, then changing the gain value by 10 dB should get you pretty close. The example you provided is essentially your "calibration". You know that your input is at -20dBm and you know the RMS value of your samples at the given settings so the difference is the calibration value. If you then apply a new signal with the same settings, you can measure the RMS value and you will know the actual power level at the input. This can change with frequency - the curves in the performance data give you an idea how much frequency dependency exists. Rob
On Thu, Nov 5, 2020 at 7:08 PM Marcus D. Leech via USRP-users < [email protected]> wrote: > 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". > > > > _______________________________________________ > USRP-users mailing list > [email protected] > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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