Hi Marcus, > From: "Marcus D. Leech" <[email protected]> > 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.
Yes, I am aware of that but my question related specifically to https://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf because I want to sanity-check my measurements (and I want to know how much off I am compared to the figures). Hence my question is how the "Gain" in the "RX Figure" in this document is defined. It's not written anywhere. > 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". Thank you, that I did not know. For me the result is: TX Frontend 0, RX Frontend 0: Gain range PGA0: 0.0 to 31.5 step 0.5 dB RX Codec A, RX Codec B: Gain range digital: 0.0 to 6.0 step 0.5 dB TX Codec A, TX Codec B: None Is the "RX Codec" the "VGA on some motherboards ahead of the ADCs" you were referring to above? A related question (of course, I studied the the schematic https://files.ettus.com/schematics/ubx/UBX-160_revE.pdf as well): For both TX and RX path the only programmable element are attenuators (HMC624LP4E, 6-bit). That would fit nicely the "PGA0" from above: 0 to 31.5dB in 0.5dB steps are 64 steps (6 bit). The other gain elements (TX: NBB-400, PHA-1+, RX: MGA-62563, NBB-400 are static). Why would you painstakingly gain up a signal, just to attenuate it again? Why a programmable attenuator instead of a real programmable amplifier? For the TX path I can somewhat see for simplicity ... but for RX path? The attenuator unnecessarily generates noise (the noise figure of a passive attenuator is identical to its attenuation). Thanks, Lukas _______________________________________________ USRP-users mailing list [email protected] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
