My knowledge is limited, therefore, read this with a grain of salt. However, I wanted to try to help and if something I say does not make sense then double-check it or someone else may come along and correct me.
I had this same problem when I started with these types of systems. I had trouble understanding what the numbers meant in terms of a physical measurement. People would give me a short summary of it but I still failed to completely understand until I dug down into what the system actually does from the time the RF energy is presented to the time it hands back a sequence of complex numbers. Yes, Marcus is correct not only because he is an expert/professional but because what he says aligns with what I have learned. The USRP does not provide dBm. All it provides is a complex vector of 32-bits (16-bit) or 16-bits (8-bit). This is then normalized and scaled between 0.0 and 1.0. That 16-bit I and Q are what the ADC outputs - although I know it is manipulated by the FPGA with filters and decimation but for what it is worth that is where they come from. The ADC and FPGA really have no idea what the original signal power/voltage level was before being amplified unless they calculate it. You *can* do your own calculation but then the accuracy is questionable unless you calibrate it. I also know the FPGA can control various amplifiers so it is not just a single component, therefore, that must be what makes it difficult to know for certain the accuracy. I think calibration is difficult because of lots of complex factors. I could only suspect this would be in relation to distortions and interactions between components with different gains, temperatures, and ... well that is what I think. But, if you used a loop back you could see why you get units less than makes sense. As in, you could see that the antenna had little effect at the same gain and such, or maybe the antenna is making a big difference. On Thu, Oct 19, 2017 at 6:00 PM, Kevin McGuire <kmcg3...@gmail.com> wrote: > I have an idea. Connect them directly to each other. Perhaps the device > has a built-in local loop back. This eliminates any cables or antennas as > the problem. > > On Thu, Oct 19, 2017 at 10:07 PM, Nirmala Soundararajan via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> Hi Konstantin and Mike, >> >> In fact I started with 0 gains for both transmitter and receiver with >> different amplitudes of input signal. The received power is always in the >> range of -80 dbm to -100 dbm. >> >> I am not sure how to say that a certain received power (in dbm) 'is >> acceptable' when given an input signal (that evaluates approx to 0 dbm in >> fft) indoors when the transmitting and receiving antenna are very close say >> just 0.5 meters apart for a carrier frequency of around 800 MHz. >> >> regards >> >> Nirmala >> >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >> >
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