[USRP-users] interpretation of received signal

Kevin McGuire kmcg3413 at gmail.com
Sat Oct 21 11:59:33 EDT 2017


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 <kmcg3413 at 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 at 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
>>
>> _______________________________________________
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>>
>>
>
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