[USRP-users] interpretation of received signal

Nirmala Soundararajan nsound1 at students.towson.edu
Wed Oct 18 13:47:16 EDT 2017


Yeah got it!  But don't you think the tool has those capabilities to
consider the entire chain and then display the output?  Since I don't have
actual spectrum analyzer in the lab I am forced to use the software
spectrum analyzer.

The initial phase is to first confirm that the transmit and receive are
proper enough to comprehend the results. I can send a table of input
voltages and gains that I set and the results that I observed. I would
atleast expect that given the same numbers as input , the results do not
vary beyond a certain margin. That way I can declare /  set that my channel
of interest has passed the loop back test of transmission and reception!

I am only confused that the results do not match the expected values as per
path loss, power transmitted and gain equation or it could be that I am not
looking the right way!

regards

Nirmala


>
>
>
>
> On 2017-10-18 12:29, Nirmala Soundararajan via USRP-users wrote:
>
> Hi Marcus,
>
> I made a simple model using Matlab Simulink and used spectrum analyser
> block to observe  the transmitted and received waveform. Since 0 dbm was
> the maximum input power, I just calculated (input volatge)^2 / 2. It comes
> to 0.045V.  I specified block parameters and gave different values.
> Actually with 0 gain of transmitter and receiver I was getting -97  to
> -105 dbm for the same input voltage 45mV.
>
> regards
>
> Nirmala
>
> On Wed, Oct 18, 2017 at 3:03 AM, Neel Pandeya via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
>> Also note that at 0 dBm, you're at the maximum safe input power, so your
>> received signal might be saturated.
>>
>> --Neel Pandeya
>>
>>
>> On Oct 17, 2017 22:30, "Marcus D. Leech via USRP-users" <
>> usrp-users at lists.ettus.com> wrote:
>>
>> On 10/18/2017 12:08 AM, Nirmala Soundararajan via USRP-users wrote:
>>
>>> Hi all,
>>>
>>> I am trying to transmit and receive a simple 10 kHz tone using a single
>>> USRP B205mini-i. The input power is 0 dbm ( The amplitude of 10kHz tone
>>> being 45mV). The transmitting and receiver antenna are same omnidirectional
>>> type each having 3 dbi gain. I set the transmitter gain as 8 and receiver
>>> gain as 70. The carrier frequency is around 800 MHz. The path loss comes to
>>> around 20 db considering the fact that transmitting and receiving antenna
>>> are just a foot apart. ( I took far field and did not consider near field)
>>>
>>> The received spectrum shows -26 dbm. How should I interpret these
>>> results?
>>>
>>> regards
>>>
>>> Nirmala
>>>
>>> How did you get -26dBm?
>>
>> Unless you have painstakingly *calibrated* your USRP B205mini, and wrote
>> code that converts the received data into dBm, given your carefully-derived
>>   calibration tables, an FFT will simply show received power relative to
>> the mathematical maximum in the system.
>>
>> Further, given that modulating signals in the SDR world are purely
>> digital number, how did you derive a figure of 45mV of modulation?
>>
>>
>>
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