0dBm is the maximum power that is recommended to prevent damage to the
hardware. But the actual response of the entire signal processing
chain, including variable-gain elements, the ADCs, and such, cannot
simply be "modeled" in such a simple manner.
In order to map the digital-domain results back through the chain into
"what is the antenna port actually seeing", you MUST do a calibration
exercise, using signal sources of known power. You have to do this
exercise over your frequency/gain/sample-rate settings of interest.
Real-world RF hardware does not have a perfectly-uniform response over
its operating range, neither is it always perfectly-linear.
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@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@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?
>
> _______________________________________________
> USRP-users mailing list
> USRP-users@lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com [1]
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