[USRP-users] interpretation of received signal

mleech at ripnet.com mleech at ripnet.com
Wed Oct 18 12:41:50 EDT 2017


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 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?
> 
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
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