[USRP-users] interpretation of received signal

Marcus D. Leech mleech at ripnet.com
Sat Oct 21 11:56:28 EDT 2017

On 10/21/2017 09:20 AM, Marcus Müller via USRP-users wrote:
> Hi Nirmala,
> I'm confused, so what *calibrated measurement device* did you use to 
> get "-80 dbm to -100 dbm"? As Marcus L has pointed out, the values you 
> get from the USRP are *not* relative to a physical unit.
> Please simply don't think the plots that *any* software gives you have 
> something to do with dBm. They don't, simple as that. You can MAKE 
> them do so, by converting digital amplitude to a physical meaning. For 
> that, you need to CALIBRATE first.
> Best regards,
> Marcus
Perhaps this is just confusion between dB and dBm.

dB == a unitless measure of ratios between voltages or powers (or, 
really anything that can be measured, like sound pressure)

            dB == 10log10(ratio)

dBm == a measure of RF power, relative to 1.0e-3 watts, that 0dBm == 
1.0e3 watts,  10dBm == 1.0e-2 watts,  20dBm == 1.0e-1 watts, etc

So, dB is a ratio, which means that it's taken relative to something.    
In Gnu Radio and SDR land, it is often scaled to somewhere around the 
maximum value of an ADC or whatever numeral system is in use.

This means that in order to derive dBm (a physical, real-world, unit) 
you have to have some way of turning those dB values (that are just 
relative to your numerical system) into real-world values. This is where 
having a source, or sources, with *precisely-known* power levels becomes 
exceedingly important.
You need to painstakingly take measurements over your entire operating 
parameter space (frequency, gain, sample-rate, filtering) to create a 
map that maps dB values into real-world measurements, like dBm.

> On 20.10.2017 00:07, Nirmala Soundararajan via USRP-users 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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>> USRP-users at lists.ettus.com
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