Dear Fikrat,
the physical power depends on your waveform. Generally, the power is
always $P(t) = U(t)\cdot I(t)$, which, thanks to Ohm's law ($U=R\cdot
I\rightarrow I = \frac UR$) is $P(t) = \frac{U^2(t)}{R}$. As you might
know from the basics of electrical engineering, one can represent
harmonic functions such as a voltage sine generated by a function
generator as complex number with magnitude $A$ and phase $\varphi$ ,
i.e. as ${\underline U}(t) = A \cdot e^{j\varphi}$; notice that for the
power consideration, you can omit the $e^{j\varphi}$, it always having
the magnitude 1. Use your math basics to find the average power by
integrating over a period. For harmonic signals you'll find that if you
set $A=U_{eff}=\frac 1{\sqrt 2} U_{max}$.
> 2. Moreover, am I supposed to connect the signal generator directly
> to the TX/RX port?
If you can make sure your signal generator doesn't push more than -15dBm
into the USRP, then sure. Otherwise, use a calibrated attenuator and
adjust your measurement.
I don't know which signal generator you use, but most RF signal
generators I know accept both, either voltage/amplitude or power as setting.
Also make sure your signal generator is set to 50Ohm impedance, if that
is adjustable.
> 3. Finally, if that was the case, how do I observe the digital power
> on the USRP n210?
Well, the magnitude of the imaginary and real part of the digital
samples are proportional to the voltage on the I and Q input of the
ADC... Soooo: Digital power is just I²+Q² = |s|², the magnitude squared.
All in all, these are pretty basic questions; we're constantly working
on making GNU Radio more beginner-friendly, but to do that, we might at
times need to refer people to adequate literature.
So: May I ask what background you come from?
Best regards,
Marcus
PS: could you also try to keep the [email protected] mailing list
at least in CC:? It's always better to ask the whole list instead of
individual people. I might not always have the time...
On 22.03.2016 17:48, Fikrat Al-Kazimi wrote:
> Dear Marcus,
>
> Thanks a lot for your reply. I'm really grateful!
>
> I have a few more inquiries I wish to get your help with if you don't
> mind. I just got access to a function generator and I plan on
> generating my injected signal using it.
>
> 1. The physical power of the injected signal is measured as Vmax^2 / 2R ?
> 2. Moreover, am I supposed to connect the signal generator directly
> to the TX/RX port?
> 3. Finally, if that was the case, how do I observe the digital power
> on the USRP n210?
>
> I apologize for the basic questions, but I'm fairly new to this field.
>
> Best regards,
> Fikrat
>
> On Tue, Mar 22, 2016 at 12:24 AM, Marcus Müller
> <[email protected] <mailto:[email protected]>> wrote:
>
> Dear Fikrat,
>
> Feed in a known power, note down the digital power, repeat for
> another known power.
> You'll get three input power->digital power mappings.
>
> Now, assume the power transfer function is a linear one:
>
> $P_{digital} = G\cdot P_{analog} + P_{noise}$
>
> With the two $(P_{digital}, P_{analog})$ measurements you can
> simply deduce the slope $G$ of the above function; simple math,
> subtract the equations:
>
> $P_{digital,1}-P_{digital,2}=(G\cdot P_{analog,1} +
> P_{noise})-(G\cdot P_{analog,2} +
> P_{noise})=G(P_{analog,1}-P_{analog,2})$
>
> and find $G$ and the offset $P_{noise}$.
>
> Repeat with a few other known powers to make sure you're in the
> linear region.
>
> Whatever you do, never feed in more than -15dBm into your device!
>
> Best regards,
> Marcus
>
>
> On 21.03.2016 23:05, Fikrat Al-Kazimi wrote:
>> Hi guys,
>>
>> I hope you're all doing well.
>>
>> I'm been searching a lot and I read that if I want to measure the
>> absolute power ( in W or dBm ) using the usrp_spectrum_sense.py,
>> then I must calibrate the USRP by injecting a signal of known
>> physical power.
>>
>> Can someone please walk me through the calibration steps? How can
>> I accomplish this and what do I edit in the code after
>> calibration is complete to help me sense the absolute power
>> instead of power_dB?
>>
>> Thank you for your help!
>>
>>
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>
>
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