# Signal power w.r.t the amplitude of sample sent to USRP?

**URL:** <https://www.ruby-forum.com/t/signal-power-w-r-t-the-amplitude-of-sample-sent-to-usrp/210126>\
**Category:** GNU Radio\
**Created:** [July 31, 2011, 6:24pm UTC](https://www.ruby-forum.com/t/signal-power-w-r-t-the-amplitude-of-sample-sent-to-usrp/210126 "2011-07-31T18:24:18Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\
**Post date:** [July 31, 2011, 6:24pm UTC](https://www.ruby-forum.com/t/signal-power-w-r-t-the-amplitude-of-sample-sent-to-usrp/210126/1 "2011-07-31T18:24:18Z")

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Hi,  
How can we determine the transmitted signal power with respect to the  
amplitude value of the sample sent to the USRP and also how can we  
measure  
the power of the received signal at the USRP?

Thanks

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**Author:** ![Marcus\_M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@Marcus\_M](https://www.ruby-forum.com/u/Marcus_M)\
**Post date:** [October 9, 2011, 1:32pm UTC](https://www.ruby-forum.com/t/signal-power-w-r-t-the-amplitude-of-sample-sent-to-usrp/210126/2 "2011-10-09T13:32:43Z")

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Hi Marcus,  
I saw your question “Signal power w.r.t the amplitude of sample sent to  
USRP?” Have u figured it out? If so, please tell me.  
Thank u very much!

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**Author:** ![Marcus\_M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@Marcus\_M](https://www.ruby-forum.com/u/Marcus_M)\
**Post date:** [August 2, 2011, 3:46am UTC](https://www.ruby-forum.com/t/signal-power-w-r-t-the-amplitude-of-sample-sent-to-usrp/210126/3 "2011-08-02T03:46:44Z")

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On 07/31/2011 12:23 PM, Marcus M wrote:

> Hi,  
> How can we determine the transmitted signal power with respect to the  
> amplitude value of the sample sent to the USRP and also how can we  
> measure the power of the received signal at the USRP?
> 
> Thanks  
> In general, at least under UHD, an envelope value of 1.0 delivers  
> maximum baseband power to the transmit up-converter, whose actual  
> power output is dependant on the TX gain setting, and the delivered  
> output power of the amplifier at any given frequency at that  
> gain setting. Actual measured RMS power on a precision RF power  
> measurement instrument will also depend very much on the  
> specifics of the waveform being transmitted.

There can be batch-to-batch variability in RF power amplifiers of 1dB or  
more, and RF power amplifiers also have frequency-dependant  
gain.

So, it’s hard to give you a straightforward numerical answer. The only  
thing you can do is measure and calibrate, that is also true in the  
receive direction as well.

Received signal power is _proportional_ to AVG((I**2) + (Q**2)). By  
injecting calibrated signal sources at various gain settings, you can  
determine what the proportionality will be for the specific device  
under test. Due, again, to batch-to-batch variability in the gain of  
various elements in the analog portion of the system, you can’t get  
exact values that are reproducible across multiple instances of  
the same device. This is why laboratory instruments for measuring RF  
power are calibrated at the factory, and must be recalibrated  
on a regular basis thereafter (typically every couple of years).

–  
Marcus L.  
Principal Investigator  
Shirleys Bay Radio Astronomy Consortium

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