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Re: [Discuss-gnuradio] Signal power w.r.t the amplitude of sample sent t

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Signal power w.r.t the amplitude of sample sent to USRP?
Date: Mon, 01 Aug 2011 21:45:49 -0400
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On 07/31/2011 12:23 PM, Marcus M wrote:
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?

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

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 Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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