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 Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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