On Fri, Jul 06, 2012 at 01:24:12PM -0400, mle...@ripnet.com wrote: > uhd_fft *is not in any way* showing real-world dBm values -- it's merely > showing FFT magnitude values in dB -- but it's dB relative to a > pretty-arbitrary scaling. > > Different sample rates result from different decimation values in the FPGA, > which amounts to different amounts of *filtering* between the ADC and the > towards-the-host data stream. > > In order to get actual dBm values, you need to calibrate, at various > frequencies, adn if you're going to be running at various sample rates, you'll > have to calibrate for that sample-rate as well. And the calibration will vary > from card-to-card due to inherent variability in analog gain components. > > The reason your $10K spectrum analyser shows actual dBm values is that it has > been calibrated in the factory prior to shipping to you, and it has to be > recalibrated every couple of years.
Marcus, you're way too helpful. One could think you're Canadian :) Eric, your question pops up a lot here and is thus on the gnuradio.org FAQ: http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal The wiki gets a lot of flak for being crap, but there's actually a good amount of useful information on there. Please check it out. MB -- Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin Braun Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT -- University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association
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