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Testing of DBBC power measurement device reveals that it has a %red%logarithmic output response!!%black%.
%red%Update:13/6/2011: Logarithmic response verified, 37 mV/dB according to data sheet %black%Attach:HMC610LP4-datasheet.pdf
Presumably the DC output from this chip is sampled by an ADC. Would not think it likely that this (ADC) has significantly non linear behaviour. Conclude that the numbers provided by DBBC are proportional to power in dB
See attached spreadsheet and notes below;
Attach:linearity-test-jun-2011.xls
Attach:dbbc-linearity.jpg