data={Table[Sin[2 Pi 697 t]+Sin[2 Pi 1209 t]+RandomReal[{-0.02,0.02}],{t,0.,0.1,1/8000.}],Table[Sin[2 Pi 852 t]+Sin[2 Pi 1336 t]+RandomReal[{-0.02,0.02}],{t,0.,0.1,1/8000.}]};
Periodogram[data]