can anyone explain function mix in harmonic simulation in RF

can anyone explain function mix in harmonic simulation in RFDE?
I am confused by expression "IF_d=mix(Spectrum,{-1,1},Mix}"
can anyone explain the mean of it? What is 1 and -1, 1 means base frequency? how about -1? also how about spectrum? Is there a good tutorial about harmonic simulation on RFDE or ADS? If do, can you give me a link, please?
thanks a lot

If you have nonlinear circuit or a linear time variant circuit and you excite it with, for example, two tones f1 and f2. The output will contain frequencies in the form n.f1±m.f2. Now in ADS, to specify a certain frequency, you use mix(signal_name, {n,m}). mix(Vout, {1,-1}) will result in the output frequency component at f1-f2 and so on so forth

Added after 2 minutes:

For a good tutorial about ADS, google "ADSexp.ppt" It's a 200-slide presentation that covers all the simulators in ADS

as mbyoussef said
let us say , u have entered 2 frequncuies F1 , F2 in the HB engine , and in the mix functon u typed mix {1,1}

this meanu u get F1+F2 ,
and if u typed mix {-1,1} you get F2-F1

general care
mix {m,n} get mF1+nF2

khouly

Got it, thanks a lot