In an article, I see that it says the spectral level (in Watts/Hz) of a pulse with pulse repetition frequency f (in Hzz) and pulse duration t (in seconds) and pulse peak power P (in Watts) is given as below.

spectral_level (Watts/Hz) = f.P.t^2 

Then, the article says that we convert the spectral level to dBm/kHz as below where F is f in kHz and T is t in microsecond. I know that dBm is 10log(power in milliwatts / 1 milliwatt), but I'm not sure how the term -60 is added in this equation.

spectral_level(dBm/kHz) = P(dBm) + 10log(F) + 20log(T) - 60

The -60 is compensating for T being measured in μs rather the s of your first equation.

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