CNR and SNR

Carrier to Noise Ratio (CNR) is a measure of the received carrier strength relative to the strength of the received noise.

CNR[dB] = 10 log10(Pc/Pn)
where
Pc - incoming carrier strength in microwatts
Pn - noise level in microwatts


Signal to Noise Ratio (SNR), is a measure of signal strength relative to background noise.

SNR[dB] = 20 log10(Vs/Vn)
where
Vs - incoming signal strength in microvolts
Vn - noise level in microvolts

Why CNR use 10log10(xxx) but SNR use 20log10(xxx) ?

Because in this case one uses ratio of powers, and other ratio of voltages.