the noise power level. Many will recognize this as Boltzmann's constant expressed in dBW. We can calculate the thermal noise level of a perfect receiver operating at room tem- perature using the following formula: Pn = -228.6 dBW/Hz + 10 log 290 (K) = -204 dBW/Hz. (9.9) The value, 290 K (kelvins), is room temperature, or about 17°C or 68°F. Noise figure simply tells us how much noise has been added to a signal while passing through a device in question. Noise figure (dB) is the difference in signal-to-noise ratio between the input to the device and the output of that same device. We can convert noise figure to noise temperature in kelvins with the following formula: NFdb = 10 log(1 +Te/290) (9.10) where Te is the effective noise temperature of a device. Suppose the noise figure of a device is 3 dB. What is the noise temperature? 3 dB = 10 log(1 + Tc/290), 0.3 = log(1 + Tc/290), 1.995 = 1 + Te/290. 9.2 RADIO SYSTEMS We round 1.995 to 2; thus - 1 = Te/290, Te = 290 K. The thermal noise power level of a device operating at room temperature is Pn -204 dBW/Hz + NFdb + 10 log BWHz, (9.11) where BW is the bandwidth of the device in Hz. Example. A microwave receiver has a noise figure of 8 dB and its bandwidth is 10 MHz. What is the thermal noise level (sometimes called the thermal noise threshold)? Pn = -204 dBW/Hz + 8 dB + 10 log(10 x 106) = -204 dBW/Hz + 8 dB + 70 dB =-126 dBW. Calculation of Eb/No in Digital Radio Systems. In Section 3.2.1 signal-to-noise ratio (S/N) was introduced. S/N is widely used in analog transmission systems as one measure of signal quality. In digital systems the basic measure of transmission quality is BER. With digital radio links, we will introduce and employ the ratio Eb/No as a measure of signal quality. Given a certain modulation type, we can derive BER from an Eb/No curve. In words, Eb/No means energy per bit per noise spectral density ratio. No is simply the thermal noise in 1 Hz of bandwidth or No = -204 dBW/Hz + NFdB. (9.12) NF, as defined above, is the noise figure of the receiver 