 to this end the term noise 10.11.5 Noise factor measurement figure is used, which is the noise factor expressed in decibels, as in Equation 10.41. The most convenient method of measurement of noise factor is using a noise diode, as in Figure 10.18, as this method does not require the bandwidth of the system to be known. The system Noise figure = 10 log (Signal noise ratio at input ) bandwidth is very difficult to measure to any degree of accuracy. (10.41) A noise diode produces mainly shot noise, which may be adjusted - 10 log ( Signal noise ratio at output by varying the input current to it. It produces a broad spectrum of frequencies up to at least 5MHz. The input resistance of the active It may be seen from this equation that the noise figure is the network is Rg, the output power is P o' and the noise factor is F. The number of decibels by which the signal to noise ratio is degraded by an amplifier. Active Noise Noise network 10.11.2 Noise factor in terms of equivalent diode Attenuator power source meter noise resistance As has already been stated, for calculation purposes the shot noise produced by valves and transistor devices may be represented by the the thermal noise produced by an "equivalent resistor". Figure 10.18 Method of measurement of noise factor The relationship between the equivalent resistance and the noise factor is: the higher the equivalent resistance the higher the noise factor or noisiness of a device. For triode valve circuits the relation- following method is used. First measure P when the diode current ship is given by Equation 10.42, where F is the noise factor, Rcy is and the attenuation are zero. The diode current is then adjusted to the equivalent resistance, and R is the resistance in series with the give the same power reading with an attenuation of 3dB (a doubling input supply. of the output power). By using the same meter reading any discrep- ancies caused by the meter are avoided. Equation 10.47 can be (10.42) obtained, and this reduces to Equation 10.49 if Equation 10.48 hol