VE3KL Session  5
 Noise Figure of Multi-Stage (Cascaded) Systems   This session focuses on measuring the Noise Figure of cascaded systems. Here we will measure the Noise Figure of the 3 dB attenuator and a 10 dB attenuator that you built  and measured in previous sessions. A good discussion of multistage systems can be found in Chapter 2 of the  Agilent Application Note-57-1 The basic formula for cascaded systems is: (1)   Fsys = F1 + (F2-1)/G1 [not dB] In the above formula  Fsys is the Noise Factor of the complete system as measured. The system includes the attenuator cascaded with the Dongle.  F1 = the Noise Factor of the  attenuator under test. F2 is the Noise Factor of the Dongle. G1= 1/L, the gain of the attenuator. According to the above formula, if the Noise Factor of an attenuator is equal to its loss L and if its gain equals 1/L, then: (2) Fsys = L*F2........ Fsys(dB) = L(dB) + F2(dB).   Let’s see if this addition occurs. The measurement process to follow is: (a) Set up the noise test set and measure the Dongle NF (F2) at a couple of frequencies:  250 MHz and 350 MHz or other frequencies of interest.  Make sure to avoid harmonics of the crystal controlled oscillator in the Dongle. (28.8 MHz).  Include this new measurement and date on your data collection NF spreadsheet. (b) Repeat the above measurements with a 3 dB and then a 10 dB attenuator  cascaded to the the Dongle. ( c) Check to see if difference in the measurements between (a) and (b) is  3 dB with the 3 dB attenuator and 10 dB with the 10 dB attenuator according to (2) above.  
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