You have the noise figure of the filter/preamp combination correct at 5 dB.
However, you may not be able to ignore the contribution of the spectrum analyzer. Some (many? most?) have rather high noise figures, well over 20 dB. Your amplifier gain is effectively only 19 dB, so the spectrum analyzer contribution may not be negligible. The noise power of a 50 ohm resistor in a 1 Hz bandwidth is -173 dBm. The noise power scales directly with bandwidth, so in (for example) 1 kHz bandwidth, the noise power would be -143 dBm. Look at the noise floor of your spectrum analyzer, using its noise power function, and see high much higher you are than the noise power of a 50 ohm resistor in the selected bandwidth. This will tell you the noise figure of the spectrum analyzer. The antenna is another matter. The antenna factor is a transducer factor. It is used to tell you the magnitude of the signal produced given a certain field strength. There is no noise figure as such for the antenna. Since the antenna is looking at a room temperature ambient (i.e., not looking to space), its noise is that of a 50 ohm resistor at room temperature (assuming the antenna impedance is 50 ohms). The signal level is determined >from the field strength and the antenna factor. So you will have a certain signal-to-noise ratio at the output of the antenna, given a bandwidth. The signal-to-noise ratio will be worsened dB for dB by the noise figure of the filter/amplifier/spectrum analyzer system that follows. Don Borowski Schweitzer Engineering Labs Pullman, WA [email protected]@majordomo.ieee.org on 04/10/2003 05:38:55 AM Please respond to [email protected] Sent by: [email protected] To: [email protected] cc: Subject: Noise Factor I need verification that I am computing the noise factor (NF) of my emissions receiver correctly. The system will measure emissions in the 1-10 GHz band and will use a horn antenna followed by a high pass filter followed by a preamp feeding a spectrum analyzer. I can ignore the NF of the spectrum analyzer. since gain of the preamp is 20 dB, so the total NF will be set by the other three elements. The NF of the preamp is 4 dB, the insertion loss of the filter is 1 dB and the antenna factor of the horn is 39 dB at 10 GHz. I understand that the NF of the filter will be equal to the insertion loss, 1 dB. And the NF of the antenna will be equal to its insertion loss (antenna factor), 39 dB at 10 GHz. Therefore the total NF of the three elements will be the sum of their NFs - i.e., 39 + 1 + 4 = 44 dB. It seems that I don't have to be overly concerned about the NF when I select a preamp, since the only way of reducing the NF significantly would be to improve the antenna factor. Are my calculations correct? This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
