I don't understand how an antenna factor figures in a noise factor calculation. As I understand it, noise factor or figure is defined in the following basic equation:
Noise power (dBm) = k T B F, where: k = Boltzmann's constant which is the ideal gas law constant divided by Avogadro's number T = absolute temperature (Kelvin scale) B = bandwidth in Hz F = noise figure or factor F = 1 is the limiting case (can be approached but not achieved). At room temperature: Noise power (dBm) = -174 dBm + B (Hz) + F (dB) For instance, the noise floor of the HP8566 and its kindred analyzers was -135 dBm in a 10 Hz bandwidth. That computes to a 29 dB noise figure. But if you read the fine print, that is with full video averaging. If you eliminated video averaging, which you need to do to make a peak measurement, the noise floor can rise as much as 12 dB, so the effective noise figure for a peak EMI measurement can be as high as 41 dB with that family of equipment. If you have a cascaded measurement system, such as a preamp in front of a receiver, the following equation (numeric, not logarithmic) predicts the measurement system nose floor: F system = F preamp + F rcvr/Gain preamp I did not render this equation exactly, because it might be hard to read in e-mail, but it is more than precise enough for this calculation. Let's apply this equation using your preamp and the HP 8566: F preamp = 4 dB = 2.5 numeric Gain preamp = 20 dB = 100 numeric F rcvr = 41 dB = 12,600 numeric Then, F sys (numeric) = 2.5 + 12,600/100 = 128.5 F sys dB = 21 dB Even if you used full video averaging, to get the most sensitivity, figuring that you were only looking at narrowband signals in this frequency range, the rcvr noise figure is still 29 dB, or 794 numeric, and F sys, average = 2.5 + 794/100 = 10.4 numeric F sys, average dB = 10 dB In either case, the receiver noise figure dominates and CANNOT be neglected. Incidentally, if your receiver happened to be the HP 8566 these calculations cannot be used, because above 2.2 GHz, the analyzer uses harmonic mixing, which further deteriorates the noise figure from the values used above. Based on the cascaded system noise figure relationship, the preamp noise figure can only be assumed to dominate when the preamp gain vastly exceeds the receiver noise figure. Any attenuation in front of the preamp simply increases the system noise figure. I don't think you can apply the concept of antenna factor to calculating a noise figure directly. The way I do it is to calculate what my adjusted noise floor needs to be by subtracting the antenna factor from the limit, allowing for a reasonable signal to noise ratio (typically 6 dB) and then looking at my receiver noise floor relative to that. Then I can calculate how much if any pre-amplification I need. Ken Javor > From: [email protected] > Reply-To: [email protected] > Date: Thu, 10 Apr 2003 08:38:55 -0400 > To: [email protected] > 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 > 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
