Ralph:
That's a workable explanation for the variation from flatness. Those who spend a lot of their time acquiring emission data get very accustomed to the shape of the corrected noise floor, and any variation from expected is a reason to stop the test and examine everything that is going on in the signal and the data processing paths. However, there is a problem with your statement #2. A pre-amplifier is chosen primarily for it having a first stage which has a Noise Figure lower than that of the input stage of your spectrum analyzer. You could well have a pre-amp with a small-signal gain of 35 dB and a noise figure of 1 dB, but if you put that pre-amp onto a spectrum analyzer that has a noise floor of -15 dBuV, you will not be able to see emissions of -50dBuV at the input of that pre-amp. Regardless of pre-amp gain, the Minimum Discernable Signal through the pre-amp and SA system will be limited by the pre-amp's 1st stage Noise Figure. You can prove this, without any math, just by getting a signal generator that puts out say 60 dBuV. Now add another 60 or so dB attenuation on the signal generator output, the idea to be able to apply a very smal signal to your SA. Now, put the pre-amp into the circuit and see what happens to the SA amplitude. Then, begin reducing the signal generator amplitude while noting the SA response. This will show you where the Minimum Discernable Signal fades into the pre-amp Noise Figure. Perhaps Ken or Ghery can give us a more elegant mathematical description of this effect. Ed Price WB6WSN Chula Vista, CA USA From: McDiarmid, Ralph [mailto:[email protected]] Sent: Tuesday, February 23, 2016 4:43 PM To: [email protected] Subject: Re: [PSES] Spectrum analyzer and noise floor I would like to explain to a colleague why the noise floor on a SA does not look flat as it sweeps across a given frequency range after antenna factors, cable factors, external gain and external attenuation are programmed into its display function. I think it breaks down to these fundamental points: 1. the SA receiver has noise in its attenuator, mixer and filter circuits (say -80 dBm, and maybe flat within a limited frequency range) 2. the external amplifier has some noise too, but its gain lowers the noise floor created by #1 (also flat within a limited frequency range) 3. the cables have losses which are frequency dependant, and those can be entered as loss factors into the SA (shapes the noise floor a little and those losses raise the noise floor) 4. the antenna has a gain which is frequency dependant with several dB of hills and valleys across its usable frequency range (that really shapes the noise floor more than 1, 2 or 3 above) 5. noise floor shape caused by #4 is the mirror image of the antenna factor vs frequency Is that a decent summary? . Ralph McDiarmid Compliance Engineering Residential/Commercial Solar Business Schneider Electric D +1 (604) 422 2622 x62622 E <mailto:[email protected]> [email protected] 3700 Gilmore Way Burnaby BC Canada - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
