Not wanting to sound like too much of a dinosaur, but back in the 1960's and early 70's, emission data was acquired by slowly tuning a receiver across the frequency range while listening to headphones and watching a meter. You would record the frequency and amplitude at several frequencies per octave, or more if you were feeling charitable, and also record any uniquely "interesting" emission peaks. When you were finished acquisition, you might have several pages of tabular data (perhaps 75-100 frequency / amplitude data pairs).
Then, you could go back to your desk and break out your correction data, a set of graphs for each of the parameters of cable loss, gain flatness, bandwidth variation, pre-amplifier gain, antenna factors and maybe filter attenuation (nothing was flat in those days). Typically, you might have 5 of these variables, so that meant writing down (5 x 75) 375 numbers on your data sheet. Then, you added up the raw amplitude and all factors for each frequency and entered that corrected amplitude. Next, you used the limit of the standard to visually interpolate and enter a limit value for each measurement frequency. And lastly, you compared the amplitude value to the limit value, and entered the over-limit values if necessary. And, if this was a very formal test, you often had to break out that old pre-printed multi-cycle logarithmic graph paper and plot the corrected data and the limit. Data correction took as long, or longer, than data acquisition. I dread to think of the error budget! The first automation that I encountered was a box using a variable speed motor and a flexible speedometer cable connected to the tuning knob of a receiver. You would set a "reasonable" scanning speed and connect an analog plotter to plot detector amplitude versus scan progress (or the Volts/MHz receiver output if you had a very advanced receiver). In this case, you would make a master plot with a derived limit (as Ken Javor described), then make photocopies of that master for the working measurement plots. We have come a very long way in 40 years. Ed Price WB6WSN Chula Vista, CA USA From: Ken Javor [mailto:[email protected]] Sent: Tuesday, February 23, 2016 5:13 PM To: [email protected] Subject: Re: [PSES] Spectrum analyzer and noise floor I agree. A modern machine adjusts the raw data or noise floor for any and all transducer/amplifier/attenuator factors, and if the transducer factor is not flat, neither will be the adjusted noise floor. The only reason we do things this way is an "embarrassment of riches" in processing power. Absent a digital controller, the sane way to take data would be to adjust the limit for the transducer factors and arrive at an adjusted limit in terms of dBuV, or dBm. Consider the number of computations involved in adjusting a thousand data points across a screen for the transducer factors, vs. a simple flat or log-linear limit and transducer factors that only need be reentered at the next frequency at which they have changed by some set amount from the last frequency, such as 1 dB, or 0.5 dB. The adjusted limit represents orders of magnitude less computation, plus one can reverse engineer an adjusted limit if one knows the transducers in use, whereas adding factors to a signal above noise renders this impossible. Ken Javor Phone: (256) 650-5261 _____ From: "McDiarmid, Ralph" <[email protected]> Reply-To: "[email protected]" <[email protected]> Date: Tue, 23 Feb 2016 16:42:47 -0800 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 [email protected] <mailto:[email protected]> <mailto:[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> <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) <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]>
