Ed, As one who has done all that manually, I can second your comments. But it's important not to bypass the QP adaptor.
The QP adaptor ALSO provides correct bandwidth (200 Hz, 9 KHz, 120 KHz) and detector time constants for each frequency range. This means that the adaptor has to be used during all record testing BUT the QP detector function is only selected when a QP reading must be taken. One common error made by folks unaccustomed to doing manual readings is leaving the analyzer in logarithmic mode for QP or average measurements. It's important to first switch to linear mode before doing a QP or video averaging reading. The analyzer might switch units (uV or volts) instead of DB when you go to linear mode; this can be overcome by selecting dBuV _after_ switching to linear mode. Another error is sweeping at too fast a rate for the QP adaptor bandwidth selected. The QP adaptor is narrower than the analyzer -- it has to be, to get the right shape factor -- and this means that the "automatic" sweep time setting will be too fast. The newer QP adaptors have the correct analyzer BW and sweep times printed on the front panel. When the QP detector is selected, the sweep must be even more drastically slowed down (time constants, remember?). When using the QP detector, a 100 KHz sweep might take a couple of minutes above 30 MHz. You can imagine how long it would take below! For this reason, a practical QP sweep will examine only the peaks close to the limit or above. The next obstacle is determining the actual center of the energy bandwidth. SOme emissions will have a rather high peak to average presentation at an edge, while being close to a carrier at another (combined FM and AM of the emission, for example). Different operators may have different ways to resolve this, Some folks set a narrow limit to the sweep, and accept a slightly slower reading from the QP detector. I prefer to find the amplitude peak, switch to zero span (receiver mode) then (in QP mode) manually tune the analyzer through the signal for max QP reading. When you are using zero span, you can make the sweep as fast as you want it to be. But people who know what they are doing will end up with the same results no matter what method they use. Video averaging is a rather different matter. There is no average detector in this analyzer. One repeatedly measures the signal and averages the value present on the screen. Still, this does require we take enough samples for a stable reading, and that the analyzer not drift off frequency when the average is being taken. However, if the emission is drifting, that's OK! It also helps to set reference level so the signal being examined is as close as possible to the top of the screen, but not higher. This assures that peaks get measured without clipping. Finally, I can say that doing ALL testing in manual mode, as at a former employer we did, that you will end up replacing some of the switches on the analyzer annually, maybe more often. A skilled technician can actually compete with computer control in taking readings, but he hits those buttons several times per second when he's doing so, and the force of rapid inputs is just more than they can take. Cortland Ed Price write: I have an HP-8566B, configured as an HP-8571A Receiver system. Since I use mine for Mil testing, I didn't buy the Quasi-peak adapter option. However, the QP adapter is used ONLY to obtain amplitudes in compliance with CISPR bandwidths and QP detector time constants. An IF pickoff (at 21MHz?) is made at the spectrum analyzer, and is routed to the QP adapter, which applies the appropriate CISPR bandwidth and the QP detection. The SA handles the Peak and Average detection all by itself. So, the answer to your question "Can I do it all from the front panel?" is a Yes. With a qualifying "But it ain't easy!" Let's take the case of when you want to scan a frequency range using QP detection. First, set the SA to the factory defaults. Then set the freq scan desired. Then set the QP adapter to the proper CISPR BW. Now, let the SA do the scan and command a plot (ont! o a GPIB plotter). You now have an amplitude vs frequency plot. Unfortunately, it is not corrected for antenna factor nor cable loss nor external attenuators nor external pre-amps. Also, it is PEAK amplitude, not QP. Next, go to manual tuning on the SA, and tune to the SA to the first peak of interest. Force manual on the SA BW, and make sure you are at a slightly larger RBW than the CISPR requirement. Force manual on Span and retune to center the emission. Now go to Zero Hz Span. Your SA has just become a receiver, tuned to the emission frequency with nearly the right RBW. You are now seeing an amplitude vs time view on the SA display. Now look at the QP adapter and adjust attenuation on the adapter and SA to get a QP amplitude display on the QP adapter. Hand annotate the plot with the QP emission level. Repeat for all other emission freqs of interest. When you get finished, you will have an annotated plot (or maybe a table of freq/QP amplitude pairs). Now, get into the Wayback Machine and set it for 1975. Get copies of your antenna calibration data, your cable loss data, and any other frequency dependent correction factors for filters, pre-amps and other stuff that may have been in your signal path ahead of the SA. At each frequency, add the SA amplitude, AF, cable loss and any other stuff, to arrive at a corrected field strength in dBuV/m. (Conducted emission technique works the same.) You may have a lot of freqs, so you are allowed to cheat a bit by putting this on a spreadsheet to automate the arithmetic. This certainly isn't a desirable way to go, but it will yield progress for you when the computer dies. Considering the costs involved, I think that it would be a good idea to have a spare computer, software pre-loaded, just sitting on the shelf as a back-up. I still use the HP-85869A software, which runs on archaic HP series 360 controllers. I have a whole set of 360 series hardware as a backup against computer disaster. Assuming that you have the newer PC version software, extra computer hardware is even cheaper. I'm a bit concerned about the Average detection function. In an EMI receiver, the average detector function is achieved in an analog circuit with component values selected for a specific charge/discharge time constant. From what I understand of the HP SA, the detection is done with Peak values, and then an average of the set of peaks is performed. They call this Video Averaging. I don't know if these are equivalent techniques. Maybe your software produces the Average detector function (in some magical way, huh?). Finally, I do know that if I command my SA to go to manual mode after a Peak scan, and I further command a remeasure over a small frequency span using Average detection, the time for this Average segment is much longer than for an entire Peak scan. Ths SA firmware is really busy doing SOMETHING. A scan of a large frequency range, using what my HP software calls Average detector function, would be very, very slow. Ed ------------------------ From: Steve Kuiper <[email protected]> Subject: Conducted Emissions, average method. Date: Thu, 24 Sep 1998 14:36:37 -0700 To: EMC-PSTC <[email protected]> > Dear esteemed colleagues, > > How would one perform conducted emissions testing manually using the front > panel keys on a HP 8568B and 85650A Spec. Analyzer and Adapter? Our lab > recently lost three days to down-time because of EMC software and hardware > problems encountered with one of our test computers. For backup purposes it > would be helpful to have some procedure in writing detailing the manual > steps and spec. analyzer settings necessary to perform conducted emissions > for peak, quasi-peak and more importantly AVERAGE> > > It is my understanding that the instrument settings and values for > controlling RES-BW, VBW, SWP-RATE, and SPAN greatly differ between a PEAK > measurement vs. AVERAGE. > > Any thoughts or suggestions would be appreciated. > > Kind regards, > > Steve Kuiper > [email protected] -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 09/25/1998 Time: 13:55:31 -------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). 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