RF spectrum analyzer - Computing quasi-peak? - Page 1
Forum section: electronics / testgear

author: MarkL
author_rank: Unknown
timestamp: January 19, 2014, 08:06:12 pm
content: Does anyone know of a way to compute the quasi-peak using a spectrum analyzer that does not have a quasi-peak detector? The model used for quasi-peak detection is a damped mechanical meter movement, and the specifics of the model is defined by CSPR 16-1-1.I have an HP 8595E that does not have the quasi-peak option (opt 103), and I'm wondering if it's possible to implement a quasi-peak computation externally. It would obviously be slower (and maybe MUCH slower), but I'm not too concerned with time since it would only be used to make more precise measurements if peak detector levels were exceeded during EMC compliance pre-testing.Has anyone done this and/or know of an algorithmic implementation of the model? It seems that it should be possible, perhaps by stepping through zero-span chunks using "sample" detection mode, or sweeping the range many times also in "sample" detection mode, and then crunching on the data to produce a quasi-peak spectra.HP used to sell a quasi-peak retrofit kit (11946A) which consisted of a card and driver software, but of course that's nowhere to be found now, and no one is selling opt 103 parts separately (been watching for over a year). I'd like to not invest in another SA if at all possible. Thanks...

author: nctnico
author_rank: Super Contributor
timestamp: January 19, 2014, 08:35:15 pm
content: IMHO you don't need quari peak detection to solve EMC problems. You'll get a graph showing which frequencies are exceeding the maximum levels by X dB. You can measure those levels with your spectrum analyser and modify the circuit so those frequencies are X db lower and therefore meet the spec.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 19, 2014, 09:29:19 pm
content: Hi MarkL and the group,I have been down this path. I was looking for option 103 for my HP8595E spectrum analyzer. I was also looking for the HP85650A quasi peak detector for the HP8568A/B spectrum analyzer.I eventually found the HP85650A. The HP85650A contains the quasi-peak detector and the EMC IF filters (120 Hz, 9kHz and 120 kHz). The unit connects to the HP8568A with cables so the video and IF signals are routed through the HP85650A.The full schematic and theory of operation are in the HP85650A manual. They give the QP detector time constants as:In the real circuit there are electronic switches for changing the time constants for the three bands. I was mainly interested in the 150 kHz to 30 MHz band so I came up with this model, based on the component values in the HP schematics:If you know the shape of the EMC signal, you can apply that signal to this filter, and you can see what the (meter) reading would be. In this example the I am applying a 100us pulse at a repetition frequency of 60Hz. The output is 0.18VThis means that the QP detector would be 15dB below the peak.The QP detector output will always be greater or equal to the average value and less than or equal to the peak. The QP detector gives the largest attenuation for short duration, low repetition rate signals. If you know what the signal looks like in zero span, you can get an idea what the benefit of using a QP detector would be.You can download the attached LTspice model and play with the pulse width and frequency to see the effect of the QP detector.So you don't really need a QP detector.Regards,Jay_Diddy_B

author: MarkL
author_rank: Unknown
timestamp: January 20, 2014, 05:05:19 pm
content: Hi Jay_Diddy_B,Thanks very much for the great info on the 85650A. This really fills the gap between the CISPR spec and just pushing a "QP" button. I wish all manuals were written like these old HP manuals.I especially appreciate the extracted SPICE model.I would need to use the SA in peak detector mode, not sample mode as I was thinking previously.Thanks,-mark

author: Bhagwan
author_rank: Newbie
timestamp: June 18, 2014, 09:34:41 pm
content: Attached is modified schematic for 30 MHz -1 GHz.Changed R3 to 84.12K and added resistors in parallel with R4 and R5.I need to try this in LTspice.But for your model of 150 KHz - 30 MHz I think the 20K for R3 may be wrong though. Per HP schematic 21.5K in series with 1.62K = “about” 23.12K. I say "about" because there are some other resistors in parallel with 1.62k but they’re high value so insignificant. How did you arrive at 20K?I’m also wondering if a buffer (unity gain amplifier) should be inserted between QP time constants and meter LP filter just like the HP schematic?