More Fun Stories, Physicists and Engineering Nomenclature - Page 1
Forum section: electronics / general

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 07:23:43 pm
content: While commenting on another thread wrt to Resonances and Antiresonance or as we recall Series and Parallel Resonances we recalled the many technical "discussions" we've had with Physicists, one in particular comes to mind.Way back in ~1980 a development program called XM-21, a Remote Chemical Agent Detector for battlefield deployment, was under scrutiny and going to be canceled because the Chief Science Advisor to US President Regan said the concept of passively detecting chemical agents at distance was totally impossible, and many folks agreed. However the XM-21 program lobbied for a real time field test which was conducted and the test instrument passed with flying colors and the program development continued.Soon after we were addressing with the Point Blank Thermo Nuclear Detonation requirement, which was ridiculous and tried to convince this wasn't necessary. We lost the argument and had to meet these requirements for a survival of such which we eventually did which probably alone quadrupled the instrument cost!!The full Military Electromagnetic Compatibility as well as the EMP from Nuclear Blast above requirements were also imposed and this created some issues with the large Optical Input Window which was designated to be ZnSe. The EMC experts wanted to impose the use of a metallic screen across the window which would reduce the sensitive of the instrument by 20~30% ruining all the efforts with the Split-Cycle Sterling Cryogenically Cooled Ultra-Sensitve IR HgCdTe Detector (which was performing beyond theoretical predictions at the time, later IR detector theory was adapted (another story)).To us this seemed unacceptable throwing away all that hard earned sensitivity and set out to find another solution. We are not Optical Experts, never were, and never will be, altho we do (did) have a good grasp of EM wave theory. After much research we came across the developments of the HgCdTe detectors and realized that Germanium metal might be useful at 8-12um region where the XM-21 operated and could replace the ZnSe Optical Window.At first everyone thought this was ridiculous and the hard core Optical Physicists discarded it because we were an EE and how could an EE come up with something to replace the tried and true ZnSe. We informed them during a meeting that Ge has an optical transmission good enough in the 8-12um region for use and should be investigated. This set up another meeting later in the week, where the customer attended with their experts.In this later meeting we had an ace under our sleeve and presented our concept of the optical window made from a Ge sheet thick enough to survive the nuclear blast wave. After a few "attacks" we asked how they were going to pass the EMC/EMP tests with the ZnSe window, and the EMC experts presented their screen approach which significantly reduced the instrument sensitivity.Then we pulled out the Ace and said the Ge window can be doped with an impurity to increase the lower frequency conductivity such that an analysis based upon 3D transmission line theory showed enough impedance mismatch at the window would reflect the Interference Waves to pass the EMC/EMP requirements, so no metallic screen required!! We also showed how this would reduce the 8-12um transmission a small amount with an outside and inside Ge surface impedance matching 3D 1/4 wave coating to match free space at 8-12um. We caught all sorts of flack about this, included an argument about the Index of Refraction and the Complex Dielectric Constant effects/meaning/relationship and how we had done the analysis. We were getting hammered from all sides, the EMC experts, the Optical Physicists and others, so we proposed a solution to this standoff. Hire an industry expert to access the various approaches and rate them.In the meantime we ordered a Germanium wafer and had it doped to increase the conductivity. By the time the next meeting was scheduled and the expert (University Dean, MIT PhD in Microwave Theory) was to present, we had collected some preliminary test results comparing the ZnSe window with the doped Ge window. At the meeting the consultant expert presented his findings (within +-1dB of ours), the EMC experts got up and left the meeting at the break and did not return. We were invited to present the data we had collected in the meantime which showed Interferrograms collected from the XM-21 Test System with various test sources. The results were almost identical between the ZnSe window (without the required EMC metal screen) and the doped Germanium window. At that point the decision was to forgo the ZnSe window and replace with doped Germanium with an Impedance Matching (Antireflective) 1/4 wave front and rear coatings. The reason for the front and rear coatings on the Ge window is to impedance match into the Ge from free space, then pass thru the Ge and then exit to free space again (instrument interior was open space for the optical path to the scanning mirror). Of course this is just Antireflective coatings in optical terms just like you see on your Nikon Macro Lens, front and back coatings or as we like to infer....just impedance matchingAnyway, this is one story and we have a few others if interested.Best