Radio/Microwave CCDs - Page 1
Forum section: electronics / general

author: Phaedrus
author_rank: Frequent Contributor
timestamp: September 30, 2014, 01:25:13 am
content: of Dave's gave me a thought. Are there charge couple devices that can be used for imaging in the Radio and Microwave bands? Imagine being able to use something like a digital camera to take a picture of your board in RF andseewhere it's leaking EMI. Or be able to visualize the data transmission from wireless devices. It seems like this would be quite cool/useful. Do these exist, and if not, why not?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 30, 2014, 01:37:56 am
content: The wavelength is much too large to get much useful information.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2014, 08:15:25 am
content: In a fundamental sense?CCDs work by displacing charge carriers (electrons, and the absence of electrons = holes) when a photon is absorbed. At visible frequencies, this works great, silicon responds strongly and it's cheap and popular to work with. Down to maybe 1000nm (~300THz), silicon is still okay, but to go any lower, you need a different material. Why? The electrons in silicon are held in energy bands with a fixed spacing, the band gap: electrons can't move about (and couple charge and be scanned and read) unless they're given at least that much energy. Anything less is a hard cutoff, at best getting helped along by the high-energy fringes of ambient thermal energy.Which gives you two ideas right away. One, a lower bandgap material (like InSb, or HgCdTe, or..) will require less energy to push electrons around. But, thermal energy becomes an ever larger relative fraction of that, which leads to random charges, which means at best, a noisy sensor with poor sensitivity, but at worst, something which simply doesn't work at all (because the N/P doping process that leads to useful semiconductors is just overwhelmed by thermally generated carriers, and the whole lump looks like a dumb resistor!). So the second idea is, to do the job, it must be cool. Cold. Cryogenic, if possible.Another, more thermodynamical, way of thinking about it: a sensor at 300K isn't going to be very sensitive to a warm body at 300K. In fact, it will be emitting exactly the same (far IR) radiation as the subject! Sensitivity is necessarily no better than 50%, i.e., half the photons "detected" are its own (or stray electrons, or leakage, or who knows what).Now, if you average over a difference, you can address this a bit -- instead of looking for absolute emission, you look for small changes in it. The noise floor gets magnified, which sucks (in an already rather noisy system to start with!), but if you have big enough sensors and low enough bandwidth, maybe you can make something of it. This is, to some extent, why thermal cams are so slow -- fundamental limitations of the detector, or the signal processing.Now, suppose you wanted to detect 10um photons (~room temp, 30THz). Silicon really just doesn't detect anything at all. You need 1/10th the band gap of silicon, and potentially, 1/10th the  temperature. If you select such a material, build a sensor out of it, and hide it behind a cryostat (liquid hydrogen or neon, or maybe vacuum pumped nitrogen or argon), or a seriously sick Peltier stack, you can do pretty well, and this is exactly what's used (for CCD sensors, that is; there are other methods used in things like the FLIR cams, which don't require cooling, or much anyway, but suck more -- frequent calibration steps, for example).So how about even lower? Well, obviously to get into the 3THz (100um) range, you need 1/10th the bandgap and temperature again. Well, 1/100th of room temperature, now we're talking liquid helium, and things just got a whole lot uglier. Besides that, compounds with such a minuscule band gap are hard to come by: at room temperature, they're indistinguishable from ordinary metals, they're so full of  electrons! What to do?Now we're getting into some of the more esoteric ranges of optics, where light and radio meet. Today, "circuit" and "optic" methods both exist for the generation and detection of THz waves, and imaging technology (not necessarily passive thermal) is being produced.What about lower? Can we do microwaves? Absolutely -- of course, as the wavelength continues to stretch, the pixels need to stretch, too. THz sensors are about at the limit of what monolithic ICs can offer -- if the pixels are more than, say, tens of micrometers on a side, you're wasting so much space that you're starting to be better off fabricating it with individual pixels on a PCB! (An 0201 package is only 600 x 300um.) We're also running out of cheap materials to use for optics -- or rather, not that materials are hard to come by (e.g., the index of refraction of something like polyethylene is just about constant from DC to light, except for a few holes in the IR range where the hydrogen bonds resonate), but that your lens gets so massive that you want to look at other possibilities!What might a microwave imaging sensor look like? An array of antennas on a structure; perhaps something more robust than a PCB, like this:http://en.wikipedia.org/wiki/File:PAVE_PAWS_Radar_Clear_AFS_Alaska.jpg(Although what they're doing is more akin to a lens of antennas, than a dull sensor.)How low can you go? Thehttp://en.wikipedia.org/wiki/Very_Large_Arraysays down to 73MHz (~4m?), not bad at all. Of course, they're doing things even more esoteric; as I understand it, it's not that they're piping coax cables in from each antenna and projecting them somehow onto a screen: rather, they record the observations wholesale (on "tape", which I'm sure has long since been changed to something more reliable and electronic), then match up all the recordings based on coincidences first, then do a bunch of math to form an image (which includes phase as well as magnitude information).Tim

author: jeremy
author_rank: Super Contributor
timestamp: September 30, 2014, 10:42:04 am
content: Interesting stuff Tim! As an aside, some people I work with deal with 10um lasers, and you most certainly need liquid helium for those, hydrogen wouldn't cut it. At least with current technology.

author: Lycagon
author_rank: Newbie
timestamp: October 13, 2025, 06:25:06 am
content: Actually there is a way. It's called ccp (not chinese communist part) ; charge coupling polarization. In other words, columns of charge are coupled using one or another type of polarizing material charged with 500kv, for example. Now, I know this was done in the early 90's using a 200k fiber optic. It was slid into a hole in a block of lithium tantalate which was charged with 500kv. the 200,000 pixels from that many fiber optic cables were possible by taking single fiber optic cables and re-melting them on a tower to make micron size fibers all in one that was about 1.5mm across or so. The fiber could see through walls, solid brick walls, because the 500kv causes all the properties of each individual strand to produce a copy of it's signature emf field to a good distance from the cable. Now, that's for optical and I heard the navy might have used that for some spy project which is why we don't hear about it.So, if you want to do something similar with emf, you simply need gold fiber in glass I suppose, idk. Maybe an element that can light up from electrostatic, like phosphor at the receiving tip just by a lens to project it's image.