Talk:Distributed-element filter
Latest comment: 15 years ago by Spinningspark in topic Prose comments by Cryptic C62
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Rewrote the lead
The old lead had some problems.
- A distributed element filter is an electronic filter designed for frequencies above the VHF band or thereabouts. At these frequencies the wavelengths of the operative signals are shorter than the size of the device being constructed and it is no longer possible to use the more familiar lumped element model. Filters and other passive circuitry are instead designed using the distributed element model in which the components of the circuit are treated as being transmission lines, which indeed, they effectively are.
Note that at the top of VHF band the wavelength in air is ~1 meter. Lumped elements crap out about here but clearly "wavelengths of the operative signals" are not "shorter than the size of the device being constructed". I've also added some helpful wikilinks. I think the new language flows a bit better as well but YMMV. JPatterson (talk) 20:34, 15 February 2010 (UTC)
Things to improve
- Front figure: "for suppressing non-TEM modes of transmission" needs a cite. Adding a note on image width would help understanding the size - not everyone knows this connector type. Image file needs a better description, preferably explaining why rubber seal (moisture ?).
- I have addressed the "through holes for TEM mode" issue. Image size is a bit problematic; it is not my image so I cannot retake it with a rule included. It would, of course, be possible to estimate the size from some of the circuit structures, but that is almost the definition of OR. I am against explaining the rubber seal (it is to give the enclosure an IP rating because the product is intended for outdoor use) - this is going way off-topic; there are any number of features that could be explained here. The only one mentioned, the through holes, are only described because they can easily be mistaken for a filter structure. SpinningSpark 20:48, 8 April 2010 (UTC)
- Regarding the via fences: there will be an internal metal lid cast with a pattern of walls on the underside, which corresponds to the pattern of via-fences. This lid is clamped down tightly onto the circuit by a large number of bolts - notice all the screw holes associated with the via fences (it can't have been screwed down very well in this example as the walls have not made a visible indentation in the metallization). The via fences connect the base of the walls to the ground-plane, so forming a number of near perfectly isolated enclosures, communicating only where the microstrip passes through a notch in the cast walls. This internal lid is in addition to a simple external lid which makes a seal with the rubber gasket.--catslash (talk) 10:41, 29 April 2010 (UTC)
- Regarding the photo: I've just put a more close-up view of a (just) a filter at File:Microstrip-bandpass-filter.jpg. I was going to use this to illustrate edge-coupling on the microstrip page, but never got around to writing about it. I haven't put the picture on this page, as you may feel that the existing one is better - take a look and see what you think. --catslash (talk) 16:35, 29 April 2010 (UTC)
- Nice picture. That would be great for the parallel-coupled lines section. I don't think we should expand on via fences in this article, it is only mentioned for clarification purposes. I have red-linked it with the intention that an article should go there someday. SpinningSpark 07:35, 30 April 2010 (UTC)
- OK. I just felt that the phrase for suppressing non-TEM modes of transmission though strictly speaking true, is rather misleading. --catslash (talk) 11:55, 30 April 2010 (UTC)
- Nice picture. That would be great for the parallel-coupled lines section. I don't think we should expand on via fences in this article, it is only mentioned for clarification purposes. I have red-linked it with the intention that an article should go there someday. SpinningSpark 07:35, 30 April 2010 (UTC)
- There is some imbalance between figure captions - one cites references and others don't. I probably understand why, but this could be picked up at FAC.
- There is usually no need to give citations to image captions unless they introduce facts not in the main text. All the figures here are described in the text where a citation is given. I have made an exception for figure 2 because it is a composite from a number of different sources and I wished to make it clear which filter structure came from which source. SpinningSpark 20:30, 9 April 2010 (UTC)
- Add history subsection?
- I am actively looking into this. The history of the various technologies (co-ax, stripline, microstrip, waveguide etc) is well documented, mostly of them starting in radar in WWII and the filters in these technologies followed shortly after their first use. However, sources dealing with distributed element filtering as a general topic are a bit harder to find and I need to see if enough material can be gathered to write a worthwile section. SpinningSpark 20:30, 9 April 2010 (UTC)
- I have now started a history section. There is still a bit more that could go in, but I think the point is now addressed. SpinningSpark 18:13, 11 April 2010 (UTC)
- Need to clarify or wikilink "quarter wavelength stubs", "LC (anti-)resonators" (even LC needs some wikilink at first appearance), "impedance or admittance transformers" Materialscientist (talk) 04:21, 6 April 2010 (UTC)
Prose comments by Cryptic C62
Resolved comments
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Unresolved comments
Here are some comments on the article's prose intended to help make the article accessible and precise:
- "they have the drawback that it is impossible to create an ideal open circuit" Because I'm a physics nerd, I automatically assume that "ideal" has some special meaning, as is the case for ideal gas. What meaning is it intended to convey here?
- Yes, exactly in the sense of an ideal gas which perfectly obeys some law of physics. Even more relevant, it is the same sense in which we talk of ideal voltage or current generators in electrical analysis. Specifically here is meant a circuit element through which zero current flows regardless of the voltage across it. SpinningSpark 17:56, 1 July 2010 (UTC)
- Is it ever possible to create an ideal circuit? I only ask because ideal gasses do not exist, so I would assume that the same is true of ideal circuits. If such is the case, then the highlighted sentence doesn't really make any sense. If it is possible to create an ideal circuit, I think this sentence would benefit from a wikilink (I couldn't find any appropriate articles) or a brief explanation—the article should be accessible to everyone, not just to circuitry nerds. --Cryptic C62 · Talk 00:06, 4 July 2010 (UTC)
- Ideal circuit elements do not in reality exist but it is perfectly valid to examine how well a real component corresponds to the ideal. Electrical analysis is entirely based on networks of ideal elements so it is not only unavoidable, but actually helpful, to discuss ideal elements here and there. Even when it is desired to model a non-ideal component, this is done by adding additional ideal elements to its network representation - a real non-ideal resistor might be modelled, for instance, by adding a capacitor in parallel, and an inductor in series with it. In the case of a printed line being left open-circuit, this is a non-ideal open-circuit because of the unavoidable dielectric effect of the board on which it is printed. Consequently, this non-ideal open circuit is frequently modelled as shown in figure 4b as a parallel capacitor. This is to be compared to a line terminated with a short-circuit achieved with a strap through a via hole to the ground plane beneath. To be sure, this will not be an ideal element either, the strap must possess some resistance for instance. However, it is close enough to the ideal to be treated as ideal for most design purposes. The dielectric effect causing such a problem for open-circuits is not a problem here since anything in parallel with a short-circuit is still a short-circuit; the capacitive effect has been shorted out. I will try tweaking the wording though, to make this a bit clearer. SpinningSpark 09:47, 4 July 2010 (UTC)
- Is it ever possible to create an ideal circuit? I only ask because ideal gasses do not exist, so I would assume that the same is true of ideal circuits. If such is the case, then the highlighted sentence doesn't really make any sense. If it is possible to create an ideal circuit, I think this sentence would benefit from a wikilink (I couldn't find any appropriate articles) or a brief explanation—the article should be accessible to everyone, not just to circuitry nerds. --Cryptic C62 · Talk 00:06, 4 July 2010 (UTC)
- The following two sentences seem to contradict each other: "While open-circuit stubs are easier to manufacture in planar technologies, they have the drawback that it is impossible to create an ideal open circuit (see figure 4(b)), often leading to a preference for short-circuit stubs (one can always be used in place of the other by adding or subtracting λ/4 to or from the length)" and "Coupled lines tend to be preferred in planar technologies, where they are easy to implement, whereas stubs tend to be preferred elsewhere." If open-circuit stubs are easy to manufacture in planar technologies, then how can it be that coupled lines are preferred in planar technologies because they are easy to implement?
- There is no contradiction; coupled lines are preferred over stubs especially for higher order (which means more complex) filters if all other considerations are equal. Where stubs are being used (and there can be good reasons for using stubs) the open-circuit version is easier to manufacture, but has less accuracy than the short-circuit version. The example in the lede image shows these forms: in the full monty wide-band high-order filters, the designer has used parallel coupled resonators. For the simple bias filters where only one resonator is used, s/he has chosen to use a stub (parallel coupled lines would not actually work at all in this application because dc is not passed by such a structure). In both cases s/he can't be asked to bother with the complication of providing short-circuits in the filter (again, in the case of the bias filter this is ruled out in any case by the need to pass dc) and has settled for the less accurate open-circuit version. SpinningSpark 17:56, 1 July 2010 (UTC)
- I think the most pertinent piece of information which you just explained to me is that coupled lines are preferred for higher-order filters, as this is not made clear in the article. --Cryptic C62 · Talk 00:06, 4 July 2010 (UTC)
- I think the point you are missing is that the first sentence you highlight is comparing different types of stub in a paragraph discussing stubs. The second sentence is comparing stubs as a whole to coupled lines as a whole in a completely different section of the article. SpinningSpark 09:47, 4 July 2010 (UTC)
More to come. As you make changes in the article, please respond below individual concerns so I know which are done and which need further discussion. Thanks! --Cryptic C62 · Talk 00:52, 15 June 2010 (UTC)